Unix shell scripting with ksh/bash

Course Handout: (last update 22 March 2012)

These notes may be found at http://www.dartmouth.edu/~rc/classes/ksh. The online version has many links to additional information and may be more up to date than the printed notes

UNIX shell scripting with ksh/bash

The goals of this class are to enable you to:

These notes are intended for use in a 2-part class, total duration 3 hours.

It is assumed that you already know how to:

Example commands are shown like this. Many commands are shown with links to their full man pages (sh)
Output from commands is shown like this; optional items are [ in brackets ].

Some descriptions in these notes have more detail available, and are denoted like this:

More details of this item would appear here. The printed notes include all of the additional information

Permission is granted to download and use these notes and example scripts, as long as all copyright notices are kept intact. Some of the examples are taken from texts or online resources which have granted permission to redistribute.
These notes are updated from time to time. The "development" set of notes are http://northstar-www.dartmouth.edu/~richard/classes/ksh (Dartmouth only)

Richard Brittain, Dartmouth College Computing Services.
© 2003,2004,2010 Dartmouth College.
Comments and questions, contact Richard.Brittain @ dartmouth.edu

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Table of Contents

1.What is a shell script
2.Why use shell scripts
4.Feature comparison
5.Other scripting languages
6.ksh/bash vs sh
8.Filename Wildcards
10.Preset Variables
12.Shell options
13.Command substitution
14.I/O redirection and pipelines
15.Input and output
16.Conditional Tests
17.Conditional Tests (contd.)
18.Flow control
19.Flow control (contd.)
20.Conditional test examples
22.Manipulating Variables
24.Advanced I/O
25.Wizard I/O
32.Common external commands


What is a Shell Script

A shell script can be as simple as a sequence of commands that you type regularly. By putting them into a script, you reduce them to a single command.

Example: ex0 display, text

   1: #!/bin/sh
   2: date
   3: pwd
   4: du -k


Why use Shell Scripts

Typical uses



History of Shells

aka "Bourne" shell, written by Steve Bourne at AT&T Bell Labs for Unix V7 (1979). Small, simple, and (originally) very few internal commands, so it called external programs for even the simplest of tasks. It is always available on everything that looks vaguely like Unix.

The "C" shell. (Bill Joy, at Berkeley). Many things in common with the Bourne shell, but many enhancements to improve interactive use. The internal commands used only in scripts are very different from "sh", and similar (by design) to the "C" language syntax.

The "TC" shell. Freely available and based on "csh". It has many additional features to make interactive use more convenient.
We use it as the default interactive shell for new accounts on all of our public systems.
Not many people write scripts in [t]csh. See Csh Programming Considered Harmful by Tom Christiansen for a discussion of problems with programming csh scripts.

The "Korn" shell, written by David Korn of AT&T Bell Labs (now AT&T Research). Written as a major upgrade to "sh" and backwards compatible with it, but has many internal commands for the most frequently used functions. It also incorporates many of the features from tcsh which enhance interactive use (command line history recall etc.).
It was slow to gain acceptance because earlier versions were encumbered by AT&T licensing. This shell is now freely available on all systems, but sometimes not installed by default on "free" Unix. There are two major versions. ksh88 was the version incorporated into AT&T SVR4 Unix, and may still be installed by some of the commercial Unix vendors. ksh93 added more features, primarily for programming, and better POSIX compliance.

POSIX 1003.2 Shell Standard.
Standards committees worked over the Bourne shell and added many features of the Korn shell (ksh88) and C shell to define a standard set of features which all compliant shells must have.
On most systems, /bin/sh is now a POSIX compliant shell. Korn shell and Bash are POSIX compliant, but have many features which go beyond the standard. On Solaris, the POSIX/XPG4 commands which differ slightly in behaviour from traditional SunOS commands are located in /usr/xpg4/bin

The "Bourne again" shell. Written as part of the GNU/Linux Open Source effort, and the default shell for Linux and Mac OS-X. It is a functional clone of sh, with additional features to enhance interactive use, add POSIX compliance, and partial ksh compatability.

A freeware functional clone of sh, with parts of ksh, bash and full POSIX compliance, and many new interactive command-line editing features. It was installed as the default shell on early MacOSX systems.


Comparison of shell features

All the shells just listed share some common features, and the major differences in syntax generally only affect script writers. It is not unusual to use one shell (e.g. tcsh) for interactive use, but another (sh or ksh) for writing scripts.

Core Similarities (and recap of basic command line usage)

Each of these items is discussed in more detail later.

Principal Differences

between sh (+derivitives), and csh (+derivitives).


Other Scripting Languages

There are many other programs which read a file of commands and carry out a sequence of actions. The "#!/path/to/program" convention allows any of them to be used as a scripting language to create new commands. Some are highly specialized, and some are much more efficient than the equivalent shell scripts at certain tasks. There is never only one way to perform a function, and often the choice comes down to factors like: Some major players (all of these are freely available) in the general purpose scripting languages are:


ksh/bash vs sh

Ksh and bash are both supersets of sh. For maximum portability, even to very old computers, you should stick to the commands found in sh. Where possible, ksh or bash-specific features will be noted in the following pages. In general, the newer shells run a little faster and scripts are often more readable because logic can be expressed more cleanly user the newer syntax. Many commands and conditional tests are now internal.
The philosophy of separate Unix tools each performing a single operation was followed closely by the designers of the original shell, so it had very few internal commands and used external tools for very trivial operations (like echo and [). Ksh and bash internally performs many of the basic string and numeric manipulations and conditional tests. Occasional problems arise because the internal versions of some commands like echo are not fully compatible with the external utility they replaced.

The action taken every time a shell needs to run an external program is to locate the program (via $PATH), fork(), which creates a second copy of the shell, adjust the standard input/output for the external program, and exec(), which replaces the second shell with the external program. This process is computationally expensive (relatively), so when the script does something trivial many times over in a loop, it saves a lot of time if the function is handled internally.

If you follow textbooks on Bourne shell programming, all of the advice should apply no matter which of the Bourne-derived shells you use. Unfortunately, many vendors have added features over the years and achieving complete portability can be a challenge. Explicitly writing for ksh (or bash) and insisting on that shell being installed, can often be simpler.

The sh and ksh man pages use the term special command for the internal commands - handled by the shell itself.


Basic sh script syntax

The most basic shell script is a list of commands exactly as could be typed interactively, prefaced by the #! magic header. All the parsing rules, filename wildcards, $PATH searches etc., which were summarized above, apply.
In addition:
# as the first non-whitespace character on a line
flags the line as a comment, and the rest of the line is completely ignored. Use comments liberally in your scripts, as in all other forms of programming.

\ as the last character on a line
causes the following line to be logically joined before interpretation. This allows single very long commands to be entered in the script in a more readable fashion. You can continue the line as many times as needed.
This is actually just a particular instance of \ being to escape, or remove the special meaning from, the following character.

; as a separator between words on a line
is interpreted as a newline. It allows you to put multiple commands on a single line. There are few occasions when you must do this, but often it is used to improve the layout of compound commands.
Example: ex1 display, text

   1: #!/bin/ksh
   2: # For the purposes of display, parts of the script have 
   3: # been rendered in glorious technicolor.
   4: ## Some comments are bold to flag special sections
   6: # Line numbers on the left are not part of the script.
   7: # They are just added to the HTML for reference.
   9: # Built-in commands and keywords (e.g. print) are in blue
  10: # Command substitutions are purple. Variables are black
  11: print "Disk usage summary for $USER on `date`"
  13: # Everything else is red - mostly that is external 
  14: # commands, and the arguments to all of the commands.
  15: print These are my files       # end of line comment for print
  16: # List the files in columns
  17: ls -C
  18: # Summarize the disk usage
  19: print
  20: print Disk space usage
  21: du -k 
  22: exit 0

Exit status

Every command (program) has a value or exit status which it returns to the calling program. This is separate from any output generated. The exit status of a shell script can be explicitly set using exit N, or it defaults to the value of the last command run.
The exit status is an integer 0-255. Conventionally 0=success and any other value indicates a problem. Think of it as only one way for everything to work, but many possible ways to fail. If the command was terminated by a signal, the value is 128 plus the signal value.


