Use of Linux sed command

1. Function Introduction

sed (Stream EDitor) is a stream file editor that processes content one line at a time. During processing, the currently processed line is stored in a temporary buffer, called the "Pattern Space". Then the sed command is used to process the contents of the buffer. After the processing is completed, the contents of the buffer are sent to the screen, and then the next line is processed until the end of the file. The file contents are not changed unless the -i option is used. sed is mainly used to edit one or more files, simplify repetitive operations on files, or write conversion programs.

The function of sed is similar to that of awk, but the difference is that sed is simpler and its column processing function is less powerful, while awk is more complex and its column processing function is more powerful.

2. Command format

sed [OPTION]... {script-only-if-no-other-script} [input-file]...

OPTION is the command option, script-only-if-no-other-script is the processing action, and multiple ones can be specified by -e, and input-file is the input file, and multiple ones can be specified.

3. Option Description

Options:
-n, --quiet, --silent: Use quiet mode. In the general usage of sed, all data from STDIN will generally be printed to the terminal. If -n is added, only the line that has been specially processed by sed will be listed.
-e <script>,--expression=<script>: specifies the sed action. Multiple actions can be specified by multiple -e.
-f <script-file>, --file=<script-file>: directly write the sed action in a file, -f filename can run the sed action in filename;
-r, --regexp-extended: sed supports extended regular expressions (the default is basic regular expressions).
-i: Modify the contents of the read file directly instead of outputting it to the terminal.
--help: Display help.
--version: Display version.

Action description: [n1[,n2]]function
n1, n2: may not exist, generally represents "select the number of lines to perform the action". For example, if my action needs to be performed between 10 and 20 lines, then write "10,20 action behavior".

function:
a : Added, a can be followed by a string, and these strings will appear in a new line (the next line to the current one)~
c: Replacement, c can be followed by a string, which can replace the lines between n1 and n2!
d: delete. Because it means to delete, there is usually nothing after d.
i : insert, i can be followed by a string, and these strings will appear in a new line (the previous line of the current one);
p: Print, that is, print out a selected data. Usually p will be run with the parameter sed -n~
s: Replacement, usually this s action can be used with regular expressions! For example, 1,20s/old/new/g.

4. Typical Examples

4.1 Delete Row Operation

(1) List the contents of /etc/passwd and print the line numbers. At the same time, delete lines 2 to 5.

[b3335@MIC ~]$ nl -n ln /etc/passwd | sed '2,5d'
 root:x:0:0:root:/root:/bin/bash
 sync:x:5:0:sync:/sbin:/bin/sync
 shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
.....(Omitted later).....

Note: Originally, you should issue sed -e. When there is only one action, it is OK without -e, but when there are more than one action, you must use the -e option to specify the action. Also note that the actions following sed must be enclosed in two single quotes.

(2) Simply delete line 2.

nl /etc/passwd | sed '2d'

(3) To delete the 3rd to the last line

nl /etc/passwd | sed '3,$d'

4.2 Add Row Operation

(1) Add “I like drinking tea” after the second line.

[b3335@MIC ~]$ nl -n ln /etc/passwd | sed '2a I like drinking tea'
 root:x:0:0:root:/root:/bin/bash
 bin:x:1:1:bin:/bin:/sbin/nologin
I like drinking tea
 daemon:x:2:2:daemon:/sbin:/sbin/nologin
.....(Omitted later).....

(2) If you want to add it before the second line.

nl /etc/passwd | sed '2i drink tea'
//or nl /etc/passwd | sed '1a drink tea'

(3) Add two lines after the second line: “I like drinking tea” and “I like drinking beer”.

