In this article of sed series, we will see the examples of how to remove or delete characters from a file. The syntax of sed command replacement is:

$ sed 's/find/replace/' file

This sed command finds the pattern and replaces with another pattern. When the replace is left empty, the pattern/element found gets deleted.

Let us consider a sample file as below:

$ cat file

Linux

Solaris

Ubuntu

Fedora

RedHat

1. To remove a specific character, say 'a'

$ sed 's/a//' file

Linux

Solris

Ubuntu

Fedor

RedHt

This will remove the first occurence of 'a' in every line of the file. To remove all occurences of 'a' in every line,

$ sed 's/a//g' file

2. To remove 1st character in every line:

$ sed 's/^.//' file

inux

olaris

buntu

edora

edHat

.(dot) tries to match a single character. The ^ tries to match a pattern(any character) in the beginning of the line. Another way to write the same:

$ sed 's/.//' file

This tells to replace a character with nothing. Since by default, sed starts from beginning, it replaces only the 1st character since 'g' is not passed.

3. To remove last character of every line :

$ sed 's/.$//' file

Linu

Solari

Ubunt

Fedor

RedHa

The $ tries to match a pattern in the end of the line.

4. To remove the 1st and last character of every line in the same command:

$ sed 's/.//;s/.$//' file

inu

olari

bunt

edor

edHa

Two commands can be given together with a semi-colon separated in between.

5. To remove first character only if it is a specific character:

$ sed 's/^F//' file

Linux

Solaris

Ubuntu

edora

RedHat

This removes the 1st character only if it is 'F'.

6. To remove last character only if it is a specific character:

$ sed 's/x$//' file

Linu

Solaris

Ubuntu

Fedora

RedHat

This removed the last character only if it s 'x'.

7. To remove 1st 3 characters of every line:

$ sed 's/...//' file

ux

aris

ntu

ora

Hat

A single dot(.) removes 1st character, 3 dots remove 1st three characters.

8. To remove 1st n characters of every line:

$ sed -r 's/.{4}//' file

x

ris

tu

ra

at

.{n} -> matches any character n times, and hence the above expression matches 4 characters and deletes it.

9. To remove last n characters of every line:

$ sed -r 's/.{3}$//' file

Li

Sola

Ubu

Fed

Red

10. To remove everything except the 1st n characters in every line:

$ sed -r 's/(.{3}).*/\1/' file

Lin

Sol

Ubu

Fed

Red

.* -> matches any number of characters, and the first 3 characters matched are grouped using parantheses. In the replacement, by having \1 only the group is retained, leaving out the remaining part.

11. To remove everything except the last n characters in a file:

$ sed -r 's/.*(.{3})/\1/' file

nux

ris

ntu

ora

Hat

Same as last example, except that from the end.

12. To remove multiple characters present in a file:

$ sed 's/[aoe]//g' file

Linux

Slris

Ubuntu

Fdr

RdHt

To delete multiple characters, [] is used by specifying the characters to be removed. This will remove all occurences of the characters a, o and e.

13. To remove a pattern :

$ sed 's/lari//g' file

Linux

Sos

Ubuntu

Fedora

RedHat

Not just a character, even a pattern can be removed. Here, 'lari' got removed from 'Solaris'.

14. To delete only nth occurrence of a character in every line:

$ sed 's/u//2' file

Linux

Solaris

Ubunt

Fedora

RedHat

By default, sed performs an activity only on the 1st occurence. If n is specifed, sed performs only on the nth occurence of the pattern. The 2nd 'u' of 'Ubuntu' got deleted.

15. To delete everything in a line followed by a character:

$ sed 's/a.*//' file

Linux

Sol

Ubuntu

Fedor

RedH

16. To remove all digits present in every line of a file:

$ sed 's/[0-9]//g' file

[0-9] stands for all characters between 0 to 9 meaning all digits, and hence all digits get removed.

17. To remove all lower case alphabets present in every line:

$ sed 's/[a-z]//g' file

L

S

U

F

RH

[a-z] represents lower case alphabets range and hence all lower-case characters get removed.

18. To remove everything other than the lower case alphabets:

$ sed 's/[^a-z]//g' file

inux

olaris

buntu

edora

edat

^ inside square brackets negates the condition. Here, all characters except lower case alphabets get removed.

19. To remove all alpha-numeric characters present in every line:

$ sed 's/[a-zA-Z0-9]//g' file

All alpha-numeric characters get removed.

20. To remove a character irrespective of the case:

$ sed 's/[uU]//g' file

Linx

Solaris

bnt

Fedora

RedHat

By specifying both the lower and upper case character in brackets is equivalent to removing a character irrespective of the case.