Linux grep Command

The Linux grep command is used to search for a string of characters in a specified file.

The grep command searches files for content that matches a specified pattern. If it finds a match in a file, the default behavior of grep is to print the line containing the pattern. If no file names are specified, or if the file name is -, grep reads from the standard input device.

Syntax

grep [-abcEFGhHilLnqrsvVwxy][-A<number-of-lines>][-B<number-of-columns>][-C<number-of-columns>][-d<action>][-e<pattern>][-f<pattern-file>][--help][pattern][file or directory...]

Parameters:

--a or --text: Do not ignore binary data.

--A<number-of-lines> or --after-context=<number-of-lines>: Display the line matching the pattern and the lines after it.

--b or --byte-offset: Display the byte offset of the line containing the match.

--B<number-of-lines> or --before-context=<number-of-lines>: Display the line matching the pattern and the lines before it.

--c or --count: Count the number of lines that match the pattern.

--C<number-of-lines> or --context=<number-of-lines> or -<number-of-lines>: Display the line matching the pattern and the lines before and after it.

--d <action> or --directories=<action>: Specify the action to take if a directory is specified instead of a file.

--e<pattern> or --regexp=<pattern>: Specify a pattern to search for in the file content.

--E or --extended-regexp: Use extended regular expressions for the pattern.

--f<rule-file> or --file=<rule-file>: Specify a file containing rules (one per line) to match against the file content.

--F or --fixed-regexp: Treat the pattern as a fixed string list.

--G or --basic-regexp: Use basic regular expressions for the pattern.

--h or --no-filename: Do not display the file name for matching lines.

--H or --with-filename: Display the file name for matching lines.

--i or --ignore-case: Ignore case distinctions.

--l or --file-with-matches: List the names of files containing matches.

--L or --files-without-match: List the names of files not containing matches.

--n or --line-number: Display the line number for matching lines.

--o or --only-matching: Display only the matched parts of the lines.

--q or --quiet or --silent: Do not output any information.

--r or --recursive: Equivalent to specifying "-d recurse".

--s or --no-messages: Do not display error messages.

--v or --invert-match: Display lines that do not match the pattern.

--V or --version: Display version information.

--w or --word-regexp: Display lines that match whole words.

--x --line-regexp: Display lines that match the entire line.

--y: Equivalent to specifying "-i".

Linux Command Manual

Examples

Search for the string "test" in files with a "file" suffix in the current directory and print the lines containing the string. Use the following command:
```
grep test *file
```

Output:

$ grep test test* # Search for files with the prefix "test" containing the string "test"
testfile1:This a Linux testfile! # Lines from testfile1 containing "test"
testfile_2:This is a linux testfile! # Lines from testfile_2 containing "test"

List the lines in the file testfile_2 that contain the character string "test" using the command:

grep test testfile_2

Recursively search for files containing the string "update" in the specified directory /etc/acpi and its subdirectories (if any), and print the lines containing the string. The command used is:
```
grep -r update /etc/acpi
```

The output is as follows:

$ grep -r update /etc/acpi # Recursively search for files in "etc/acpi" containing "update"
/etc/acpi/ac.d/85-anacron.sh:# (Things like the slocate updatedb cause a lot of IO.)
Rather than
/etc/acpi/resume.d/85-anacron.sh:# (Things like the slocate updatedb cause a lot of IO.) Rather than
/etc/acpi/events/thinkpad-cmos:action=/usr/sbin/thinkpad-keys--update

Inverse search. The previous examples searched for and printed lines that matched the condition. Using the -v parameter, you can print lines that do not match the condition.

To find lines in files containing "test" in their names that do not contain "test", use the command:

grep -v test *test*

The results are as follows:

$ grep -v test *test* # Find lines in files containing "test" in their names that do not contain "test"
testfile1:helLinux!
testfile1:Linis a free Unix-type operating system.
testfile1:Lin
testfile_1:HELLO LINUX!
testfile_1:LINUX IS A FREE UNIX-TYPE OPTERATING SYSTEM.
testfile_1:THIS IS A LINUX TESTFILE!
testfile_2:HELLO LINUX!
testfile_2:Linux is a free unix-type opterating system.

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