Filename Wildcards

The following characters are interpreted by the shell as filename wildcards, and any word containing them is replaced by a sorted list of all the matching files.
Wildcards may be used in the directory parts of a pathname as well as the filename part. If no files match the wildcard, it is left unchanged. Wildcards are not full regular expressions. Sed, grep, awk etc. work with more flexible (and more complex) string matching operators.
Match zero or more characters.
Match any single character
Match any single character from the bracketed set. A range of characters can be specified with [ - ]
Match any single character NOT in the bracketed set.
chapter[1-5].* could match chapter1.tex, chapter4.tex, chapter5.tex.old. It would not match chapter10.tex or chapter1


Shell Variables

Scripts are not very useful if all the commands and options and filenames are explicitly coded. By using variables, you can make a script generic and apply it to different situations. Variable names consist of letters, numbers and underscores ([a-zA-Z0-9_], cannot start with a number, and are case sensitive. Several special variables (always uppercase names) are used by the system -- resetting these may cause unexpected behaviour. Some special variables may be read-only. Using lowercase names for your own variables is safest.

Setting and exporting variables

Creates (if it didn't exist) a variable named "srcfile" and sets it to the value "dataset1". If the variable already existed, it is overwritten. Variables are treated as text strings, unless the context implies a numeric interpretation. You can make a variable always be treated as a number. Note there must be no spaces around the "=".
Display all the variables currently set in the shell
unset srcfile
Remove the variable "srcfile"
Give the variable a null value, (not the same as removing it).
export srcfile
Added srcfile to the list of variables which will be made available to external program through the environment. If you don't do this, the variable is local to this shell instance.
List all the variables currently being exported - this is the environment which will be passed to external programs.

Using variables

Prefacing the variable name with $ causes the value of the variable to be substituted in place of the name.
If the variable is not surrounded by whitespace (or other characters that can't be in a name), the name must be surrounded by "{}" braces so that the shell knows what characters you intend to be part of the name.


# Tries to find $datafile_part1, which doesn't exist
echo $datafile_part1.sas 
# This is what we intended
echo ${datafile}_part1.sas  

Conditional modifiers

There are various ways to conditionally use a variable in a command.
Substitute the value of $datafile, if it has been defined, otherwise use the string "default". This is an easy way to allow for optional variables, and have sensible defaults if they haven't been set. If datafile was undefined, it remains so.
Similar to the above, except if datafile has not been defined, set it to the string "default".
If variable datafile has been defined, use the string "default", otherwise use null. In this case the actual value $datafile is not used.
${datafile?"error message"}
Substitute the value of $datafile, if it has been defined, otherwise display datafile: error message. This is used for diagnostics when a variable should have been set and there is no sensible default value to use.
Placing a colon (:) before the operator character in these constructs has the effect of counting a null value the same as an undefined variable. Variables may be given a null value by setting them to an empty string, e.g. datafile= .
Example: echo ${datafile:-mydata.dat}
Echo the value of variable datafile if it has been set and is non-null, otherwise echo "mydata.dat".

Variable assignment command prefix

It is possible to export a variable just for the duration of a single command using the syntax:
var=value command args


Preset Shell Variables

Several special variables are used by the system -- you can use these, but may not be able to change them. The special variables use uppercase names, or punctuation characters. Some variables are set by the login process and inherited by the shell (e.g. $USER), while others are used only by the shell.
Try running set or env
These are some of the more commonly used ones:

Login environment

Preset to the currently logged-in username.
The list of directories that will be searched for external commands. You can change this in a script to make sure you get the programs you intend, and don't accidentally get other versions which might have been installed.
The terminal type in which the shell session is currently executing. Usually "xterm" or "vt100". Many programs need to know this to figure out what special character sequences to send to achieve special effects.
If set, this contains the name of the program which the user prefers to use for text file viewing. Usually set to "more" or "less" or something similar. Many programs which need to present multipage information to the user will respect this setting (e.g. man). This isn't actually used by the shell itself, but shell scripts should honour it if they need to page output to the user.
If set, this contains the name of the program which the user prefers to use for text file editing. A program which needs to have the user manually edit a file might choose to start up this program instead of some built-in default (e.g. "crontab -e". This also determines the default command-line-editing behaviour in interactive shells.

Shell internal settings

Always set the current working directory (readonly)
The previous directory (before the most recent cd command). However, changing directories in a script is often dangerous.
$? (readonly)
Set to the exit status of the last command run, so you can test success or failure. Every command resets this so it must be saved immediately if you want to use it later.
Set to the currently set options flags.
Internal Field Separators: the set of characters (normally space and tab) which are used to parse a command line into separate arguments. This may be set by the user for special purposes, but things get very confusing if it isn't changed back.

Process ID variables

$$ (readonly)
Set to the process ID of the current shell - useful in making unique temporary files, e.g. /tmp/$0.$$
$PPID (readonly)
Set to the process ID of the parent process of this shell - useful for discovering how the script was called.
$! (readonly)
Set to the process ID of the last command started in background - useful for checking on background processes.

ksh/bash additional features

$SECONDS (readonly)
Integer number of seconds since this shell was started. Can be used for timing commands.
Every time it is valuated, $RANDOM returns a random integer in the range 0-32k. RANDOM may be set to "seed" the random number generator.
$LINENO (readonly)
Always evaluates to the current line number of the script being executed - useful for debugging.


Command Line (positional) arguments

To customize the behaviour of a script at run time, you can give it any number of arguments on the command line.
These are often filenames, but can be interpreted by the script in any way. Options are often specified using the "-flag" convention used by most Unix programs, and a ksh command getopts is available to help parse them.
The shell expands wildcards and makes variable and command substitutions as normal, then parses the resulting words by whitespace (actually special variable $IFS), and places the resulting text strings into the positional variables as follows:
$0, $1, $2, ... $9
The first 9 arguments are made available directly as $1-$9. To access more than 9, use shift, or $*, $@. The variable $0 contains the name of the script itself.
${10}, ${11}, ...
Positional arguments greater than 9 are set by ksh and bash. Remember to use braces to refer to them.
discard $1 and renumber all the other variables. "shift N" will shift N arguments at once.
contains the number of arguments that were set (not including $0).
contains all of the arguments in a single string, with one space separating them.
similar to $*, but if used in quotes, it effectively quotes each argument and keeps them separate. If any argument contains whitespace, the distinction is important.
e.g. if the argument list is: a1 a2 "a3 which contains spaces" a4
then: $1=a1, $2=a2, $3=a3 which contains spaces, $4=a4
and: $*=a1 a2 a3 which contains spaces a4
and: "$@"="a1" "a2" "a3 which contains spaces" "a4"

Only using the form "$@" preserves quoted arguments. If the arguments are being passed from the script directly to some other program, it may make a big difference to the meaning.

