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About string : Extract-substring-in-Bash

Question Detail

Given a filename in the form someletters_12345_moreleters.ext, I want to extract the 5 digits and put them into a variable.

So to emphasize the point, I have a filename with x number of characters then a five digit sequence surrounded by a single underscore on either side then another set of x number of characters. I want to take the 5 digit number and put that into a variable.

I am very interested in the number of different ways that this can be accomplished.

Question Answer

You can use Parameter Expansion to do this.

If a is constant, the following parameter expansion performs substring extraction:


where 12 is the offset (zero-based) and 5 is the length

If the underscores around the digits are the only ones in the input, you can strip off the prefix and suffix (respectively) in two steps:

tmp=${a#*_}   # remove prefix ending in "_"
b=${tmp%_*}   # remove suffix starting with "_"

If there are other underscores, it’s probably feasible anyway, albeit more tricky. If anyone knows how to perform both expansions in a single expression, I’d like to know too.

Both solutions presented are pure bash, with no process spawning involved, hence very fast.

Use cut:

echo 'someletters_12345_moreleters.ext' | cut -d'_' -f 2

More generic:

SUBSTRING=$(echo $INPUT| cut -d'_' -f 2)

just try to use cut -c startIndx-stopIndx

Generic solution where the number can be anywhere in the filename, using the first of such sequences:

number=$(echo $filename | egrep -o '[[:digit:]]{5}' | head -n1)

Another solution to extract exactly a part of a variable:


If your filename always have the format stuff_digits_... you can use awk:

number=$(echo $filename | awk -F _ '{ print $2 }')

Yet another solution to remove everything except digits, use

number=$(echo $filename | tr -cd '[[:digit:]]')

Here’s how i’d do it:

[[ ${FN} =~ _([[:digit:]]{5})_ ]] && NUM=${BASH_REMATCH[1]}



  • [[ ]] indicates a conditional expression
  • =~ indicates the condition is a regular expression
  • && chains the commands if the prior command was successful

Regular Expressions (RE): _([[:digit:]]{5})_

  • _ are literals to demarcate/anchor matching boundaries for the string being matched
  • () create a capture group
  • [[:digit:]] is a character class, i think it speaks for itself
  • {5} means exactly five of the prior character, class (as in this example), or group must match

In english, you can think of it behaving like this: the FN string is iterated character by character until we see an _ at which point the capture group is opened and we attempt to match five digits. If that matching is successful to this point, the capture group saves the five digits traversed. If the next character is an _, the condition is successful, the capture group is made available in BASH_REMATCH, and the next NUM= statement can execute. If any part of the matching fails, saved details are disposed of and character by character processing continues after the _. e.g. if FN where _1 _12 _123 _1234 _12345_, there would be four false starts before it found a match.

In case someone wants more rigorous information, you can also search it in man bash like this

$ man bash [press return key]
/substring  [press return key]
[press "n" key]
[press "n" key]
[press "n" key]
[press "n" key]


              Substring Expansion.  Expands to  up  to  length  characters  of
              parameter  starting  at  the  character specified by offset.  If
              length is omitted, expands to the substring of parameter  start‐
              ing at the character specified by offset.  length and offset are
              arithmetic expressions (see ARITHMETIC  EVALUATION  below).   If
              offset  evaluates  to a number less than zero, the value is used
              as an offset from the end of the value of parameter.  Arithmetic
              expressions  starting  with  a - must be separated by whitespace
              from the preceding : to be distinguished from  the  Use  Default
              Values  expansion.   If  length  evaluates to a number less than
              zero, and parameter is not @ and not an indexed  or  associative
              array,  it is interpreted as an offset from the end of the value
              of parameter rather than a number of characters, and the  expan‐
              sion is the characters between the two offsets.  If parameter is
              @, the result is length positional parameters beginning at  off‐
              set.   If parameter is an indexed array name subscripted by @ or
              *, the result is the length members of the array beginning  with
              ${parameter[offset]}.   A  negative  offset is taken relative to
              one greater than the maximum index of the specified array.  Sub‐
              string  expansion applied to an associative array produces unde‐
              fined results.  Note that a negative offset  must  be  separated
              from  the  colon  by  at least one space to avoid being confused
              with the :- expansion.  Substring indexing is zero-based  unless
              the  positional  parameters are used, in which case the indexing
              starts at 1 by default.  If offset  is  0,  and  the  positional
              parameters are used, $0 is prefixed to the list.

