grep.md

grep

grep, egrep, fgrep - print lines matching a pattern

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References

• man grep

Quickstart

# Regular Expression
grep -e pattern file    # Use basic syntaxes
grep -E pattern file    # Use extended syntaxes
egrep pattern file      # ( shorter )
# e.g.  egrep "^#" /etc/hosts

# Common
grep -v pattern file    # Invert match
grep --invert-match pattern file

grep -i pattern file    # Ignore case
grep --ignore-case pattern file

grep -n pattern file    # Print with line numbers
grep --line-number pattern file

grep -c pattern file    # Count of matching lines
grep --count pattern file

grep -r pattern dir     # Recursive search in dir
grep -R pattern dir
grep --recursive pattern dir

# Context
grep -A NUM pattern file    # Print n lines after matching lines
grep --after-context=n pattern file

grep -B NUM pattern file    # Print n lines before matching lines
grep --before-context=n pattern file

grep -C NUM pattern file    # Print n lines before & after …
grep --context=NUM pattern file
grep -NUM pattern file      # e.g. grep -2 abc file

Synopsis

grep

grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

egrep

egrep
# same as
grep -E

fgrep

• cannot understand its usage temporarily

fgrep
# same as
grep -F

Options

Matcher Selection

Interpret PATTERN as :

• -E, --extended-regexp an extended regular expression (ERE)
• -F, --fixed-strings, --fixed-regexp a list of fixed strings
  • separated by newlines, any of which is to be matched.
  • --fixed-regexp is an obsoleted alias, please do not use it in new scripts.
• -G, --basic-regexp a basic regular expression (BRE)
  • This is the default.
• -P, --perl-regexp a Perl regular expression
  • This is highly experimental and grep -P may warn of unimplemented features.

Matching Control

• -e PATTERN, --regexp=PATTERN Use PATTERN as the pattern.
  • This can be used to specify multiple search patterns, or to protect a pattern beginning with a hyphen (-).
• -f FILE, --file=FILE Obtain patterns from FILE, one per line.
  • The empty file contains zero patterns, and therefore matches nothing.
• -i, --ignore-case Ignore case distinctions in both the PATTERN and the input files.
• -v, --invert-match Invert the sense of matching, to select non-matching lines.
• -w, --word-regexp Select only those lines containing matches that form whole words.
  • The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character.
  • Similarly, it must be either at the end of the line or followed by a non-word constituent character.
  • Word-constituent characters are letters, digits, and the underscore.
• -x, --line-regexp Select only those matches that exactly match the whole line.

General Output Control

• -c, --count Suppress normal output; instead print a count of matching lines for each input file.
  • With the -v, --invert-match option, count non-matching lines.
• -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.
• -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.
• -o, --only-matching Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.
• -q, --quiet, --silent Quiet; do not write anything to standard output.
  • Exit immediately with zero status if any match is found, even if an error was detected.
• -s, --no-messages Suppress error messages about nonexistent or unreadable files.
  • Portability note: unlike GNU grep, 7th Edition Unix grep did not conform to POSIX, because it lacked -q and its -s option behaved like GNU grep's -q option.
  • USG-style grep also lacked -q but its -s option behaved like GNU grep.
  • Portable shell scripts should avoid both -q and -s and should redirect standard and error output to /dev/null instead.

Output Line Prefix Control

• -H, --with-filename Print the file name for each match.
  • This is the default when there is more than one file to search.
• -h, --no-filename Suppress the prefixing of file names on output.
  • This is the default when there is only one file (or only standard input) to search.
• --label=LABEL Display input actually coming from standard input as input coming from file LABEL.
  • This is especially useful when implementing tools like zgrep
  • e.g., gzip -cd foo.gz | grep --label=foo -H something.
• -n, --line-number Prefix each line of output with the 1-based line number within its input file.
• -T, --initial-tab Make sure that the first character of actual line content lies on a tab stop, so that the alignment of tabs looks normal.
  • This is useful with options that prefix their output to the actual content: -H,-n, and -b.
  • In order to improve the probability that lines from a single file will all start at the same column, this also causes the line number and byte offset (if present) to be printed in a minimum size field width.
• ……

Context Line Control

• -A NUM, --after-context=NUM Print NUM lines of trailing context after matching lines.
  • Places a line containing a group separator (described under --group-separator) between contiguous groups of matches.
  • With the -o or --only-matching option, this has no effect and a warning is given.
• -B NUM, --before-context=NUM Print NUM lines of leading context before matching lines.
  • Places a line containing a group separator (described under --group-separator) between contiguous groups of matches.
  • With the -o or --only-matching option, this has no effect and a warning is given.
• -C NUM, -NUM, --context=NUM Print NUM lines of output context.
  • Places a line containing a group separator (described under --group-separator) between contiguous groups of matches.
  • With the -o or --only-matching option, this has no effect and a warning is given.
• --group-separator=SEP Use SEP as a group separator.
  • By default SEP is double hyphen (--).
• --no-group-separator Use empty string as a group separator.

File & Directory Selection

• -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, i.e., read directories just as if they were ordinary files.
  • If ACTION is skip, silently skip directories.
  • If ACTION is recurse, read all files under each directory, recursively, following symbolic links only if they are on the command line. This is equivalent to the -r option.
• --exclude-from=FILE Skip files whose base name matches any of the file-name globs read from FILE (using wildcard matching as described under --exclude).
• --exclude-dir=DIR Exclude directories matching the pattern DIR from recursive searches.
• --include=GLOB Search only files whose base name matches GLOB (using wildcard matching as described under --exclude).
• -r, --recursive Read all files under each directory, recursively, following symbolic links only if they are on the command line.
  • This is equivalent to the -d recurse option.
• -R, --dereference-recursive Read all files under each directory, recursively.
  • Follow all symbolic links, unlike -r.

Usage

Sample

$ cat input
abhishek
divyam
chitransh
naveen
harsh

Default

$ grep h input
abhishek
chitransh
harsh

Count

$ grep h input -c
3

Line Number

$ grep h input -n
1:abhishek
3:chitransh
5:harsh

Invert Match

$ grep h input -v
divyam
naveen

Context

After

$ grep t input -A 1
chitransh
naveen

Before

grep t input -B 1
divyam
chitransh

Before & After

$ grep t input -C 1
divyam
chitransh
naveen

Recursive

Follow all symbolic links ( Recommended )

grep <content> <file/directory> -R

Follow symbolic links only if they are on the command line

grep <content> <file/directory> -r

Regular Expression

egrep = grep -E

$ egrep '.*sh$' input
# same as
$ grep -E '.*sh$' input
chitransh
harsh

$ egrep '.*sh' input
abhishek
chitransh
harsh

$ egrep '[n|h]a.*' input
naveen
harsh

$ egrep '(abh|chi).*' input
abhishek
chitransh

Regular Expression

grep understands three different versions of regular expression syntax:

• basic
• extended
• perl

Implementsations

• In GNU grep, there is no difference in available functionality between basic & extended syntaxes.
• In other implementations, basic regular expressions are less powerful.
  • Differences for basic regular expressions are summarized afterwards.
  • Perl regular expressions give additional functionality, and are documented in pcresyntax(3) and pcrepattern(3), but may not be available on every system.

Basic vs Extended Regular Expressions

• In basic regular expressions the meta-characters ? + { | ( ) lose their special meaning; instead use the backslashed versions \? \+ \{ \| \( \).
• Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.
• GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification.
  • For example, the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax error in the regular expression.
  • POSIX allows this behavior as an extension, but portable scripts should avoid it.