In this article, 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.
Original article here, from The UNIX School.