I’m surprised this pure bash solution didn’t come up:

set $a
echo $2
# prints 12345

You probably want to reset IFS to what value it was before, or unset IFS afterwards!

Building on jor’s answer (which doesn’t work for me):

substring=$(expr "$filename" : '.*_\([^_]*\)_.*')

Following the requirements

I have a filename with x number of characters then a five digit
sequence surrounded by a single underscore on either side then another
set of x number of characters. I want to take the 5 digit number and
put that into a variable.

I found some grep ways that may be useful:

$ echo "someletters_12345_moreleters.ext" | grep -Eo "[[:digit:]]+" 

or better

$ echo "someletters_12345_moreleters.ext" | grep -Eo "[[:digit:]]{5}" 

And then with -Po syntax:

$ echo "someletters_12345_moreleters.ext" | grep -Po '(?<=_)\d+' 

Or if you want to make it fit exactly 5 characters:

$ echo "someletters_12345_moreleters.ext" | grep -Po '(?<=_)\d{5}' 

Finally, to make it be stored in a variable it is just need to use the var=$(command) syntax.

If we focus in the concept of:
“A run of (one or several) digits”

We could use several external tools to extract the numbers.
We could quite easily erase all other characters, either sed or tr:


echo $name | sed 's/[^0-9]*//g'    # 12345
echo $name | tr -c -d 0-9          # 12345

But if $name contains several runs of numbers, the above will fail:

If “name=someletters_12345_moreleters_323_end.ext”, then:

echo $name | sed 's/[^0-9]*//g'    # 12345323
echo $name | tr -c -d 0-9          # 12345323

We need to use regular expresions (regex).
To select only the first run (12345 not 323) in sed and perl:

echo $name | sed 's/[^0-9]*\([0-9]\{1,\}\).*$/\1/'
perl -e 'my $name='$name';my ($num)=$name=~/(\d+)/;print "$num\n";'

But we could as well do it directly in bash(1) :

regex=[^0-9]*([0-9]{1,}).*$; \
[[ $name =~ $regex ]] && echo ${BASH_REMATCH[1]}

This allows us to extract the FIRST run of digits of any length
surrounded by any other text/characters.

Note: regex=[^0-9]*([0-9]{5,5}).*$; will match only exactly 5 digit runs. 🙂

(1): faster than calling an external tool for each short texts. Not faster than doing all processing inside sed or awk for large files.

Without any sub-processes you can:

shopt -s extglob

A very small variant of this will also work in ksh93.

Here’s a prefix-suffix solution (similar to the solutions given by JB and Darron) that matches the first block of digits and does not depend on the surrounding underscores:

s1="${str#"${str%%[[:digit:]]*}"}"   # strip off non-digit prefix from str
s2="${s1%%[^[:digit:]]*}"            # strip off non-digit suffix from s1
echo "$s2"                           # 12345

My answer will have more control on what you want out of your string. Here is the code on how you can extract 12345 out of your string

echo $str

This will be more efficient if you want to extract something that has any chars like abc or any special characters like _ or -. For example: If your string is like this and you want everything that is after someletters_ and before _moreleters.ext :


With my code you can mention what exactly you want.

#* It will remove the preceding string including the matching key. Here the key we mentioned is _
% It will remove the following string including the matching key. Here the key we mentioned is ‘_more*’

Do some experiments yourself and you would find this interesting.

