Linux
grep command

Linux Command – Grep

Grep command on Linux process text line by line and prints any line match the pattern

Generic Syntax

grep [Options] pattern [files]

Grep, which stands for “global regular expression print,” is a powerful tool for matching a regular expression against text in a file, multiple files, or a stream of input. It searches for the PATTERN of text that you specify on the command line, and outputs the results for you.

Pattern is one or more text seperated by newline character “\n”

Typically Pattern should be in quotes


NOTE

General Options

–helpPrint a help message briefly summarizing command-line options, and exit.
-V, –versionPrint the version number of grep, and exit.

Some Examples

  • You want to search a keyword in a file
    • grep “keyword” file.txt
    • the “keyword” here is a regular expression so , we put spaces in here it all will get interpreted
  • if we want to view the grep output in color
    • grep –color “keyword” file.txt
  • searching in multiple files
    • grep “keyword” *.txt
    • this will check keyword in all the text files
  • Recursively search all the subdirectories and all the files in it.
    • grep -r “keyword” *

Match Selection Options

-E, –extended-regexpInterpret PATTERN as an extended regular expression (see Basic vs. Extended Regular Expressions).
-F, –fixed-stringsInterpret PATTERN as a list of fixed strings, separated by newlines, that is to be matched.
-G, –basic-regexpInterpret PATTERN as a basic regular expression (see Basic vs. Extended Regular Expressions). This is the default option when running grep.
-P, –perl-regexpInterpret PATTERN as a Perl regular expression. This functionality is still experimental, and may produce warning messages.

Matching Control Options

-e PATTERN, –regexp=PATTERNUse PATTERN as the pattern to match. This can be used to specify multiple search patterns, or to protect a pattern beginning with a dash ().
-f FILE, –file=FILEObtain patterns from FILE, one per line.
-i, –ignore-caseIgnore case distinctions in both the PATTERN and the input files.
-v, –invert-matchInvert the sense of matching, to select non-matching lines.
-w, –word-regexpSelect 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. Or, 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 underscores.
-x, –line-regexpSelect only matches that exactly match the whole line.
-yThe same as -i.

General Output Control

-c, –countInstead of the normal output, 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 fields 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 older environment variable GREP_COLOR is still supported, but its setting does not have priority. WHEN is never, always, or auto.
-L, –files-without-matchInstead of the normal output, 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-matchesInstead of the normal output, 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=NUMStop 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-matchingPrint only the matched (non-empty) parts of a matching line, with each such part on a separate output line.
-q, –quiet, –silentQuiet; 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-messagesSuppress error messages about nonexistent or unreadable files.

Output Line Prefix Control

-b, –byte-offsetPrint 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-filenamePrint the file name for each match. This is the default when there is more than one file to search.
-h, –no-filenameSuppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.
–label=LABELDisplay 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-numberPrefix each line of output with the 1-based line number within its input file.
-T, –initial-tabMake 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. 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-offsetsReport 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, –nullOutput 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.

File and Directory Selection

-a, –textProcess a binary file as if it were text; this is equivalent to the –binary-files=text option.
–binary-files=TYPEIf 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. 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=ACTIONIf 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=ACTIONIf 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=GLOBSkip 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=FILESkip files whose base name matches any of the file-name globs read from FILE (using wildcard matching as described under –exclude).
–exclude-dir=DIRExclude directories matching the pattern DIR from recursive searches.
-IProcess a binary file as if it did not contain matching data; this is equivalent to the –binary-files=without-match option.
–include=GLOBSearch only files whose base name matches GLOB (using wildcard matching as described under –exclude).
-r, –recursiveRead 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-recursiveRead all files under each directory, recursively. Follow all symbolic links, unlike -r.

Repetition

A regular expression may be followed by one of several repetition operators:

?The preceding item is optional and matched at most once.
*The preceding item will be matched zero or more times.
+The preceding item will be matched one or more times.
{n}The preceding item is matched exactly n times.
{n,}The preceding item is matched n or more times.
{n,m}The preceding item is matched at least n times, but not more than m times.

Environment Variables

The behavior of grep is affected by the following environment variables.

The locale for category LC_foo is specified by examining the three environment variables LC_ALL, LC_foo, and LANG, in that order. The first of these variables that is set specifies the locale. For example, if LC_ALL is not set, but LC_MESSAGES is set to pt_BR, then the Brazilian Portuguese locale is used for the LC_MESSAGES category. The C locale is used if none of these environment variables are set, if the locale catalog is not installed, or if grep was not compiled with national language support (NLS).

One more example


The following example outputs the location and contents of any line
containing “f” and ending in “.c”, within all files in the current di‐
rectory whose names contain “g” and end in “.h”. The -n option outputs
line numbers, the — argument treats expansions of “g.h” starting
with “-” as file names not options, and the empty file /dev/null causes
file names to be output even if only one file name happens to be of the
form “g.h”.

     $ grep -n -- 'f.*\.c$' *g*.h /dev/null
     argmatch.h:1:/* definitions and prototypes for argmatch.c

   The only line that matches is line 1 of argmatch.h.  Note that the reg‐
   ular expression syntax used in the pattern differs  from  the  globbing
   syntax that the shell uses to match file names.

Full Documentation


A complete manual ⟨https://www.gnu.org/software/grep/manual/⟩ is avail‐
able.

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