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