Example: ex7 display, text

   1: #!/bin/sh
   2: #
   3: # Check positional argument handling
   4: echo "Number of arguments: $#"
   5: echo "\$0 = $0"
   7: echo "Loop over \$*"
   8: for a in $*; do
   9:    echo \"$a\"
  10: done
  12: echo "Loop over \"\$@\""
  13: for a in "$@"; do
  14:    echo \"$a\"
  15: done

Setting new positional arguments

The set command, followed by a set of arguments, creates a new set of positional arguments. This is often used, assuming the original arguments are no longer needed, to parse a set of words (possibly using different field separators). Arguments may be reset any number of times.

Example: ex2 display, text

   1: #!/bin/sh
   2: # Find an entry in the password file
   3: pwent=`grep '^root:' /etc/passwd`
   4: # Turn off globbing - passwd lines often contain '*'
   5: set -o noglob
   6: # The "full name" and other comments are in
   7: # field 5, colon delimited.  Get this field using shell word splitting
   8: OIFS=$IFS; IFS=: ; set $pwent; IFS=$OIFS
   9: echo $5

Example: pickrandom display, text
Selects a random file from a directory. Uses the ksh RANDOM feature.

   1: #!/bin/ksh
   3: # Select a random image from the background logo collection
   4: # This could be used to configure a screen saver, for example.
   5: #
   6: # This works even if the filenames contain spaces.
   8: # switch to the logos directory to avoid long paths
   9: logos=/afs/northstar/common/usr/lib/X11/logos/backgrounds
  10: cd $logos
  12: # '*' is a filename wildcard to match all files in the current directory
  13: set *
  15: # Use the syntax for arithmetic expressions.  "%" is the modulo operator
  16: # Shift arguments by a random number between 0 and the number of files
  17: shift $(($RANDOM % $#))
  19: # Output the resulting first argument
  20: echo "$logos/$1"


Shell options

Startup options. ksh -options scriptname
echo line to stderr before executing it
read commands and check for syntax errors, but do not execute.
all variables are automatically exported
disable wildcard filename expansion (globbing)
set -x
Set an option within a shell script
contains the currently set option letters
There are many other options, not often needed. Options in ksh and bash can also be set using long names (e.g. -o noglob instead of -f). Many options are unique to ksh or bash.


Command Substitution

sh syntax

A command (plus optional arguments) enclosed in backticks is executed and the standard output of that command is substituted. If the command produces multiline output, the newlines are retained. If the resultant string is displayed, unquoted, using echo, newlines and multiple spaces will be removed.

ksh/bash syntax

This syntax is functionally the same as backticks, but commands can be more easily nested.
This is equivalent to `cat file`, but implemented internally for efficiency.

Example: ex3 display, text

   1: #!/bin/ksh
   3: echo Today is `date`
   5: file=/etc/hosts
   6: echo The file $file has $(wc -l < $file) lines
   8: hostname -s > myhostname
   9: echo This system has host name $(<myhostname)


I/O redirection and pipelines

Any simple command (or shell function, or compound command) may have its input and output redirected using the following operators. This is performed by the shell before the command is run.

Output redirection

> filename
Standard ouput (file descriptor 1) is redirected to the named file. The file is overwritten unless the noclobber option is set. The file is created if it does not exist.
The special device file /dev/null can be used to explicitly discard unwanted output. Reading from /dev/null results in an End of File status.
>> filename
Standard ouput is appended to the named file. The file is created if it does not exist.
>| filename
Output redirect, and override the noclobber option, if set.

Input redirection

< filename
Standard input (file descriptor 0) is redirected to the named file. The file must already exist.

Command pipelines

command | command [ | command ...]
Pipe multiple commands together. The standard output of the first command becomes the standard input of the second command. All commands run simultaneously, and data transfer happens via memory buffers. This is one of the most powerful constructs in Unix. Compound commands may also be used with pipes. Pipes play very nicely with multiprocessor systems.
No more than one command in a pipeline should be interactive (attempt to read from the terminal). This construct is much more efficient than using temporary files, and most standard Unix utilities are designed such that they work well in pipelines.

The exit status of a pipeline is the exit status of the last command. In compound commands, a pipeline can be used anywhere a simple command could be used.


Input and Output

Shell scripts can generate output directly or read input into variables using the following commands:

Script output

Print arguments, separated by spaces, and terminated by a newline, to stdout. Use quotes to preserve spacing. Echo also understands C-like escape conventions.
Beware that the shell may process backslashes before echo sees them (may need to double backslash). Internal in most shells, but was originally external.
\b backspace \c print line without new-line (some versions)
\f form-feed \n new-line
\r carriage return \t tab
\v vertical tab \\ backslash
\0n where n is the 8-bit character whose ASCII code is the 1-, 2- or 3-digit octal number representing that character.
suppress newline
print (ksh internal)
Print arguments, separated by spaces, and terminated by a newline, to stdout. Print observes the same escape conventions as echo.
suppress newline
raw mode - ignore \-escape conventions
raw mode - ignore \-escape conventions and -options except -n.

Script input

read var1 var2 rest
read a line from stdin, parsing by $IFS, and placing the words into the named variables. Any left over words all go into the last variable. A '\' as the last character on a line removes significance of the newline, and input continues with the following line.
raw mode - ignore \-escape conventions

Example: ex4a display, text

   1: #!/bin/sh
   2: echo "Testing interactive user input: enter some keystrokes and press return"
   3: read x more
   4: echo "First word was \"$x\""
   5: echo "Rest of the line (if any) was \"$more\""


Conditional tests for [...] and [[...]] commands

Most of the useful flow-control operators involve making some conditional test and branching on the result (true/false). The test can be either the test command, or its alias, [, or the ksh/bash built-in [[ ... ]] command, which has slightly different options, or it can be any command which returns a suitable exit status. Zero is taken to be "True", while any non-zero value is "False". Note that this is backwards from the C language convention.

File tests

-e file
True if file exists (can be of any type).
-f file
True if file exists and is an ordinary file.
-d file
True if file exists and is a directory.
-r file
True if file exists and is readable
Similarly, -w = writable, -x = executable, -L = is a symlink.
-s file
True if file exists and has size greater than zero
-t filedescriptor
True if the open filedescriptor is associated with a terminal device. E.g. this is used to determine if standard output has been redirected to a file.

Character string tests

-n "string"
true if string has non-zero length
-z "string"
true if string has zero length
With [, the argument must be quoted, because if it is a variable that has a null value, the resulting expansion ( [ -z ] ) is a syntax error. An expansion resulting in "" counts as a null string.
For [ only, a quoted string alone is equivalent to the -n test, e.g. [ "$var" ]. In older shells for which [ is an external program, the only way to test for a null string is:
if [ "X$var" = "X" ]
This is rarely needed now, but is still often found.
$variable = text
True if $variable matches text.
$variable < text
True if $variable comes before (lexically) text
Similarly, > = comes after


More conditional tests for [...] and [[...]] commands

Arithmetic tests

$variable -eq number
True if $variable, interpreted as a number, is equal to number.
$variable -ne number
True if $variable, interpreted as a number, is not equal to number.
Similarly, -lt = less than, -le = less than or equal, -gt = greater than, -ge = greater than or equal

Additional tests for [[...]] (ksh and bash)

$variable = pattern
True if $variable matches pattern. If pattern contains no wildcards, then this is just an exact text match. The same wildcards as used for filename matching are used.
The pattern must not be quoted. Since [[...]] is internal to the shell, the pattern in this case is treated differently and not filename-expanded as an external command would require.
file1 -nt file2
True if file1 is newer than file2.
Similarly -ot = older than
file1 -ef file2
true if file1 is effectively the same as file2, after following symlinks and hard links.

