= Grep =

'''`grep(1)`''' is a searching tool.

<<TableOfContents>>

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== Installation ==

`grep(1)` will be pre-installed on any [[Linux]] or [[BSD]] operating system, as a POSIX utility.

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== Usage ==

`grep(1)` reads lines from files and prints any that match a pattern. The name is an homage to a functionally-equivalent [[Ed|ed(1)]] script: `g/REGEX/p`.

Try:

{{{
grep --regexp='regex' somefile
#or: grep -e 'regex' somefile
}}}


To recursively read lines from all files under a directory, try:

{{{
grep --regexp='regex' --recursive somedir
#or: grep -e 'regex' -r somedir
}}}

Files can be named as either positional arguments or as named arguments on `--file` or `-f` flags.

If no file is named as a target, or if `-` is named as the target, then `grep(1)` operates on `STDIN`.

Note that if no file is named as a target, or if `-` is named as the target, ''and'' the `--recursive` flag was given, then `grep(1)` will instead read all files in the working directory.



=== Fixed Patterns ===

In some cases, regular expression patterns are unnecessary. To instead search for a fixed pattern, try:

{{{
grep --fixed-strings='literal' somefile
#or: grep -F 'literal' somefile
}}}



=== Perl Patterns ===

On the other hand, sometimes GNU regular expressions are insufficient. To instead search with [[Perl|PCRE]], try:

{{{
grep --perl-regexp='regex' somefile
#or: grep -P 'regex' somefile
}}}



=== Multiple Patterns ===

To search with multiple patterns, i.e. a logical OR, simply specify multiple pattern flags. Try:

{{{
grep --regexp='regex' --fixed-strings='literal' --perl-regexp='regex' somefile
}}}

To search for the intersections of multiple patterns, i.e. a logical AND, try piping distinct commands.

{{{
grep --regexp='regex' somefile | grep --fixed-strings='literal'
}}}



=== Inversion ===

to invert the behavior of `grep(1)`, i.e. print lines that do not match, try:

{{{
grep --regexp='regex' --invert-match somefile
#or: grep --regexp='regex' -v somefile
}}}



=== Extraction ===

To print only the part of a line that matches the pattern, try:

{{{
grep --regexp='regex' --only-matching somefile
#or: grep --regexp='regex' -o somefile
}}}

Note that for lines that match repeatedly, each match is printed on a new line.



=== File Identification ===

`grep(1)` also supports functionality for identifying files that match.

To print the names of files that match, try:

{{{
grep --regexp='regex' --files-with-matches somefile
#or: grep --regexp='regex' -l somefile
}}}

The inverted behavior, i.e. printing the names of files that do not match, is triggered with `--files-without-match` (`-L`).

To print the names of files and the total count of matches in that file, try:

{{{
grep --regexp='regex' --count somefile
#or: grep --regexp='regex' -c somefile
}}}

The inverted behavior, i.e. printing the total count of non-matches, is triggered with the same `--invert-match` (`-v`) as before.

Note that files with 0 matches are also printed in this mode.



=== Programming Tips ===

`grep(1)` can be used programmatically to test for the existence of a match. The return code is set to success (0) if a match is found. To suppress output though, use the `--quiet` (`-q`) flag.

To suppress warnings about non-existent and unreadable files, use the `--no-messages` (`-s`) flag.

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== See also ==

[[https://man.archlinux.org/man/grep.1|grep(1)]]



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