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grep command

The grep command filters the content of single or multiple files. Some variants of this command tool exist, such as egrep (grep -E) and fgrep (grep -f). For information not covered, see the grep manual.

The usage of the grep command is:

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

The options are mainly divided into four parts:

  • match control
  • output control
  • content line control
  • directory or file control

match control：

options description
-E
--extended-regexp		 Enable ERE
-P
--perl-regexp		 Enable PCRE
-G
--basic-regexp		 Enable BRE by default
-e
--regexp=PATTERN		 Pattern matching, multiple -e options can be specified.
-i Ignore case
-w Accurately match the entire word
-f FILE Obtain patterns from FILE, one per line
-x Pattern matching entire line
-v Select content lines that do not match

output control:

options description
-m NUM Output the first few matching results
-n Print line numbers on output
-H When matching the file content of multiple files, display the file name at the beginning of the line. This is the default setting
-h When matching the file content of multiple files, the file name is not displayed at the beginning of the line
-o Print only matching content without outputting the entire line
-q Not outputting normal information
-s Do not output error messages
-r Recursive matching for directories
-c Prints the number of lines matched by each file based on the content of the user

content line control:

options description
-B NUM Print NUM lines of leading context before matching lines
-A NUM Print NUM lines of trailing context after matching lines
-C NUM Print NUM lines of output context

directory or file control:

options description
--include=FILE_PATTERN Searches only files that match FILE_PATTERN. Wildcard characters for file names support *, ?, [], [^], [-], {..}, {,}
--exclude=FILE_PATTERN Skips files and directories matching FILE_PATTERN. Wildcard characters for file names support *, ?, [], [^], [-], {..}, {,}
--exclude-dir=PATTERN Excludes the specified directory name. Directory name support *, ?, [], [^], [-], {..}, {,}
--exclude-from=FILE Excludes the specified directory from the file content.

Examples of usage

  1. -f option and -o option

    Shell > cat /root/a
abcdef
123456
338922549
24680
hello world

Shell > cat /root/b
12345
test
world
aaaaa

# Treat each line of file b as a matching pattern and output the lines that match file a.
Shell > grep -f /root/b /root/a
123456
hello world

Shell > grep -f /root/b /root/a -o
12345
world

  2. Multiple pattern matching (using the -e option)

    Shell > echo -e "a\nab\nbc\nbcde" | grep -e 'a' -e 'cd'
a
ab
bcde

    or:

    Shell > echo -e "a\nab\nbc\nbcde" | grep -E "a|cd"
a
ab
bcde

  3. Remove blank lines and comment lines from the configuration file

    Shell > grep -v -E "^$|^#" /etc/chrony.conf
server ntp1.tencent.com iburst
server ntp2.tencent.com iburst
server ntp3.tencent.com iburst
server ntp4.tencent.com iburst
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync
keyfile /etc/chrony.keys
leapsectz right/UTC
logdir /var/log/chrony

  4. Print the top 5 results that all match

    Shell > seq 1 20 | grep -m 5 -E "[0-9]{2}"
10
11
12
13
14

    or:

    Shell > seq 1 20 | grep -m 5  "[0-9]\{2\}"
10
11
12
13
14

  5. -B option and -A option

    Shell > seq 1 20 | grep -B 2 -A 3 -m 5 -E "[0-9]{2}"
8
9
10
11
12
13
14
15
16
17

  6. -C option

    Shell > seq 1 20 | grep -C 3 -m 5 -E "[0-9]{2}"
7
8
9
10
11
12
13
14
15
16
17

  7. -c option

    Shell > cat /etc/ssh/sshd_config | grep  -n -i -E "port"
13:# If you want to change the port on a SELinux system, you have to tell
15:# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
17:#Port 22
99:# WARNING: 'UsePAM no' is not supported in RHEL and may cause several
105:#GatewayPorts no

Shell > cat /etc/ssh/sshd_config | grep -E -i "port" -c
5

  8. -v option

    Shell > cat /etc/ssh/sshd_config | grep -i -v -E "port" -c
140

  9. Filter files in a directory that have lines that match the string (Exclude files in subdirectories)

    Shell > grep -i -E "port" /etc/n*.conf -n
/etc/named.conf:11:     listen-on port 53 { 127.0.0.1; };
/etc/named.conf:12:     listen-on-v6 port 53 { ::1; };
/etc/nsswitch.conf:32:# winbind                 Use Samba winbind support
/etc/nsswitch.conf:33:# wins                    Use Samba wins support

  10. Filter files in a directory that have lines that match the string (include or exclude files or directories in subdirectories)

    Include syntax for multiple files:

    Shell > grep -n -i -r -E  "port" /etc/ --include={0..20}_*
/etc/grub.d/20_ppc_terminfo:26:export TEXTDOMAIN=grub
/etc/grub.d/20_ppc_terminfo:27:export TEXTDOMAINDIR=/usr/share/locale
/etc/grub.d/20_linux_xen:26:export TEXTDOMAIN=grub
/etc/grub.d/20_linux_xen:27:export TEXTDOMAINDIR="${datarootdir}/locale"
/etc/grub.d/20_linux_xen:46:# Default to disabling partition uuid support to maintian compatibility with
/etc/grub.d/10_linux:26:export TEXTDOMAIN=grub
/etc/grub.d/10_linux:27:export TEXTDOMAINDIR="${datarootdir}/locale"
/etc/grub.d/10_linux:47:# Default to disabling partition uuid support to maintian compatibility with

Shell > grep -n -i -r -E  "port" /etc/ --include={{0..20}_*,sshd_config} -c
/etc/ssh/sshd_config:5
/etc/grub.d/20_ppc_terminfo:2
/etc/grub.d/10_reset_boot_success:0
/etc/grub.d/12_menu_auto_hide:0
/etc/grub.d/20_linux_xen:3
/etc/grub.d/10_linux:3

    If you need to exclude a single directory, use the following syntax:

    Shell > grep -n -i -r -E  "port" /etc/ --exclude-dir=selin[u]x

    If you need to exclude multiple directories, use the following syntax:

    Shell > grep -n -i -r -E  "port" /etc/ --exclude-dir={selin[u]x,"profile.d",{a..z}ki,au[a-z]it}

    If you need to exclude a single file, use the following syntax:

    Shell > grep -n -i -r -E  "port" /etc/ --exclude=sshd_config

    If you need to exclude multiple files, use the following syntax:

    Shell > grep -n -i -r -E  "port" /etc/ --exclude={ssh[a-z]_config,*.conf,services}

    If you need to exclude multiple files and directories at the same time, use the following syntax:

    Shell > grep -n -i -r -E  "port" /etc/ --exclude-dir={selin[u]x,"profile.d",{a..z}ki,au[a-z]it} --exclude={ssh[a-z]_config,*.conf,services,[0-9][0-9]*}

  11. Count all IPv4 addresses of the current machine

    Shell > ip a | grep -o  -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}" | grep -v -E "127|255"
192.168.100.3

Author: tianci li