Linux grep Command Usage and Examples
This post will guide you how to use grep command on Linux system. How do I serach the given file for lines containg a match to the given pattern.
NAME
grep, egrep, fgrep, rgrep – print lines matching a pattern
SYNOPSIS
grep [OPTIONS] PATTERN [FILE...] grep [OPTIONS] [-e PATTERN]... [-f FILE]... [FILE...]
DESCRIPTION
grep searches the named input FILEs for lines containing a match to the given PATTERN. If no files are specified, or if the file “-” is given, grep searches standard input. By default, grep prints the matching lines.
In addition, the variant programs egrep, fgrep and rgrep are the same as grep -E, grep -F, and grep -r, respectively. These variants are deprecated, but are provided for backward compatibility.
OPTIONS
Generic Program Information
–help Output a usage message and exit.
-V, –version
Output the version number of grep and exit.
Matcher Selection
-E, –extended-regexp
Interpret PATTERN as an extended regular expression (ERE, see below).
-F, –fixed-strings
Interpret PATTERN as a list of fixed strings (instead of regular expressions), separated by newlines, any of which is to be matched.
-G, –basic-regexp
Interpret PATTERN as a basic regular expression (BRE, see below). This is the default.
-P, –perl-regexp
Interpret the pattern as a Perl-compatible regular expression (PCRE). This is highly experimental and grep -P may warn of unimplemented features.
Matching Control
-e PATTERN, –regexp=PATTERN
Use PATTERN as the pattern. If this option is used multiple times or is combined with the -f (–file) option, search for all patterns given. This option can be used to protect a pattern beginning with “-”.
-f FILE, –file=FILE
Obtain patterns from FILE, one per line. If this option is used multiple times or is combined with the -e (–regexp) option, search for all patterns given. 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. For a regular expression pattern, this is like parenthesizing the pattern and then surrounding it with ^ and $.
-y Obsolete synonym for -i.
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 (see below), count non-matching lines.
–color[=WHEN], –colour[=WHEN]
Surround the matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for ields and groups of context lines) with escape sequences to display them in color on the terminal. The colors are defined by the environment variable GREP_COLORS. The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority. WHEN is never, always, or auto.
-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. Also see the -s or –no-messages option.
-s, –no-messages
Suppress error messages about nonexistent or unreadable files.
Output Line Prefix Control
-b, –byte-offset
Print the 0-based byte offset within the input file before each line of output. If -o (–only-matching) is specified, print the offset of the matching part itself.
-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. See also the -H option.
-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.
-u, –unix-byte-offsets
Report Unix-style byte offsets. This switch causes grep to report byte offsets as if the file were a Unix-style text file, i.e., with CR characters stripped off. This will produce results identical to running grep on a Unix machine. This option has no effect unless -b option is also used; it has no effect on platforms other than MS-DOS and MS-Windows.
-Z, –null
Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name. For example, grep -lZ outputs a zero byte after each file name instead of the usual newline. This option makes the output unambiguous, even in the presence of file names containing unusual characters like newlines. This option can be used with commands like find -print0, perl -0, sort -z, and xargs
-0 to process arbitrary file names, even those that contain newline characters.
Context Line Control
-A NUM, –after-context=NUM
Print NUM lines of trailing context after matching lines. Places a line containing a 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 (–) 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 (–) between contiguous groups of matches. With the -o or –only-matching option, this has no effect and a warning is given.
File and Directory Selection
-a, –text
Process a binary file as if it were text; this is equivalent to the –binary-files=text option.
–binary-files=TYPE
If the first few bytes of a file indicate that the file contains binary data, assume that the file is of type TYPE. By default, TYPE is binary, and grep normally outputs either a one-line message saying that a binary file matches, or no message if there is no match. If TYPE is without-match, grep assumes that a binary file does not match; this is equivalent to the -I option. If TYPE is text, grep
processes a binary file as if it were text; this is equivalent to the -a option. When processing binary data, grep may treat non-text bytes as line terminators; for example, the pattern ‘.’ (period) might not match a null byte, as the null byte might be treated as a line terminator. Warning: grep –binary-files=text might output binary garbage, which can have nasty side effects if the output is a
terminal and if the terminal driver interprets some of it as commands.
-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=GLOB
Skip files whose base name matches GLOB (using wildcard matching). A file-name glob can use *, ?, and […] as wildcards, and \ to quote a wildcard or backslash character literally.
–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.
-I Process a binary file as if it did not contain matching data; this is equivalent to the –binary-files=without-match option.
–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. Note that if no file operand is given, grep searches the working directory. This is equivalent to the -d recurse option.
-R, –dereference-recursive
Read all files under each directory, recursively. Follow all symbolic links, unlike -r.
SEE ALSO
Regular Manual Pages
awk(1), cmp(1), diff(1), find(1), gzip(1), perl(1), sed(1), sort(1), xargs(1), zgrep(1), read(2), pcre(3), pcresyntax(3), pcrepattern(3),
terminfo(5), glob(7), regex(7).
POSIX Programmer’s Manual Page
grep(1p).
Full Documentation
A complete manual ⟨http://www.gnu.org/software/grep/manual/⟩ is available. If the info and grep programs are properly installed at your site, the command
info grep
should give you access to the complete manual.
Linux Grep Command Examples
Search /etc/passwd file for root user
If you want to serach root user information in /etc/passwd file, you can use the grep command to achieve it. Type the following command:
# grep root /etc/passwd
Outputs:
devops@devops-osetc:~$ grep root /etc/passwd root:x:0:0:root:/root:/bin/bash
Recursively Serach in a given Directory
If you want to recursively search a string “fio” in a given directory /home/devops, you can use the following grep command:
# grep -r "fio" /home/devops
Outputs:
devops@devops-osetc:~$ grep -r "fio" /home/devops/ Binary file /home/devops/.bash_history matches Binary file /home/devops/working/test.rar matches /home/devops/fio.txt:fio test /home/devops/fio.txt:fio test 2 /home/devops/.viminfo:'0 3 9 ~/fio.txt
