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Remove a fixed prefix/suffix from a string in Bash


Question

In my bash script I have a string and its prefix/suffix. I need to remove the prefix/suffix from the original string.

For example, let's say I have the following values:

string="hello-world"
prefix="hell"
suffix="ld"

How do I get to the following result?

result="o-wor"
2017/01/26
1
507
1/26/2017 11:35:41 AM

Accepted Answer

$ foo=${string#"$prefix"}
$ foo=${foo%"$suffix"}
$ echo "${foo}"
o-wor
2018/07/26
753
7/26/2018 7:18:13 AM

Using sed:

$ echo "$string" | sed -e "s/^$prefix//" -e "s/$suffix$//"
o-wor

Within the sed command, the ^ character matches text beginning with $prefix, and the trailing $ matches text ending with $suffix.

Adrian Frühwirth makes some good points in the comments below, but sed for this purpose can be very useful. The fact that the contents of $prefix and $suffix are interpreted by sed can be either good OR bad- as long as you pay attention, you should be fine. The beauty is, you can do something like this:

$ prefix='^.*ll'
$ suffix='ld$'
$ echo "$string" | sed -e "s/^$prefix//" -e "s/$suffix$//"
o-wor

which may be what you want, and is both fancier and more powerful than bash variable substitution. If you remember that with great power comes great responsibility (as Spiderman says), you should be fine.

A quick introduction to sed can be found at http://evc-cit.info/cit052/sed_tutorial.html

A note regarding the shell and its use of strings:

For the particular example given, the following would work as well:

$ echo $string | sed -e s/^$prefix// -e s/$suffix$//

...but only because:

  1. echo doesn't care how many strings are in its argument list, and
  2. There are no spaces in $prefix and $suffix

It's generally good practice to quote a string on the command line because even if it contains spaces it will be presented to the command as a single argument. We quote $prefix and $suffix for the same reason: each edit command to sed will be passed as one string. We use double quotes because they allow for variable interpolation; had we used single quotes the sed command would have gotten a literal $prefix and $suffix which is certainly not what we wanted.

Notice, too, my use of single quotes when setting the variables prefix and suffix. We certainly don't want anything in the strings to be interpreted, so we single quote them so no interpolation takes place. Again, it may not be necessary in this example but it's a very good habit to get into.

2016/08/09

Do you know the length of your prefix and suffix? In your case:

result=$(echo $string | cut -c5- | rev | cut -c3- | rev)

Or more general:

result=$(echo $string | cut -c$((${#prefix}+1))- | rev | cut -c$((${#suffix}+1))- | rev)

But the solution from Adrian Frühwirth is way cool! I didn't know about that!

2018/01/14

I use grep for removing prefixes from paths (which aren't handled well by sed):

echo "$input" | grep -oP "^$prefix\K.*"

\K removes from the match all the characters before it.


$ string="hello-world"
$ prefix="hell"
$ suffix="ld"

$ #remove "hell" from "hello-world" if "hell" is found at the beginning.
$ prefix_removed_string=${string/#$prefix}

$ #remove "ld" from "o-world" if "ld" is found at the end.
$ suffix_removed_String=${prefix_removed_string/%$suffix}
$ echo $suffix_removed_String
o-wor

Notes:

#$prefix : adding # makes sure that substring "hell" is removed only if it is found in beginning. %$suffix : adding % makes sure that substring "ld" is removed only if it is found in end.

Without these, the substrings "hell" and "ld" will get removed everywhere, even it is found in the middle.

2018/09/08

Using the =~ operator:

$ string="hello-world"
$ prefix="hell"
$ suffix="ld"
$ [[ "$string" =~ ^$prefix(.*)$suffix$ ]] && echo "${BASH_REMATCH[1]}"
o-wor