What linux shell command returns a part of a string?

Question

I want to find a linux command that can return a part of the string. In most programming languages, it's the substr() function. Does bash have any command that can be used for this purpose. I want to be able to do something like this... substr "abcdefg" 2 3 - prints cde.

Subsequent similar question:

Extract substring in Bash

Not really a duplicate. [Extract substring in Bash](https://stackoverflow.com/q/428109) asks for cutting out a sequence of characters _surrounded by a certain delimiter_. This question asks for cutting out a piece of a string giving numerical values for offset and length like `substr()` does. This is not the same. — Adrian W, Oct 24 '18 at 10:34

score 204 · Accepted Answer · edited Aug 28 '15 at 10:58

204

If you are looking for a shell utility to do something like that, you can use the cut command.

To take your example, try:

echo "abcdefg" | cut -c3-5

which yields

cde

Where -cN-M tells the cut command to return columns N to M, inclusive.

edited Aug 28 '15 at 10:58

Willi Mentzel

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answered Oct 20 '08 at 18:39

Toybuilder

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Even though I have the "accepted" answer, I would like to point out that if you want to do a lot of sub-string extraction, using the built-in substring expansion (see dmckee's answer) is the more efficient way to go. That said, cut is easier to remember and use. – Toybuilder Nov 14 '11 at 17:44
what do we need to do if we want to start from 3rd char till end of the string ie: "abcdef" we need cdef then echo "abcdef" | cut -c3?" – user1731553 Apr 05 '16 at 05:46
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`cut` has some useful arguments you can use. `-f` changes it from counting characters to counting fields, delimited by TAB by default or by any character you specify following `-d`. So to get your input string up until but not including the first slash (exactly what I wanted), you can do: `cut -d/ -f-1`, which can be read as "cut on substrings delimited by /, only return the first one". – ArtOfWarfare Sep 11 '17 at 14:31
Consider to indicate quickly: what does `-c` do? – Manuel Jordan Dec 20 '21 at 13:51
@ArtOfWarfare huge thanks for your comment - it explains the following answer: https://stackoverflow.com/a/428118/3665178 – Manuel Jordan Dec 20 '21 at 13:52

dmckee --- ex-moderator kitten · Answer 2 · 2017-02-28T15:12:14.417

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From the bash manpage:

${parameter:offset}
${parameter:offset:length}
        Substring  Expansion.   Expands  to  up  to length characters of
        parameter starting at the character  specified  by  offset.
[...]

Or, if you are not sure of having bash, consider using cut.

edited Feb 28 '17 at 15:12

answered Oct 20 '08 at 18:35

dmckee --- ex-moderator kitten

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interesting, I did not know about this. For more flexibile substring options: man cut – Evan Teran Oct 20 '08 at 18:37
Shell extensions are nice, but... meh. – Clinton Pierce Oct 20 '08 at 20:03
I mostly agree. I usually write shell scripts in vanilla /bin/sh. But I find that I have to know some bashisms to _read_ shell scripts... – dmckee --- ex-moderator kitten Oct 20 '08 at 20:06
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Is there something like `${str:3:-3}` to extract a substring from char 3 to last char minus -3? – a06e Feb 28 '17 at 15:10
@becko My current bash manpage (MacOS 10.12, so the BSD lineage, I think) has considerably more than the quote above including `If offset evaluates to a number less than zero, the value is used as an offset from the end of the value of parameter.` Meaning that you would want `${str:-3:3}`. – dmckee --- ex-moderator kitten Feb 28 '17 at 15:12
It doesn't work: `str="abcdefghijklmn"; echo ${str:-2:2}` returns `abcdefghijklmn` – a06e Feb 28 '17 at 15:33
1

Oh, but `echo ${str:2:-2}` does what I want. – a06e Feb 28 '17 at 15:34
@becko I think we're tripping up on "evaluates to". Neither of your examples work for me, but `str="abcdefghijklmn"; echo ${str:(-2):2}` returns `mn`. That kind of thing is why I gave up trying to understand what the heck the shell is doing. – dmckee --- ex-moderator kitten Feb 28 '17 at 15:37
@dmckee I'm on `zsh`. – a06e Feb 28 '17 at 16:50

score 35 · Answer 3 · answered Nov 12 '11 at 21:16

In "pure" bash you have many tools for (sub)string manipulation, mainly, but not exclusively in parameter expansion :

${parameter//substring/replacement}
${parameter##remove_matching_prefix}
${parameter%%remove_matching_suffix}

Indexed substring expansion (special behaviours with negative offsets, and, in newer Bashes, negative lengths):

${parameter:offset}
${parameter:offset:length}
${parameter:offset:length}

And of course, the much useful expansions that operate on whether the parameter is null:

${parameter:+use this if param is NOT null}
${parameter:-use this if param is null}
${parameter:=use this and assign to param if param is null}
${parameter:?show this error if param is null}

They have more tweakable behaviours than those listed, and as I said, there are other ways to manipulate strings (a common one being $(command substitution) combined with sed or any other external filter). But, they are so easily found by typing man bash that I don't feel it merits to further extend this post.

Just remember, that 'indexed substring expansion' is not part of the [POSIX standard](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06). So while it is probably faster on bash than the alternatives, it might get in your way when you want to run your script on different operating systems. — JepZ, Dec 31 '18 at 10:59

Juanma · Answer 4 · 2008-10-20T18:42:06.700

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In bash you can try this:

stringZ=abcABC123ABCabc
#       0123456789.....
#       0-based indexing.

echo ${stringZ:0:2} # prints ab

More samples in The Linux Documentation Project

edited Oct 20 '08 at 18:42

answered Oct 20 '08 at 18:36

Juanma

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Is there a syntax to replace the number 2 with infinity ? – ToiletGuy Apr 14 '21 at 14:56
@ToiletGuy Yes, just discard it. `echo ${stringZ:9}` prints ABCabc – hdrz Jul 13 '21 at 09:34

score 16 · Answer 5 · answered Oct 20 '08 at 18:37

16

${string:position:length}

answered Oct 20 '08 at 18:37

Bill the Lizard

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As usual, sitting down here with no upvotes, WTF. This worked so well for me in my parse_git_branch command. Naice! – Dan Rosenstark Oct 26 '18 at 20:37
Can the length become the infinity ? – ToiletGuy Apr 14 '21 at 14:57

score 15 · Answer 6 · edited Mar 20 '16 at 12:06

15

expr(1) has a substr subcommand:

expr substr <string> <start-index> <length>

This may be useful if you don't have bash (perhaps embedded Linux) and you don't want the extra "echo" process you need to use cut(1).

edited Mar 20 '16 at 12:06

fiatjaf

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answered Oct 21 '08 at 01:05

camh

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Heredocs work without bash (`< – Charles Duffy Dec 04 '18 at 22:12

What linux shell command returns a part of a string?

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