Linux Sed Tutorial: 6 Examples of Sed Branching Operations

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Linux Sed Tutorial: 6 Examples of Sed Branching Operations

Author：JIYIK Last Updated：2025/04/06 Views：

Like any other programming language, sed also provides special branching commands to control the flow of the program.

In this article, let's review the following two types of Sed branches.

Sed unconditional branch
Sed conditional branching

Sed unconditional branch syntax:

$ sed ':label command(s) b label'

:label - Label specification.
commands – any sed commands
label – any name for the label
blabel - jump to label without checking any conditions. If label is not specified, jump to the end of the script.

Sed conditional branch syntax:

$ sed ':label command(s) t label'

:label - Label specification.
commands – any sed commands
label – any name for the label
tlabel - Jump to label only if the last substitution command modified the pattern space. If label is not specified, jump to the end of the script.

Creating a sample test file

Let us first create the jiyik_sed.txt file which will be used in the examples mentioned below .

jiyik_sed.txt

Linux
        Administration
        Scripting
                Tips and Tricks
Windows
        Administration
Database
        Administration of Oracle
        Administration of Mysql
Security
        Network
                 Online\
        Security
Productivity
        Google Search\
        Tips
        "Web Based Time Tracking,
        Web Based Todo list and
        Reduce Key Stores etc"

Check the file contents as follows

sed Example File

Sed Example of Unconditional Branching

Sed Example 1. Replace the first occurrence of a pattern in the entire file

In the file jiyik_sed.txt, replace the first occurrence of “ Administration ” with “ Supervision ”.

$ sed '/Administration/{
 s/Administration/Supervision/
 :loop
 n
 b loop
 }' jiyik_sed.txt

Let’s look at the results

sed replaces the first occurrence of a pattern in the entire file

In the above sed command, it simply reads and prints the pattern space line by line until it matches Administration .
Once Administration is matched , replace Administration with Supervision (only appears once, note there is no "g" flag replacement).
Once the first match is replaced, just read the remaining file contents and print them.
- nis a sed command that prints the pattern space and overwrites it with the next line.
- Use loopas label. nPrint the current line and overwrite the pattern space with the next line. bThe loop jumps to again :loop. So this loop prints the rest of jiyik_sed.txt .

`“”`sed Example 2. Delete the data between the double quotes of the pattern in the entire file

In our example file, “”there are three lines between.

$ sed -e ':loop
$!{
N
/\n$/!b loop
}
s/"[^"]*"//g' jiyik_sed.txt

See the following running results

sed deletes the data between double quotes in the entire file

The above command keep appends all the lines of the file until the end of the file.
- $! – if it is not end of file.
- N - Append the next line, with the pattern space \nseparated by
- /\n$/!b loop - If this is not the last line of the file, jump to the loop again.
Now all the lines will be available in the pattern space separated by newline characters. Replace “”all the occurrences of data in between with nothing.

Sed Example 3. Remove HTML tags from a file

For example, I have a file with the following html content

index.html

<html><body>
<table
border=2><tr><td valign=top
align=right>1.</td>
<td>Line 1 Column 2</
td>
</table>
</body></html>

The following sed command removes all html tags from a given file

$ sed '/</{
:loop
s/<[^<]*>//g
/</{
N
b loop
}
}' index.html

See the following running results

sed removes HTML tags from files

Each time a line containing '<' is found, all HTML tags in that line are first deleted.
If the pattern space now contains "<", this means a multi-line tag. Now repeat the following loop:

Join next line
Remove all HTML tags until there is no single "<"

When "<" is not present in the pattern space, we print it out and start a new loop.

Sed Example of Conditional Branching

Sed Example 4. If a line ends with a backslash, append the next line to it

Our sample file has two lines ending with a backslash, and now we have to append the next line to it.

$ sed '
:loop
/\\$/N
s/\\\n */ /
t loop' jiyik_sed.txt

See the following running results

sed if a line ends with a backslash then append the next line to it

Checks if the line (/\\$/)ends with a backslash , and if so, reads the next line and appends it to the pattern space, \\replacing the at the end of the line and the number of spaces that follow with a single space.
If the replacement is successful, repeat the above steps. The branch will only be executed if the replacement is successful.
Conditional branches are mainly used in recursive patterns.

Sed Example 5. comify a numeric string

$ sed '
 :loop
 s/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/
 t loop'
12342342342343434
12,342,342,342,343,434

See the results below

sed comify a string of numbers

Divide the numbers into two groups.
The first group is all the numbers up to the last three digits. The last three digits are captured in the second group.
The two matching groups are then separated by a comma. The same rule is then applied to the line again and again until all the numbers have been grouped into groups of three.
For example, in the first iteration it will be12342342342343,434
In the next iteration 12342342342,343,434, until there are fewer than three digits.

Sed Example 6. Formatting: Replace each leading space in a line with "+"

$ sed '
s/^ */&\n/
:loop
s/^\n//;s/ \n/\n+/
t loop' jiyik_sed.txt

See the following running results

sed replaces each leading space on a line with a plus sign

Separate all leading spaces and other characters of the line with a newline character.
Now replace spaces and newlines with +and . So, from right to left, spaces will be replaced with +and newlines will be shifted one character to the left.
Finally at the beginning of the line \nwill be there, so delete that new line.

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