[b3335@MIC ~]$ nl -n ln /etc/passwd | sed '2a I like drinking tea\nI like drinking beer'
  root:x:0:0:root:/root:/bin/bash
  bin:x:1:1:bin:/bin:/sbin/nologin
I like drinking tea
I like drinking beer
  daemon:x:2:2:daemon:/sbin:/sbin/nologin
…(omitted later)…

Or you can use backslash \ to separate each line, and then you can enter a command in multiple lines in the command line, as follows:

[b3335@MIC ~]$ nl -n ln /etc/passwd | sed '2a I like drinking tea\
> I like drinking beer'

4.3 Replace Line Operation

(1) Replace the contents of lines 2-5 with “No 2-5 number”.

[b3335@MIC ~]$ nl -nln /etc/passwd | sed '2,5c No 2-5 number'
  root:x:0:0:root:/root:/bin/bash
No 2-5 number
  sync:x:5:0:sync:/sbin:/bin/sync
.....(Omitted later).....

4.4 Select Line Printing

(1) List only lines 5-7 in the /etc/passwd file.

[root@www ~]# nl -nln /etc/passwd | sed -n '5,7p'
 lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
 sync:x:5:0:sync:/sbin:/bin/sync
 shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown

4.5 Searching for data and performing related operations

(1) Searching and displaying data

Search /etc/passwd for lines with the keyword root and output them.

[b3335@MIC ~]$ nl /etc/passwd | sed -n '/root/p'
 root:x:0:0:root:/root:/bin/bash
 operator:x:11:0:operator:/root:/sbin/nologin

(2) Find and delete data Delete all lines containing root in /etc/passwd and output the other lines.

[b3335@MIC ~]$ nl /etc/passwd | sed '/root/d'
 daemon:x:1:1:daemon:/usr/sbin:/bin/sh
 bin:x:2:2:bin:/bin:/bin/sh
…(ignore below)…

If you want to delete the matching string, use the following command:

[b3335@MIC ~]$ nl /etc/passwd | sed 's/root//g'

(3) Searching and replacing data In addition to the whole-line processing mode, sed can also use line units to search and replace partial data. Basically, sed's search and replacement are quite similar to vi! He's a bit like this:

sed 's/replaced string/new string/g'

(4) Search for data and execute commands Search /etc/passwd, find the line corresponding to root, and execute the set of commands in the curly braces. Each command is separated by a semicolon. Replace bash with blueshell here, and then output this line:

nl /etc/passwd | sed -n '/root/{s/bash/blueshell/;p}'
 root:x:0:0:root:/root:/bin/blueshell

If only the first bash keyword in /etc/passwd is replaced with blueshell, exit

nl /etc/passwd | sed -n '/bash/{s/bash/blueshell/;p;q}' 
root:x:0:0:root:/root:/bin/blueshell

4.6 Edit a sed command to delete the data from the third line to the end of /etc/passwd and replace bash with blueshell

nl /etc/passwd | sed -e '3,$d' -e 's/bash/blueshell/'
 root:x:0:0:root:/root:/bin/blueshell
 daemon:x:1:1:daemon:/usr/sbin:/bin/sh

-e means multi-point editing. The first editing command deletes the data from the third line to the end of /etc/passwd. The second command searches for bash and replaces it with blueshell.

4.7 Modify files directly

sed can modify the contents of files directly without using pipe commands or data stream redirection! However, since this action will directly modify the original file, please do not use the system configuration for testing casually and be cautious when using it. Let's use the downloaded regular_express.txt file to test it!

Use sed to replace every line in regular_express.txt that ends with . with !

[root@www ~]# sed -i 's/\.$/!/g' regular_express.txt

Use sed to add "# This is a test" directly to the last line of regular_express.txt.

[root@www ~]# sed -i '$a # This is a test' regular_express.txt

Since $ represents the last line and the action of a is to add, "# This is a test" is added at the end of the file.

The -i option of sed can modify the file contents directly, which is very helpful! For example, if you have a file with 1 million lines and you want to add some text on line 100, using vim might drive you crazy! Because the file is too big! What to do then? Just use sed! With sed's ability to modify/replace directly, you don't even need to use vim to make revisions!

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