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(31) Common external commands

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.

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.

tee

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).

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.

cut

Extract selected fields from each line of a file. Often awk is easier to use, even though it is a more complex program.

compress; gzip, zip; tar *

Various utilities to compress/uncompress individual files, combine multiple files into a single archive, or do both.

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.

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.

id

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)

who

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.

logger

Place a message in the central system logging facility. Scripts can submit messages with all the facilities available to compiled programs.

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.

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.

tr

Transliterate - perform very simple single-character edits on a file.

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

expr *

The "expr" command takes an numeric or text pattern expression as an argument, evaluates it, and returns a result to stdout. Bourne shell has no built-in arithmetic operators or string manipulation. 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\.*'

dc

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

bc

A preprocessor for dc which provides infix notation and a C-like syntax for expressions and functions.

paste

Merge lines from multiple files into tab-delimited columns.

join

Perform a join (in the relational database sense) of lines in two sorted input files.