grep command in Linux w/ examples

This grep command guide is a follow-up of my previous 90 Linux Commands frequently used by Linux Sysadmins article. Every week, or as time allows, I will publish articles on around 90 commands geared toward Linux sysadmins and Linux power users. Let’s continue this series with the grep command.

Grep (global regular expression printer) searches through a file for a specific pattern of characters. When it finds a match in a line, grep copies the line to standard output or whatever output you select using options. grep was initially developed for the Unix operating system but eventually made available for all Unix-like systems, such as Linux.

The general syntax of the grep command is:

grep [option...] [patterns] [file...]

Linux grep command examples

…grep man page.

To search for a pattern within a file, use the following:

grep "pattern" /path/to/file

To search all files in the current directory, use the following:

grep pattern *

To search for an exact pattern, use the following:

grep -F "pattern" /path/to/file

To search directories recursively, use the following:

grep -r 'hello' /path/to/dir

To search for a whole word, not a part of a word, use:

grep -w 'word' /path/to/file

To perform a case insens­itive (ignore case) search, use:

grep -i 'pattern' filename

To display the filename which contains the pattern, use the following:

grep -l 'pattern' /var/log/*

To display x lines after matching pattern, use:

grep -A x 'pattern' filename

To display x lines before the matching pattern, use:

grep -B x 'pattern' filename

To display x lines around the matching pattern, use:

grep -C x 'pattern' filename

To display the matching part of the pattern, use the following:

grep -o 'pattern' filename

To display the line number of the matching pattern, use the following:

grep -n 'pattern' filename

To return all lines which don’t match the pattern, use the following:

grep -v 'warning' /var/log/nginx/error.log

To display the lines starting with ‘er’, use:

grep -e '^er' filename

To display lines with 3 w’s in a row (www), use:

grep -E 'w{3}' filename

Related commands:

• find – Find files or directories under the given directory tree recursively.
• pgrep – searches running processes and lists the process IDs which match the selection criteria to stdout.
• ngrep – grep applied to the network layer.
• locate – search files in Linux.

Useful links/reference: 

• About grep.
• Why GNU grep is fast.
• GNU Grep

Conclusion

grep helps find patterns within files or the file system hierarchy, so it’s worthwhile to learn its options and syntax.