Negating and Combining tests

Tests may be negated by prepending the ! operator, and combined with boolean AND and OR operators using the syntax:
conditional -a conditional, conditional -o conditional
AND and OR syntax for test and [

conditional && conditional, conditional || conditional
AND and OR syntax for [[ ... ]]
Parentheses may be inserted to resolve ambiguities or override the default operator precedence rules.


if [[  -x /usr/local/bin/lserve && \
       -w /var/logs/lserve.log ]]; then
   /usr/local/bin/lserve >> /var/logs/lserve.log &

pwent=`grep '^richard:' /etc/passwd`
if [ -z "$pwent" ]; then
   echo richard not found


Flow Control and Compound Commands

A list in these descriptions is a simple command, or a pipeline. The value of the list is the value of the last simple command run in it.
A list can also be a set of simple commands or pipelines separated by ";,&,&&,||,|&". For the compound commands which branch on the success or failure of some list, it is usually [ or [[, but can be anything.

Conditional execution: if/else

list && list
Execute the first list. If true (success), execute the second one.
list || list
Execute the first list. If false (failure), execute the second one.


mkdir tempdir && cp workfile tempdir

sshd || echo "sshd failed to start"
You can use both forms together (with care) - they are processed left to right, and && must come first.
mkdir tempdir && cp workfile tempdir || \
 echo "Failed to create tempdir"
if list; then list ; elif list; then list; else list; fi
Execute the first list, and if true (success), execute the "then" list, otherwise execute the "else" list. The "elif" and "else" lists are optional.


if [ -r $myfile ]
   cat $myfile
   echo $myfile not readable

Looping: 'while' and 'for' loops

while list; do list; done
until list; do list; done
Execute the first list and if true (success), execute the second list. Repeat as long as the first list is true. The until form just negates the test.

Example: ex4 display, text

   1: #!/bin/ksh
   2: count=0
   3: max=10
   4: while [[ $count -lt $max ]]
   5: do 
   6:   echo $count
   7:   count=$((count + 1))
   8: done
   9: echo "Value of count after loop is: $count"
for identifier [ in words ]; do; list; done
Set identifier in turn to each word in words and execute the list. Omitting the "in words" clause implies using $@, i.e. the identifier is set in turn to each positional argument.


for file in *.dat
    echo Processing $file
As with most programming languages, there are often several ways to express the same action. Running a command and then explicitly examining $? can be used instead of some of the above.

Compound commands can be thought of as running in an implicit subshell. They can have I/O redirection independant of the rest of the script. Setting of variables in a real subshell does not leave them set in the parent script. Setting variables in implicit subshells varies in behaviour among shells. Older sh could not set variables in an implicit subshell and then use them later, but current ksh can do this (mostly).

Example: ex11 display, text
Reading a file line by line. The book by Randal Michael contains 12 example ways to read a file line by line, which vary tremendously in efficiency. This example shows the simplest and fastest way.

   1: #!/bin/sh
   3: # Demonstrate reading a file line-by-line, using I/O
   4: # redirection in a compound command
   5: # Also test variable setting inside an implicit subshell.
   6: # Test this under sh and ksh and compare the output.
   8: line="TEST"
   9: save=
  11: if [ -z "$1" ]; then
  12:    echo "Usage: $0 filename"
  13: else
  14:    if [ -r $1 ]; then
  15:       while read line; do
  16:          echo "$line"
  17:          save=$line
  18:       done < $1
  19:    fi
  20: fi
  21: echo "End value of \$line is $line"
  22: echo "End value of \$save is $save"


Flow Control and Compound Commands (contd.)

Case statement: pattern matching

case word in pattern) list;; esac
Compare word with each pattern) in turn, and executes the first list for which the word matches. The patterns follow the same rules as for filename wildcards.
(ksh and bash only) A pattern-list is a list of one or more patterns separated from each other with a |. Composite patterns can be formed with one or more of the following:
Optionally matches any one of the given patterns.
Matches zero or more occurrences of the given patterns.
Matches one or more occurrences of the given patterns.
Matches exactly one of the given patterns.
Matches anything, except one of the given patterns.


    case $filename in
        echo Processing a .dat file
        echo Processing a .sas file
        # catch anything else that doesn't match patterns
        echo "Don't know how to deal with $filename"

Miscellaneous flow control and subshells

break [n]
Break out of the current (or n'th) enclosing loop. Control jumps to the next statement after the loop

continue [n];
Resume iteration of the current (or n'th) enclosing loop. Control jumps to the top of the loop, which generally causes re-evaluation of a while or processing the next element of a for.

. filename
Read the contents of the named file into the current shell and execute as if in line. Uses $PATH to locate the file, and can be passed positional parameters. This is often used to read in shell functions that are common to multiple scripts. There are security implications if the pathname is not fully specified.

( ... ) Command grouping
Commands grouped in "( )" are executed in a subshell, with a separate environment (can not affect the variables in the rest of the script).


Conditional Test Examples

As with most aspects of shell scripting, there are usually several possible ways to accomplish a task. Certain idioms show up commonly. These are five ways to examine and branch on the initial character of a string.
Use case with a pattern:
case $var in
/*) echo "starts with /" ;;
Works in all shells, and uses no extra processes

Use `cut`:
if [ "`echo $var | cut -c1`" = "/" ] ; then .
Works in all shells, but inefficiently uses a pipe and external process for a trivial task.

Use POSIX variable truncation:
if [ "${var%${var#?}}" = "/" ]; then
Works with ksh, bash and other POSIX-compliant shells. Not obvious if you have not seen this one before. Fails on old Bourne shells. Dave Taylor in "Wicked Cool Shell Scripts" likes this one.

Use POSIX pattern match inside of [[...]]:
if [[ $var = /* ]]; then
Works with ksh, bash and other POSIX-compliant shells. Note that you must use [[...]] and no quotes around the pattern.
The [[...]] syntax is handled internally by the shell and can therefore interpret "wildcard" patterns differently than an external command. An unquoted wildcard is interpreted as a pattern to be matched, while a quoted wildcard is taken literally. The [...] syntax, even if handled internally, is treated as though it were external for backward compatability. This requires that wildcard patterns be expanded to matching filenames.

Use ksh (93 and later) and bash variable substrings:
if [ "${var:0:1}" = "/" ]; then
ksh93 and later versions, and bash, have a syntax for directly extracting substrings by character position. ${varname:start:length}

Example: ex17 display, text


Miscellaneous internal commands

The shells (ksh in particular) have many more internal commands. Some are used more in interactive shells. The commands listed here are used in scripts, but don't conveniently fit elsewhere in the class.
eval args
The args are read as input to the shell and the resulting command executed. Allows "double" expansion of some constructs. For example, constructing a variable name out of pieces, and then obtaining the value of that variable.

NETDEV_1=hme0         # As part of an initialization step defining multiple devices

devnum=1              # As part of a loop over those devices
ifname=$netdev$devnum # construct a variable name NETDEV_1
eval device=\$$ifname # evaluate it - device is set to hme0
exec command args
The command is executed in place of the current shell. There is no return from an exec. I/O redirection may be used. This is also used to change the I/O for the current shell.
The line is variable-expanded, but otherwise treated as a comment. Sometimes used as a synonym for "true" in a loop.
while :; do
  # this loop will go forever until broken by 
  # a conditional test inside, or a signal

unset var ...
Remove the named variables. This is not the same as setting their values to null.

typeset [+/- options] [ name[=value] ] ... (ksh only, bash uses declare for similar functions)
Set attributes and values for shell variables and functions. When used inside a function, a local variable is created. Some of the options are:
Left justify and remove leading blanks. The variable always has length n if specified.
Right justify and fill with leading blanks. The variable always has length n if specified.
The named variable is always treated as an integer. This makes arithmetic faster. The reserved word integer is an alias for typeset -i.
As for -R, but fill with zeroes if the value is a number
Lower-case convert the named variables
Upper-case convert the named variables
Mark the variables as readonly
Export the named variables to the enviroment
The variables are taken as function names. Turn on execution tracing.


