grep explained

The word grep is sometimes used as a verb to mean, find or search for a string. Often this implies searching for a fixed string. But grep (Global Regular Expression Print) as the command acronym indicates, searches for a regular expression. It’s a highly versatile *nix command to match strings. See Computerphile: Where GREP Came From for a brief history.


  • Pattern – is a sequence of characters, where each character may be a literal or have a special interpretation.
  • Fixed string – is a search pattern where none of the characters have any special interpretation.
  • Regular Expression (or regex) – is a search pattern where each character is either a metacharacter, having a special meaning, or a regular character that has a literal meaning. There are two variations on the syntax of the specified pattern1. The difference is in the behavior of a few special characters, ?, +, (), {}, and |.
    • BRE (Basic Regular Expression) – these characters do not have special meaning unless prefixed with a backslash \.
    • ERE (Extended Regular Expression) – these characters are special unless they are prefixed with backslash \.
  • Glob – is a simpler search pattern used for matching filenames. The special characters *, ? and [] are referred to as wildcards. The confusing part is when the syntax of globbing and regex overlap, because these wildcard characters are the same as regex meta characters but with a different interpretation 2.
GLOB is for files. GREP is for strings.
ERE is the de facto variation of regex you teach, learn and use.


Specify target files to search.


Specify all files in target directories recursively to search.


The PATTERN is interpreted depending on the matcher selection option.
-E, –extended-regexp
-F, –fixed-strings
-G, –basic-regexp (default)

It’s this option that can make usage of grep frustrating if you are not aware of it.
* The PATTERN by default is interpreted as a regex and not a fixed string.
* Though regex typically implies ERE, the default is BRE.

In addition, there are variant programs egrep and fgrep that are the same as grep -E and grep -F, respectively. These variants are deprecated, but are provided for backward compatibility.

Using the default or -G is very confusing. Always use -E ( egrep ) or -F ( fgrep ).


Here are some useful options, especially when searching through source code.

Matching control
-r, --recursive Read all files under each directory, recursively.
-i, --ignore-case Ignore case distinctions in both the PATTERN and the input files.
-w Match whole word.
-v, --invert-match Invert the sense of matching, select non matching lines.
Output control
-c, --count Print count of matching lines for each input file.
-n, --line-number Prefix each line of output with the 1-based line number within its input file.
-l, --files-with-matches Print only the name of each input file where pattern is matched.
Helpful when there are many matches in each file making the output very long.
-color, --color Color the output.
Useful in distinguishing the matched strings, matching lines, file names, line numbers.
File and directory control
-I Ignore binary files.
Useful in reducing output verbosity like Binary file ./.git/index matches.
--exclude=GLOB Use GLOB wildcard matching to skip files when searching.
--include=GLOB Use wildcard matching to search only files that match GLOB.
--exclude-dir=GLOB Use GLOB wildcard matching to skip directories when searching recursively.

find | grep

Though grep provides file and directory control (--include, -exclude, --exclude-dir) to filter input files, it’s a bit clunky and can be confusing and frustrating. So typically, the more powerful find file(s) command is used and piped to the grep command.

In its simplest form.

$ find path/to/start/dir [FIND-OPTIONS] FILE-PATTERN | xargs grep [GREP-OPTIONS] STRING-PATTERN

Or if the filenames have space(s).

$ find path/to/start/dir [FIND-OPTIONS] FILE-PATTERN -print0 | xargs -0 grep [GREP-OPTIONS] STRING-PATTERN

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