Sed Command

Sed is a Stream Editor used for modifying the files in unix (or linux). Whenever you want to make changes to the file automatically, sed comes in handy to do this. Most people never learn its power; they just simply use sed to replace text. You can do many things apart from replacing text with sed. 

Consider the below text file as an input.

>cat file.txt
unix is great os. unix is opensource. unix is free os.
learn operating system.
unixlinux which one you choose.

Sed Command Examples

1. Replacing or substituting string

Sed command is mostly used to replace the text in a file. The below simple sed command replaces the word "unix" with "linux" in the file.

>sed 's/unix/linux/' file.txt
linux is great os. unix is opensource. unix is free os.
learn operating system.
linuxlinux which one you choose.

Here the "s" specifies the substitution operation. The "/" are delimiters. The "unix" is the search pattern and the "linux" is the replacement string.

By default, the sed command replaces the first occurrence of the pattern in each line and it won't replace the second, third...occurrence in the line.

2. Replacing the nth occurrence of a pattern in a line.

Use the /1, /2 etc flags to replace the first, second occurrence of a pattern in a line. The below command replaces the second occurrence of the word "unix" with "linux" in a line.

>sed 's/unix/linux/2' file.txt
unix is great os. linux is opensource. unix is free os.
learn operating system.
unixlinux which one you choose.

3. Replacing all the occurrence of the pattern in a line.

The substitute flag /g (global replacement) specifies the sed command to replace all the occurrences of the string in the line.

>sed 's/unix/linux/g' file.txt
linux is great os. linux is opensource. linux is free os.
learn operating system.
linuxlinux which one you choose.

4. Replacing from nth occurrence to all occurrences in a line.

Use the combination of /1, /2 etc and /g to replace all the patterns from the nth occurrence of a pattern in a line. The following sed command replaces the third, fourth, fifth... "unix" word with "linux" word in a line.

>sed 's/unix/linux/3g' file.txt
unix is great os. unix is opensource. linux is free os.
learn operating system.
unixlinux which one you choose.

Replacing string on a specific line number.

You can restrict the sed command to replace the string on a specific line number. An example is

>sed '3 s/unix/linux/' file.txt
unix is great os. unix is opensource. unix is free os.
learn operating system.
linuxlinux which one you choose.

Replacing string on a range of lines.

You can specify a range of line numbers to the sed command for replacing a string.

>sed '1,3 s/unix/linux/' file.txt
linux is great os. unix is opensource. unix is free os.
learn operating system.
linuxlinux which one you choose.

Here the sed command replaces the lines with range from 1 to 3. 

Deleting lines.

You can delete the lines a file by specifying the line number or a range or numbers.

>sed '2 d' file.txt
>sed '5,$ d' file.txt

courtesy:folkstalk