Manipulating Variables (ksh/bash only)

Text variables

The pattern in the following uses the same wildcards as for filename matching.
returns the length of $var in characters
removes the shortest suffix of $var patching pattern
removes the longest suffix of $var patching pattern
removes the shortest prefix of $var patching pattern
removes the longest prefix of $var patching pattern

Numeric variables

$(( integer expression ))
The $(( ... )) construction interprets the contents as an arithmetic expression (integer only). Variables are referenced by name without the "$". Most of the arithmetic syntax of the 'C' language is supported, including bit manipulations (*,/,+,-,|,&,<<,>>. Use parentheses for changing precedence).
filename is set to "datafile.dat" since the longest prefix pattern matching "*/" is the leading directory path (compare basename)
path is set to "/data/public/project/trials/set1" since the shortest suffix pattern matching "/*" is the filename in the last directory (compare dirname)

often used in while loops


Shell Functions

All but the earliest versions of sh allow you define shell functions, which are visible only to the shell script and can be used like any other command. Shell functions take precedence over external commands if the same name is used. Functions execute in the same process as the caller, and must be defined before use (appear earlier in the file). They allow a script to be broken into maintainable chunks, and encourage code reuse between scripts.

Defining functions

identifier() { list; }
POSIX syntax for shell functions. Such functions do not restrict scope of variables or signal traps. The identifier follows the rules for variable names, but uses a separate namespace.
function identifier { list; }
Ksh and bash optional syntax for defining a function. These functions may define local variables and local signal traps and so can more easily avoid side effects and be reused by multiple scripts.

A function may read or modify any shell variable that exists in the calling script. Such variables are global.

(ksh and bash only) Functions may also declare local variables in the function using typeset or declare. Local variables are visible to the current function and any functions called by it.

return [n], exit [n]
Return from a function with the given value, or exit the whole script with the given value.
Without a return, the function returns when it reaches the end, and the value is the exit status of the last command it ran.


   # Print an error message and exit with given status
   # call as: die status "message" ["message" ...]
   exitstat=$1; shift
   for i in "$@"; do
      print -R "$i"
   exit $exitstat

Calling functions.

Functions are called like any other command. The output may be redirected independantly of the script, and arguments passed to the function. Shell option flags like -x are unset in a function - you must explicitly set them in each function to trace the execution. Shell functions may even be backgrounded and run asynchronously, or run as coprocesses (ksh).


[ -w $filename ] || \
  die 1 "$file not writeable" "check permissions"

Example: Backgrounded function call. ex12 display, text

   1: #!/bin/sh
   3: background()
   4: {
   5:    sleep 10
   6:    echo "Background"
   7:    sleep 10
   8:    # Function will return here - if backgrounded, the subprocess will exit.
   9: }
  11: echo "ps before background function"
  12: ps
  13: background &
  14: echo "My PID=$$"
  15: echo "Background function PID=$!"
  16: echo "ps after background function"
  17: ps
  18: exit 0


   # Print or not depending on global "$verbosity"
   # Change the verbosity with a single variable.
   # Arg. 1 is the level for this message.
   level=$1; shift
   if [[ $level -le $verbosity ]]; then
      print -R $*

vprint 1 This message will appear
vprint 3 This only appears if verbosity is 3 or higher

Reuseable functions

By using only command line arguments, not global variables, and taking care to minimise the side effects of functions, they can be made reusable by multiple scripts. Typically they would be placed in a separate file and read with the "." operator.

Functions may generate output to stdout, stderr, or any other file or filehandle. Messages to stdout may be captured by command substitution (`myfunction`, which provides another way for a function to return information to the calling script. Beware of side-effects (and reducing reusability) in functions which perform I/O.


Advanced I/O

Unix I/O is performed by assigning file descriptors to files or devices, and then using those descriptors for reading and writing. Descriptors 0, 1, and 2 are always used for stdin, stdout and stderr respectively. Stdin defaults to the keyboard, while stdout and stderr both default to the current terminal window.

Redirecting for the whole script

Redirecting stdout, stderr and other file descriptors for the whole script can be done with the exec command.
exec > outfile < infile
with no command, the exec just reassigns the I/O of the current shell.
exec n>outfile
The form n<, n> opens file descriptor n instead of the default stdin/stdout. This can then be used with read -u or print -u.

Explicitly opening or duplicating file descriptors

One reason to do this is to save the current state of stdin/stdout, temporarily reassign them, then restore them.
standard output is moved to whatever file descriptor n is currently pointing to
standard input is moved to whatever file descriptor n is currently pointing to
file descriptor n is opened for writing on the named file.
file descriptor n is set to whatever file descriptor 1 is currently pointing to.
Example Sending messages to stderr (2) instead of stdout (1)
echo "Error: program failed" >&2 
Echo always writes to stdout, but stdout can be temporarily reassigned to duplicate stderr (or other file descriptors). Conventionally Unix programs send error messages to stderr to keep them separated from stdout.

Input and output to open file descriptors (ksh)

Printing to file descriptors (usually more efficient than open/append/close):
print -u n args
print to file descriptor n.
write to the pipe to a coprocess (opened by |&)

Reading from file descriptors other than stdin:

read -u n var1 var2 rest
read a line from file descriptor n, parsing by $IFS, and placing the words into the named variables. Any left over words all go into the last variable.
read from the pipe to a coprocess (opened by |&)

Closing file handles

standard input is explicitly closed
standard output is explicitly closed
For example, to indicate to another program downstream in a pipeline that no more data will be coming. All file descriptors are closed when a script exits.

I/O redirection operators are evaluated left-to-right. This makes a difference in a statement like: ">filename 2>&1". (Many books with example scripts get this wrong)

"Here" documents

<< [-]string
redirect input to the temporary file formed by everything up the matching string at the start of a line. Allows for placing file content inline in a script.
Example: ex5 display, text

   1: #!/bin/sh
   2: echo "Example of unquoted here document, with variable and command substitution"
   4: cat <<EOF
   5:  This text will be fed to the "cat" program as 
   6:  standard input.  It will also have variable
   7:  and command substitutions performed.
   8:  I am logged in as $USER and today is `date`
   9: EOF
  10: echo
  11: echo "Example of quoted here document, with no variable or command substitution"
  12: # The terminating string must be at the start of a line.
  13: cat <<"EndOfInput"
  14:  This text will be fed to the "cat" program as standard
  15:  input.  Since the text string marking the end was quoted, it does not get 
  16:  variable and command subsitutions.
  17:  I am logged in as $USER and today is `date`
  18: EndOfInput

Example: duplex display, text

   1: #!/bin/sh
   2: # Add in the magic postscript preface to perform
   3: # duplex printer control for Xerox docuprint.
   5: # To have this script send the files directly to the printer, use 
   6: # a subshell to collect the output of the two 'cat' commands.
   8: ## (
   9: cat << EOP
  10: %!PS
  11: %%BeginFeature: *Duplex DuplexTumble
  12: <</Duplex true /Tumble false>> setpagedevice
  13: %%EndFeature
  14: EOP
  15: cat "$@"
  16: ## ) | lpr


Wizard Level I/O

More complicated manipulations of file descriptors can be arranged. Two such examples are shown here:

This short test script can be used to generate suitable output.
ex13: display, text

echo "This goes to stdout"
echo "This goes to stdout and has foo in the line"
echo "This goes to stderr" >&2
exit 99

Pass stderr of a command into a pipeline for further processing

Example: ex14 display, text

exec 3>&1
./ex13.sh 2>&1 1>&3 3>&- | sed 's/stderr/STDERR/' 1>&2

We duplicate stdout to another file descriptor (3), then run the first command with stderr redirected to stdout and stdout redirected to the saved descriptor (3). The result is piped into other commands as needed. The output of the pipeline is redirected back to stderr, so that stdout and stderr of the script as a whole are what we expect.

