grep

Search file(s) for specific text.

Syntax
      grep options "Search String" [filename]

      grep options [-e PATTERN] [FILE...]

      grep options [-f FILE] [FILE...]

A simple example:
$ grep "Needle in a Haystack" /etc/*

Grep searches the named input FILEs (or standard input if no files are named, or the file name - is given) for lines containing a match to the given PATTERN. By default, grep prints the matching lines.

In addition, two variant programs egrep and fgrep are available, both are deprecated.
Egrep is the same as grep -E. Fgrep is the same as grep -F.

Options
   -A NUM
   --after-context=NUM
       Print NUM lines of trailing context after matching lines.
       Places a line containing -- between contiguous groups of matches. 
   -a
   --text
       Process a binary file as if it were text; this is equivalent
       to the --binary-files=text option.

   -B NUM
   --before-context=NUM
       Print NUM lines of leading context before matching lines.
       Places a line containing -- between contiguous groups of matches.

   -b
   --byte-offset
       Print the byte offset within the input file before each line of output. 

   --colour[=WHEN]
   --color[=WHEN]
       Surround the matching string with the marker from the GREP_COLOR environment variable.
       WHEN can be 'never', 'always', or 'auto' e.g. --color=always
       By default the matched text will be colored red.
       If grep is made to match several strings, all of the matches will be colored, one exception
       is the regex ^ (match beginning of every line), the beginning of a line has no length so will
       not be coloued.
       So to return all lines and colour only matches: egrep --color=always '^|string1|string2'

   -C NUM
   --context=NUM
       Print NUM lines of output context.
       Print num lines of leading and trailing context surrounding each match.
       The default is 2 and is equivalent to -A 2 -B 2.
       Note: no whitespace may be given between the option and its argument.

   -c
   --count
       Suppress normal output; instead print a count of matching lines for each input file.
       With the -v, --invert-match option (see below), count non-matching lines.

   -D ACTION
   --devices=ACTION
       If an input file is a device, FIFO or socket, use ACTION to process it.
       By default, ACTION is read, which means that devices are read just as if they were
       ordinary files. If ACTION is skip, devices are silently skipped.

   -d ACTION
   --directories=ACTION
       If an input file is a directory, use ACTION to process it.
       By default, ACTION is read, which means that directories are read just as if they
       were ordinary files. If ACTION is skip, directories are silently skipped.
       If ACTION is recurse, grep reads all files under each directory, recursively; this
       is equivalent to the -R and -r option.

   -E
   --extended-regexp
       Interpret PATTERN as an extended regular expression

   -e PATTERN
   --regexp=PATTERN
       Use PATTERN as the pattern; useful to protect patterns beginning with -

   --exclude
       If specified, it excludes files matching the given filename pattern from
       the search.  Note that --exclude patterns take priority over --include patterns,
       and if no --include pattern is specified, all files are searched that are not excluded.
       Patterns are matched to the full path specified, not only to the filename component.

   --exclude-dir
       If -R is specified, it excludes directories matching the given filename pattern from
       the search.  Note that --exclude-dir patterns take priority over --include-dir patterns,
       and if no --include-dir pattern is specified, all directories are searched that are not excluded.

   -F
   --fixed-strings
       Interpret PATTERN as a list of fixed strings, separated by newlines,
       any of which is to be matched.

   -f FILE
   --file=FILE
       Obtain patterns from FILE, one per line. The empty file contains zero
       patterns, and therefore matches nothing.

   -G
   --basic-regexp
       Interpret PATTERN as a basic regular expression This is the default.
 
   -H
   --with-filename
       Print the filename for each match. 
   -h
   --no-filename
       Suppress the prefixing of filenames on output when multiple files are searched. 
   --help
      Output a brief help message.

   -I
      Process a binary file as if it did not contain matching data; this is
      equivalent to the --binary-files=without-match option.
   -i
   --ignore-case
      Ignore case distinctions in both the PATTERN and the input files.

   --include
      If specified, only files matching the given filename pattern are searched.
      Note that --exclude patterns take priority over --include patterns.  Patterns are
      matched to the full path specified, not only to the filename component.

   --include-dir
      If -R is specified, only directories matching the given filename pattern are searched.
      Note that --exclude-dir patterns take priority over --include-dir patterns.

   -J, --bz2decompress
      Decompress the bzip2(1) compressed file before looking for the text.

   -L
   --files-without-match
      Suppress normal output; instead print the name of each input file from which
      no output would normally have been printed. The scanning will stop on the first match. 
   -l
   --files-with-matches
      Suppress normal output; instead print the name of each input file from which
      output would normally have been printed. The scanning will stop on the first match.

   --mmap
      Use mmap(2) instead of read(2) to read input, which can result in better performance under
      some circumstances but can cause undefined behaviour.

   -m NUM
   --max-count=NUM
      Stop reading a file after NUM matching lines.
      If the input is standard input from a regular file, and NUM matching lines are
      output, grep ensures that the standard input is positioned to just after the last
      matching line before exiting, regardless of the presence of trailing context lines.
      This enables a calling process to resume a search. When grep stops after NUM matching
      lines, it outputs any trailing context lines.
      When the -c or --count option is also used, grep does not output a count greater
      than NUM. When the -v or --invert-match option is also used, grep stops after
      outputting NUM non-matching lines. 

