Usage (GNU Grep 3.11) (2024)

grep -l 'main' test-*.c

lists names of ‘test-*.c’ files in the current directory whose contentsmention ‘main’.

grep -r 'hello' /home/gigi

searches for ‘hello’ in all filesunder the /home/gigi directory.For more control over which files are searched,use find and grep.For example, the following command searches only C files:

find /home/gigi -name '*.c' ! -type d \ -exec grep -H 'hello' '{}' +

This differs from the command:

grep -H 'hello' /home/gigi/*.c

which merely looks for ‘hello’ in non-hidden C files in/home/gigi whose names end in ‘.c’.The find command line above is more similar to the command:

grep -r --include='*.c' 'hello' /home/gigi

grep "$pattern" *

can behave unexpectedly if the value of ‘pattern’ begins with ‘-’,or if the ‘*’ expands to a file name with leading ‘-’.To avoid the problem, you can use -e for patterns and leading‘./’ for files:

grep -e "$pattern" ./*

searches for all lines matching the pattern in all the workingdirectory’s files whose names do not begin with ‘.’.Without the -e, grep might treat the pattern as anoption if it begins with ‘-’. Without the ‘./’, there mightbe similar problems with file names beginning with ‘-’.

Alternatively, you can use ‘--’ before the pattern and file names:

grep -- "$pattern" *

This also fixes the problem, except that if there is a file named ‘-’,grep misinterprets the ‘-’ as standard input.

grep -w 'hello' test*.log

searches only for instances of ‘hello’ that are entire words;it does not match ‘Othello’.For more control, use ‘\<’ and‘\>’ to match the start and end of words.For example:

grep 'hello\>' test*.log

searches only for words ending in ‘hello’, so it matches the word‘Othello’.

grep -C 2 'hello' test*.log

prints two lines of context around each matching line.

Append /dev/null:

grep 'eli' /etc/passwd /dev/null

gets you:

/etc/passwd:eli:x:2098:1000:Eli Smith:/home/eli:/bin/bash

Alternatively, use -H, which is a GNU extension:

grep -H 'eli' /etc/passwd

ps -ef | grep '[c]ron'

If the pattern had been written without the square brackets, it wouldhave matched not only the ps output line for cron,but also the ps output line for grep.Note that on some platforms,ps limits the output to the width of the screen;grep does not have any limit on the length of a lineexcept the available memory.

If grep listed all matching “lines” from a binary file, itwould probably generate output that is not useful, and it might evenmuck up your display.So GNU grep suppresses output fromfiles that appear to be binary files.To force GNU grepto output lines even from files that appear to be binary, use the-a or ‘--binary-files=text’ option.To eliminate the“Binary file matches” messages, use the -I or‘--binary-files=without-match’ option.

‘grep -lv’ lists the names of all files containing one or morelines that do not match.To list the names of all files that contain nomatching lines, use the -L or --files-without-matchoption.

grep 'paul' /etc/motd | grep 'franc,ois'

finds all lines that contain both ‘paul’ and ‘franc,ois’.

The grep command searches for lines that contain stringsthat match a pattern. Every line contains the empty string, so anempty pattern causes grep to find a match on each line. Itis not the only such pattern: ‘^’, ‘$’, and manyother patterns cause grep to match every line.

To match empty lines, use the pattern ‘^$’. To match blanklines, use the pattern ‘^[[:blank:]]*$’. To match no lines atall, use an extended regular expression like ‘a^’ or ‘$a’.To match every line, a portable script should use a pattern like‘^’ instead of the empty pattern, as POSIX does not specify thebehavior of the empty pattern.

Use the special file name ‘-’:

cat /etc/passwd | grep 'alain' - /etc/motd

The grep command follows the convention of programs likecmp and diff where an exit status of 1 is not anerror. The shell command ‘set -e’ causes the shell to exit ifany subcommand exits with nonzero status, and this will cause theshell to exit merely because grep selected no lines,which is ordinarily not what you want.

There is a related problem with Bash’s set -e -o pipefail.Since grep does not always read all its input, a commandoutputting to a pipe read by grep can fail whengrep exits before reading all its input, and the command’sfailure can cause Bash to exit.

echo 'ba' | grep -E '(a)\1|b\1'

This outputs an error message, because the second ‘\1’has nothing to refer back to, meaning it will never match anything.

Standard grep cannot do this, as it is fundamentally line-based.Therefore, merely using the [:space:] character class does notmatch newlines in the way you might expect.

With the GNU grep option -z (--null-data), eachinput and output “line” is null-terminated; see Other Options. Thus,you can match newlines in the input, but typically if there is a matchthe entire input is output, so this usage is often combined withoutput-suppressing options like -q, e.g.:

printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'

If this does not suffice, you can transform the inputbefore giving it to grep, or turn to awk,sed, perl, or many other utilities that aredesigned to operate across lines.

The name grep comes from the way line editing was done on Unix.For example,ed uses the following syntaxto print a list of matching lines on the screen:

global/regular expression/printg/re/p

The -E option stands for Extended grep.The -F option stands for Fixed grep;

7th Edition Unix had commands egrep and fgrepthat were the counterparts of the modern ‘grep -E’ and ‘grep -F’.Although breaking up grep into three programs was perhapsuseful on the small computers of the 1970s, egrep andfgrep were deemed obsolescent by POSIX in 1992,removed from POSIX in 2001, deprecated by GNU Grep 2.5.3 in 2007,and changed to issue obsolescence warnings by GNU Grep 3.8 in 2022;eventually, they are planned to be removed entirely.

If you prefer the old names, you can use your own substitutes,such as a shell script named egrep with the followingcontents:

#!/bin/shexec grep -E "$@"