   1: #!/bin/sh
   2: # Example 14
   3: # Take stderr from a command and pass it into a pipe
   4: # for further processing.
   6: # Uses ex13.sh to generate some output to stderr
   7: # stdout of ex13 is processed normally
   9: # Save a copy of original stdout
  10: exec 3>&1
  12: # stdout from ex13.sh is directed to the original stdout (3)
  13: # stderr is passed into the pipe for further processing.
  14: # stdout from the pipe is redirected back to stderr
  15: ./ex13.sh 2>&1 1>&3 3>&-  | sed 's/stderr/STDERR/' 1>&2
  17: # 3 is closed before running the command, just in case it cares
  18: # about inheriting open file descriptors.

Capture the exit status of a command in the middle of a pipeline

Example: ex15 display, text

exec 3>&1
ex13stat=`((./ex13.sh; echo $? >&4) | grep 'foo' 1>&3) 4>&1`

This script uses nested subshells captured in backtics. Again we first duplicate stdout to another file descriptor (3). The inner subshell runs the first command, then writes the exit status to fd 4. The outer subshell redirects 4 to stdout so that it is captured by the backtics. Standard output from the first command (inner subshell) is passed into the pipeline as normal, but the final output of the pipeline is redirected to 3 so that it appears on the original stdout and is not captured by the backtics.

If any of the commands really care about inheriting open file descriptors that they don't need then a more correct command line closes the descriptors before running the commands.

   1: #!/bin/sh
   2: # Example 15
   4: # Uses ex13.sh to generate some output and give us an 
   5: # exit status to capture.
   7: # Get the exit status of ex13 into $ex13stat.  
   8: # stdout of ex13 is processed normally
  10: # Save a copy of stdout
  11: exec 3>&1
  12: # Run a subshell, with 4 duplicated to 1 so we get it in stdout.  
  13: # Capture the output in ``
  14: # ex13stat=`( ...  ) 4>&1`
  15: # Inside the subshell, run another subshell to execute ex13, 
  16: # and echo the status code to 4
  17: # (./ex13.sh; echo $? >&4)
  18: # stdout from the inner subshell is processed normally, but the 
  19: # subsequent output must be directed to 3 so it goes to the 
  20: # original stdout and not be captured by the ``
  21: ex13stat=`((./ex13.sh; echo $? >&4) | grep 'foo' 1>&3) 4>&1`
  23: echo Last command status=$?
  24: echo ex13stat=$ex13stat
  26: # If any of the commands really care about inheriting open file 
  27: # descriptors that they don't need then a more correct command line 
  28: # closes the descriptors before running the commands
  29: exec 3>&1
  30: ex13stat=`((./ex13.sh 3>&- 4>&- ; echo $? >&4) | \
  31:    grep 'foo'  1>&3 3>&- 4>&- ) 4>&1`
  32: echo Last command status=$?
  33: echo ex13stat=$ex13stat

Combine the above two techniques:

Example: ex16 display, text

exec 3>&1
ex13stat=`((./ex13.sh 2>&1 1>&3 3>&- 4>&- ; echo $? >&4) | \
sed s/err/ERR/ 1>&2 3>&- 4>&- ) 4>&1`

   1: #!/bin/sh
   2: # Example 16
   4: # Uses ex13.sh to generate some output and give us an 
   5: # exit status to capture.
   7: # Get the exit status of ex13 into ex13stat. 
   8: # stderr of ex13 is processed by the pipe, stdout
   9: # is left alone.
  11: # Save a copy of stdout
  12: exec 3>&1
  14: # Run a subshell, with 4 copied to 1 so we get it in stdout.  
  15: # Capture the output in backtics`
  16: # ex13stat=`(    ) 4>&1`
  18: # In the subshell, run another subshell to execute ex13, and 
  19: # echo the status code to 4
  20: # (./ex13.sh; echo $? >&4)
  22: # stdout from the inner subshell is directed to the original stdout (3)
  23: # stderr is passed into the pipe for further processing.
  24: # stdout from the pipe is redirected back to stderr
  26: # Close the extra descriptors before running the commands
  27: exec 3>&1
  28: ex13stat=`((./ex13.sh 2>&1 1>&3 3>&- 4>&- ; echo $? >&4) | \
  29:   sed s/err/ERR/ 1>&2 3>&- 4>&- ) 4>&1`
  31: echo Last command status=$?
  32: echo ex13stat=$ex13stat
A practical application of this would be running a utility such as dd where the exit status is important to capture, but the error output is overly chatty and may need to be filtered before delivering to other parts of a script.


Coprocesses and Background jobs

Scripts can start any number of background jobs (any external command), which run in parallel with the parent script, and asynchronously. Processes which require no further interaction or synchronization (fire and forget) are easy. Interaction with background jobs is tricky. You can use signals, pipes, named pipes, or disk files for communication.
command &
Start command as a background process. Control returns immediately to the shell.
The special variable $! contains the process ID of the last background job that was started. You can save that and examine the process later (ps -p $bgpid) or send it a signal (kill -HUP $bgpid).

ksh coprocesses

Coprocesses are a way of starting a separate process which runs asychronously, but has stdin/stdout connected to the parent script via pipes.
command |&
Start a coprocess with a 2-way pipe to it
read -p var
Read from the pipe to the coprocess, instead of standard input
print -p args
Write to the pipe connected to the coprocess, instead of standard output
Multiple coprocesses can be handled by moving the special file descriptors connected to the pipes onto standard input and output, and or to explicitly specified file descriptors.
exec <&p
The input from the coprocess is moved to standard input
exec >&p
The output from the coprocess is moved to standard output

Example: ex9 display, text
A script wants to save a copy of all output in a file, but also wants a copy to the screen. This is equivalent to always running the script as
script | tee outfile

   1: #!/bin/ksh
   3: # If we have not redirected standard output, save a copy of
   4: # the output of this script into a file, but still send a
   5: # copy to the screen.
   7: if [[ -t 1 ]] ; then
   8:   # Only do this if fd 1 (stdout) is still connected
   9:   # to a terminal
  11:   # We want the standard output of the "tee" process
  12:   # to go explicitly to the screen (/dev/tty)
  13:   # and the second copy goes into a logfile named $0.out
  15:   tee $0.out >/dev/tty |&
  17:   # Our stdout all goes into this coprocess
  18:   exec 1>&p
  19: fi
  21: # Now generate some output
  22: print "User activity snapshot on $(hostname) at $(date)"
  23: print
  24: who

Example: ex10 display, text
Start a coprocess to look up usernames in some database. It is faster to run a single process than to run a separate lookup for each user.