   -n
   --line-number
      Prefix each line of output with the line number within its input file. 

   --null   Print a zero-byte after the file name.

   -O
      If -R is specified, follow symbolic links only if they were explicitly listed on the
      command line.  The default is not to follow symbolic links.

   -o
   --only-matching
      Show only the part of a matching line that matches PATTERN.

   -p
      If -R is specified, no symbolic links are followed.  This is the default.

   -q
   --quiet
   --silent
      Quiet mode: suppress normal output.  grep will only search a file until a match
      has been found, making searches potentially less expensive.

   -R
   -r
   --recursive
      Read all files under each directory, recursively
      This is equivalent to the -d recurse option. 

   -s
   --no-messages
      Silent mode.  Nonexistent and unreadable files are ignored
      (i.e. their error messages are suppressed).

   -U
   --binary
      Search binary files, but do not attempt to print them.

   -V
   --version
      Display version information and exit.

   -v
   --invert-match
      Selected lines are those not matching any of the specified patterns.

   -w
    --word-regexp
      The expression is searched for as a word
      (as if surrounded by '[[:<:]]' and '[[:>:]]'; see re_format(7)).

   -x
   --line-regexp
      Only input lines selected against an entire fixed string or regular expression
      are considered to be matching lines.

   -y      Equivalent to -i.  Obsoleted.

   -Z
   -z
   --decompress
      Force grep to behave as zgrep.

   --binary-files=value
      Controls searching and printing of binary files.  Options are binary, the
      default: search binary files but do not print them; without-match: do not search
      binary files; and text: treat all files as text.

   --context[=num]
      Print num lines of leading and trailing context.  The default is 2.

   --line-buffered
      Force output to be line buffered.  By default, output is line buffered
      when standard output is a terminal and block buffered otherwise.

If no file arguments are specified, the standard input is used.

The name grep comes from the basic ed search command: g/re/p (Global, Regular Expression re search delimited by slashes, Print).
The newer grep (Global Regular Expression Print) command adds several features, primarily the ability to handle much larger files.

Environment variables

Grep’s behavior can be affected by setting the following environment variables:

GREP_OPTIONS                 - default options
LC_ALL, LC_MESSAGES, LANG    - language for messages
LC_CTYPE                     - foreign characters
POSIXLY_CORRECT              - Posix behaviour
_N_GNU_nonoption_argv_flags_ - ignore an operand

see 'info grep' for more

Exit Status

grep exits with one of the following values:

   0     One or more lines were selected.
   1     No lines were selected.
   >1    An error occurred - syntax error in pattern, inaccessible input files, or other system error.

In other words a failure to find a matching item is reported as an error (1)

The '-v' option can be used to effectively invert the exit status.

Searching an entire hard drive with grep can be very slow, using mdfind to identify files containing text is orders of magnitude faster.

Examples

Search the file example.txt, including binary data (-a) for the string 'hunting the snark':

$ sudo grep -a 'hunting the snark' example.txt

Search the whole partition (/disk0), including binary data (-a) for the string 'hunting the snark' return all the lines
starting 25 Before the text found and 50 lines After the matching text found, this can be a way to discover fragments of deleted files:

$ sudo grep -a -B 25 -A 50 'hunting the snark' /dev/disk0> results.txt

Look for those lines starting (^) with H, then 0 or more characters and finally a 'k' at the end of the line ($), this regex assumes the text file has unix style LF line endings:

$ grep '^H.*k$' example.txt

Look for those lines starting (^) with H, then 0 or more characters and finally a 'k' at the end of the line ($), this regex assumes the text file has Windows style CR/LF line endings, Use Ctrl+V, Ctrl+M to enter a literal Carriage Return as ^M:

$ grep '^H.*k^M$' example.txt

Search the file wordlist.txt for any lines that include either 'sco' or 'sca':

$ grep "sc[oa]" wordlist.txt

Search the file wordlist.txt for any lines that don’t include at least one vowel:

$ grep -v [aeiou] wordlist.txt

Find words in the dictionary which include the phrase 'geront' and highlight in colour:

grep --color geront /usr/share/dict/words

Search all files in the folder /users/Carmen/ for any files NOT contaning the word 'snark':

$ grep -RL 'snark' /users/Carmen/

Remove lines from invoices.txt if they appear in paid.txt:

$ grep -F -x -v -f paid.txt invoices.txt >paidinvoices.txt

List all the file links in the current folder - in the ouptut of ls each symbolic directory has l permission at the begining of the permission flags, so grep ^l will list only symbolic links:

$ ls -lR | grep ^l
A less cryptic method is to use find . -type l

“The most dangerous of all falsehoods is a slightly distorted truth” ~ G. C. Lichtenberg

Related macOS commands

Local man page: grep - Command line help page on your local machine.
command -v — Display the path to the executable or the alias definition of a specific command.
find - Search for files that meet a desired criteria.
pgrep - List processes by a full or partial name.
awk - Find and Replace text within file(s).
mdfind - Spotlight search.
tr - Translate, squeeze, and/or delete characters.
whereis - Locate a command.
Equivalent PowerShell command: Select-String - Select lines matching a pattern.