Given test.txt is a file containing “ABCDEFGHIJKLMNOPQRSTUVWXYZ”

cut -b19-20 test.txt > test1.txt # This will extract chars 19 & 20 "ST" 
while read -r; do;
> x=$REPLY
> done < test1.txt
echo $x

shell cut – print specific range of characters or given part from a string

#method1) using bash

 echo ${str:11:8}

#method2) using cut

 cut -c12-19 <<< $str

#method3) when working with awk

 awk '{time=gensub(/.{11}(.{8}).*/,"\\1","g",$1); print time}' <<< $str

I love sed‘s capability to deal with regex groups:

> var="someletters_12345_moreletters.ext"
> digits=$( echo "$var" | sed "s/.*_\([0-9]\+\).*/\1/p" -n )
> echo $digits

A slightly more general option would be not to assume that you have an underscore _ marking the start of your digits sequence, hence for instance stripping off all non-numbers you get before your sequence: s/[^0-9]\+\([0-9]\+\).*/\1/p.

> man sed | grep s/regexp/replacement -A 2
    Attempt to match regexp against the pattern space.  If successful, replace that portion matched with replacement.  The replacement may contain the special  character  &  to
    refer to that portion of the pattern space which matched, and the special escapes \1 through \9 to refer to the corresponding matching sub-expressions in the regexp.

More on this, in case you’re not too confident with regexps:

  • s is for _s_ubstitute
  • [0-9]+ matches 1+ digits
  • \1 links to the group n.1 of the regex output (group 0 is the whole match, group 1 is the match within parentheses in this case)
  • p flag is for _p_rinting

All escapes \ are there to make sed‘s regexp processing work.

similar to substr(‘abcdefg’, 2-1, 3) in php:

echo 'abcdefg'|tail -c +2|head -c 3

Ok, here goes pure Parameter Substitution with an empty string. Caveat is that I have defined someletters and moreletters as only characters. If they are alphanumeric, this will not work as it is.

echo $substring

A bash solution:

IFS="_" read -r x digs x <<<'someletters_12345_moreleters.ext'

This will clobber a variable called x. The var x could be changed to the var _.

IFS="_" read -r _ digs _ <<<"$input"

May be this could help you to get desired output

Code :

your_number=$(echo "someletters_12345_moreleters.ext" | grep -E -o '[0-9]{5}')
echo $your_number

Output :


There’s also the bash builtin ‘expr’ command:

SUBSTRING=`expr match "$INPUT" '.*_\([[:digit:]]*\)_.*' `  

Inklusive end, similar to JS and Java implementations. Remove +1 if you do not desire this.

function substring() {
    local str="$1" start="${2}" end="${3}"
    if [[ "$start" == "" ]]; then start="0"; fi
    if [[ "$end"   == "" ]]; then end="${#str}"; fi
    local length="((${end}-${start}+1))"
    echo "${str:${start}:${length}}"


    substring 01234 0
    substring 012345 0
    substring 012345 0 0
    substring 012345 1 1
    substring 012345 1 2
    substring 012345 0 1
    substring 012345 0 2
    substring 012345 0 3
    substring 012345 0 4
    substring 012345 0 5

More example calls:

    substring 012345 0
    substring 012345 1
    substring 012345 2
    substring 012345 3
    substring 012345 4
    substring 012345 5
    substring 012345 6
    substring 012345 3 5
    substring 012345 3 4
    substring 012345 2 4
    substring 012345 1 3

A little late, but I just ran across this problem and found the following:

host:/tmp$ asd=someletters_12345_moreleters.ext 
host:/tmp$ echo `expr $asd : '.*_\(.*\)_'`

I used it to get millisecond resolution on an embedded system that does not have %N for date:

set `grep "now at" /proc/timer_list`
fraction=`expr $nano : '.*\(...\)......'`
$debug nano is $nano, fraction is $fraction

Here is a substring.sh file


`substring.sh $TEXT 2 3` # characters 2-3

`substring.sh $TEXT 2` # characters 2 and after 

substring.sh follows this line

#echo "starting substring"

if [[ -z $end ]]; then
  end=`echo "$chars " | wc -c`
#echo "length is " $e
a=`echo $chars | sed  's/\(.\)/\1 /g'`
#echo "a is " $a
for c in $a
  #echo "substring" $i $e $c
  if [[ i -lt $start ]]; then
    : # DO Nothing
  elif [[ i -gt $end ]]; then
#echo substring returning $o
echo $o

An easy way to use sed replace:

result=$(echo "someletters_12345_moreleters.ext" | sed 's/.*_\(.*\)_.*/\1/g')
echo $result

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