   1: #!/bin/ksh
   2: # This example uses a locally written tool for Dartmouth Name Directory lookups
   4: # Start the dndlookup program as a coprocess
   5: # Tell it to output only the canonical full name, and to not print multiple matches
   6: dndlookup -fname -u |&
   8: # move the input/output streams so we 
   9: # can use other coprocesses too
  10: exec 4>&p
  11: exec 5<&p
  13: echo "Name file contents:"
  14: cat namefile
  15: echo
  17: # read the names from a file "namefile"
  18: while read uname; do
  19:   print -u4 $uname
  20:   read  -u5 dndname
  21:   case $dndname in
  22:   *many\ matches*)
  23:     # handle case where the name wasn't unique
  24:     print "Multiple matches to \"$uname\" in DND"
  25:     ;;
  26:   *no\ match*)
  27:     # handle case where the name wasn't found
  28:     print "No matches to \"$uname\" in DND"
  29:     ;;
  30:   *)
  31:     # we seem to have a hit - process the
  32:     # canonical named retrieved from dndlookup
  33:     print "Unique DND match: full name for \"$uname\" is \"$dndname\""
  34:     ;; 
  35:   esac
  36:   sleep 2
  37: done < namefile
  39: # We've read all the names, but the coprocess
  40: # is still running.  Close the pipe to tell it
  41: # we have finished.
  42: exec 4>&-


Variable arrays

Both ksh and bash implement arrays of variables, but in somewhat different ways.

ksh distinguishes between numerically indexed (small) arrays, and string indexed (associative) arrays. bash uses integers for all array indexing, but the integers need not be consecutive and unassigned array elements do not exist. Arrays must be declared before use, e,g. typeset -A myarray (ksh associative array), or typeset -a myarray (bash).

Array elements are set with the syntax: myarray[index]=value and referenced with the syntax ${myarray[index]}

This example shows use of an array indexed by IP addresses, as strings in ksh or as non-consecutive numbers in bash. It also demonstrates use of getopt for options processing

Example: getauthlogs display, text

   1: #!/bin/bash
   2: # $Header: $
   3: # First attempt at a consolidated auth log collection from kaserver
   4: # Timestamps in the raw files are NOT designed for easy sorting.
   5: #
   6: # Options:
   7: #  -i  -- translate hex IP addresses to dotted-decimal (relatively quick)
   8: #  -h  -- translate hex IP addresses to DNS names (somewhat slower - DNS lookups)
   9: #  -u user -- filter for the named user before translating addresses
  11: hextodec()
  12: {
  13:    # convert the IP address in reverse-hex to dotted-decimal
  14:    echo $((0x${1:6:2})).$((0x${1:4:2})).$((0x${1:2:2})).$((0x${1:0:2}))
  15: }
  17: hostlookup()
  18: {
  19:    # Convert a decimal IP to hostname - calls 'host' each time
  20:    hostname=$(host $1)
  21:    case $hostname in
  22:    *\ not\ found*)
  23:       # Just echo the address we tried to look up
  24:       echo "$1"
  25:       ;;
  26:    *)
  27:       # The result is word 5.  Lower-case it for consistency
  28:       set $hostname
  29:       echo "$5" | tr 'A-Z' 'a-z'
  30:       ;;
  31:    esac
  32: }
  34: # Options
  35: iptranslate=0
  36: gethostnames=0
  37: filter=cat
  38: while getopts ihu: o ; do
  39:    case $o in
  40:    i) iptranslate=1 ;;
  41:    h) gethostnames=1; iptranslate=1 ;;
  42:    u) filter="grep $OPTARG" ;;
  43:    esac
  44: done
  45: shift $(($OPTIND-1))
  47: # We could get the DB server names from 'fs checkservers', but it isn't obvious what is from our cell.  We
  48: # could also grep CellServDB.  I cop out and hard code one known DB server and get the others from it.
  49: masterserver=halley.dartmouth.edu
  50: serverlist=$(bos listhosts -server $masterserver| grep 'Host .* is ' | awk '{print $4}')
  52: # If we want to filter usernames, it is more efficient to do it inline, before sorting, translation and hostname lookups
  54: # Array to hold IP address/name conversions (associative array, ksh only)
  55: # ksh - use -A for associative array.  bash - use -a and numeric array
  56: typeset -a hostnames
  58: (
  59: for dbserver in $serverlist; do
  60:    bos getlog -server $dbserver -file /usr/afs/logs/AuthLog
  61: done
  62: ) | grep -v 'Fetching log file' | $filter | sed -e 's/^... //' -e 's/  \([1-9]\) / 0\1 /' | sort --month-sort | \
  63:     sed '-e s/ \([0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]\)$/ 0\1/' |
  64:     while read line; do
  65:    if [[ $iptranslate == 1 ]] ; then
  66:       # Ugly!
  67:       # Sometimes we get a 7-digit hex code in the log - the kaserver apparently drops leading zeros.
  68:       # The second 'sed' in the pipe catches these are fixes them.
  69:       case $line in
  70:       *\ from\ [0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f])
  71:          # translate the reverse-hex address
  72:          iphex=${line##* from }
  73:          # bash version - index by numeric value only, but can be sparse array -- use the raw IP 
  74:          ipdec=$((0x$iphex))
  75:          frontpart=${line% from *}
  76:          if [[ $gethostnames == 1 ]]; then
  77:             # ksh - index on hex value as a string (iphex)
  78:             # bash - index on numeric value (ipdec)
  79:             index=$ipdec
  80:             if [[ -z "${hostnames[$index]}" ]]; then
  81:                hostnames[$index]="$(hostlookup $(hextodec $iphex))" 
  82:             fi
  83:             echo "$frontpart from ${hostnames[$index]}"
  84:          else
  85:             echo "$frontpart from $(hextodec $iphex)"
  86:          fi
  87:          ;;
  88:       *)
  89:          echo "$line"
  90:          ;;
  91:       esac
  92:    else
  93:       # No ip translation, just echo the whole line
  94:       echo "$line"
  95:    fi   
  96: done


Delivering and Trapping Signals

Unix signals (software interrupts) can be sent as asynchronous events to shell scripts, just as they can to any other program. The default behaviour is to ignore some signals and immediately exit on others. Scripts may detect signals and divert control to a handler function or external program. This is often used to perform clean-up actions before exiting, or restart certain procedures. Execution resumes where it left off, if the signal handler returns. Signal traps must be set separately inside of shell functions. Signals can be sent to a process with kill.
trap handler sig ...
handler is a command to be read (evaluated first) and executed on receipt of the specified sigs. Signals can be specified by name or number (see kill(1)) e.g. HUP, INT, QUIT, TERM. A Ctrl-C at the terminal generates a INT.

Example: ex8 display, text

   1: #!/bin/bash
   2: # Try this under bash, ksh and sh
   4: trap huphandler  HUP
   5: trap ''          QUIT
   6: trap exithandler TERM INT
   8: huphandler()
   9: {
  10:    echo 'Received SIGHUP'
  11:    echo "continuing"
  12: }
  14: exithandler()
  15: {
  16:    echo 'Received SIGTERM or SIGINT'
  17:    exit 1
  18: }
  19: ## Execution starts here - infinite loop until interrupted
  20: # Use ":" or "true" for infinite loop
  21: # SECONDS is built-in to bash and ksh.  It is number of seconds since script started
  22: : is like a comment, but it is evaluated for side effects and evaluates to true
  23: seconds=0
  24: while : ; do
  25: # while true; do
  26:    sleep 5
  27:    seconds=$((seconds + 5))
  28:    echo -n "$SECONDS $seconds - "
  29: done
Exit handlers can be defined to clean up temporary files or reset the state of devices. This can be useful if the script has multiple possible exit points.


Security issues in shell scripts

Shell scripts are often used by system administrators and are run as a priviledged user.

Consider the effects of a file named "myfile;cd /;rm *" if processed, unquoted, by your script.

One possible way to protect against weirdo characters in file names:
# A function to massage a list of filenames 
# to protect weirdo characters
# e.g. find ... | protect_filenames | xargs command
# We are backslash-protecting the characters \'" ?*;
   sed -es/\\\\/\\\\\\\\/g \
       -es/\\\'/\\\\\'/g   \
       -es/\\\"/\\\\\"/g   \
       -es/\\\;/\\\\\;/g   \
       -es/\\\?/\\\\\?/g   \
       -es/\\\*/\\\\\*/g   \
       -es/\\\ /\\\\\ /g
If using GNU find and xargs, there is a much cleaner option to null-terminate generated pathnames.



Shell scripts are very frequently written quickly for a single purpose, used once and discarded. They are also as frequently kept and used many times, and migrate into other uses, but often do not receive the same level of testing and debugging that other software would be given in the same situation. It is possible to apply general principles of good software engineering to shell scripts.

When not to use shell scripts


Some longer examples

The class accounts have directories with all of the examples from the books by Blinn, Michael, Rosenblatt, and Taylor. These can also be downloaded (see the References page). Some of these are linked below (but not included in the printed notes), with additional comments.

Download a compressed tar file of all example scripts used in these notes.

This entire tutorial was created from individual HTML pages using a content management system written as ksh scripts (heavily using sed to edit the pages), coordinated by make.

You can even write an entire web server as a shell script. This one is part of the LEAF (Linux Embedded Appliance Firewall) project. This wouldn't be suitable for much load, but handles occasional queries on static HTML and CGI scripts. (www.nisi.ab.ca/lrp/Packages/weblet.htm)


A Toolkit of commonly used external commands

The following commands are very frequently used in shell scripts. Many of them are used in the examples in these notes. This is just a brief recap -- see the man pages for details on usage. The most useful are flagged with *.

Most of these commands will operate on a one or more named files, or will operate on a stream of data from standard input if no files are named.

Listing, copying and moving files and directories

ls *
list contents of a directory, or list details of files and directories.
mkdir; rmdir *
Make and Remove directories.
rm; cp; mv *
Remove (delete), Copy and Move (rename) files and directories
touch *
Update the last modifed timestamp on a file, to make it appear to have just been written.
If the file does not exist, a new zero-byte file is created, which is often useful to signify that an event has occurred.
Make a duplicate copy of a data stream - used in pipelines to send one copy to a log file and a second copy on to another program. (Think plumbing).

Displaying text, files or parts of files

echo *
Echo the arguments to standard output -- used for messages from scripts. Some versions of "sh", and all csh/ksh/bash shells internalized "echo".
Conflicts sometimes arise over the syntax for echoing a line with no trailing CR/LF. Some use "\c" and some use option "-n". To avoid these problems, ksh also provides the "print" command for output.
cat *
Copy and concatenate files; display contents of a file
head, tail *
Display the beginning of a file, or the end of it.
Extract selected fields from each line of a file. Often awk is easier to use, even though it is a more complex program.
Count lines, words and characters in the input.

Compression and archiving

compress; gzip, zip; tar *
Various utilities to compress/uncompress individual files, combine multiple files into a single archive, or do both.

Sorting and searching for patterns

sort *
Sort data alphabetically or numerically.
grep *
Search a file for lines containing character patterns. The patterns can be simple fixed text, or very complex regular expressions.
The name comes from "Global Regular Expression and Print" -- a function from the Unix editors which was used frequently enough to warrant getting its own program.
uniq *
Remove duplicate lines, and generate a count of repeated lines.
wc *
Count lines, words and characters in a file.

System information (users, processes, time)

date *
Display the current date and time (flexible format). Useful for conditional execution based on time, and for timestamping output.
ps *
List the to a running processes.
kill *
Send a signal (interrupt) to a running process.
Print the user name and UID and group of the current user (e.g. to distinguish priviledged users before attempting to run programs which may fail with permission errors)
Display who is logged on the system, and from where they logged in.
uname *
Display information about the system, OS version, hardware architecture etc.
mail *
Send mail, from a file or standard input, to named recipients. Since scripts are often used to automate long-running background jobs, sending notification of completion by mail is a common trick.
Place a message in the central system logging facility. Scripts can submit messages with all the facilities available to compiled programs.
Display the hostname of the current host - usful to keep track of where your programs are running

Conditional tests

test; [ *
The conditional test, used extensively in scripts, is also an external program which evaluates the expression given as an argument and returns true (0) or false (1) exit status. The name "[" is a link to the "test" program, so a line like:
if [ -w logfile ]
actually runs a program "[", with arguments "-w logfile ]", and returns a true/false value to the "if" command.
In ksh and most newer versions of sh, "[" is replaced with a compatible internal command, but the argument parsing is performed as if it were an external command. Ksh also provides the internal "[[" operator, with simplified syntax.

Stream Editing

awk *
A pattern matching and data manipulation utility, which has its own scripting language. It also duplicates much functionality from 'sed','grep','cut','wc', etc.
Complex scripts can be written entirely using awk, but it is frequently used just to extract fields from lines of a file (similar to 'cut').
sed *
Stream Editor. A flexible editor which operates by applying editing rules to every line in a data stream in turn.
Since it makes a single pass through the file, keeping only a few lines in memory at once, it can be used with infinitely large data sets. It is mostly used for global search and replace operations. It is a superset of 'tr', 'grep', and 'cut', but is more complicated to use.
Transliterate - perform very simple single-character edits on a file.

Finding and comparing files

find *
Search the filesystem and find files matching certain criteria (name pattern, age, owner, size, last modified etc.)
xargs *
Apply multiple filename arguments to a named command and run it.
Xargs is often used in combination with "find" to apply some command to all the files matching certain criteria. Since "find" may result in a very large list of pathnames, using the results directly may overflow command line buffers. Xargs avoids this problem, and is much more efficient than running a command on every pathname individually.
diff *
Compare two files and list the differences between them.
basename pathname
Returns the base filename portion of the named pathname, stripping off all the directories
dirname pathname
Returns the directory portion of the named pathname, stripping off the filename

Arithmetic and String Manipulation

expr *
The "expr" command takes an numeric or text pattern expression as an argument, evaluates it, and returns a result to stdout. The original Bourne shell had no built-in arithmetic operators. E.g.
expr 2 + 1
expr 2 '*' '(' 21 + 3 ')'
Used with text strings, "expr" can match regular expressions and extract sub expressions. Similar functionality can be achived with sed. e.g.
expr SP99302L.Z00 : '[A-Z0-9]\{4\}\([0-9]\{3\}\)L\.*'
Desk Calculator - an RPN calculator, using arbitrary precision arithmetic and user-specified bases. Useful for more complex arithmetic expressions than can be performed internally or using expr
A preprocessor for dc which provides infix notation and a C-like syntax for expressions and functions.

Merging files

Merge lines from multiple files into tab-delimited columns.
Perform a join (in the relational database sense) of lines in two sorted input files.


References, Resources, Man pages etc.

The standard man pages for sh and ksh are quite complete, but not easy to learn from. The following is a sampling of the many available books on the subject. The Bolsky and Korn book might be viewed as the standard "reference". The Blinn book is Bourne shell, but everything in it should work for either shell.
The links are to publisher's web sites, or Amazon.com. Some links are also given to the example scripts provided with the books.


Online Resources

Unix-like shells and utilities for Microsoft Windows

Unix shell scripting with ksh/bash: Course Handout
[an error occurred while processing this directive] (last update   22 March 2012)  ©Dartmouth College     http://www.dartmouth.edu/~rc/classes/ksh