Extract substring in Bash
If x is constant, the following parameter expansion performs substring extraction:
b=${a:12:5}
where 12 is the offset (zero-based) and 5 is the length
If the underscores around the digits are the only ones in the input, you can strip off the prefix and suffix (respectively) in two steps:
tmp=${a#*_} # remove prefix ending in "_"
b=${tmp%_*} # remove suffix starting with "_"
If there are other underscores, it's probably feasible anyway, albeit more tricky. If anyone knows how to perform both expansions in a single expression, I'd like to know too.
Both solutions presented are pure bash, with no process spawning involved, hence very fast.
Use cut:
echo 'someletters_12345_moreleters.ext' | cut -d'_' -f 2
More generic:
INPUT='someletters_12345_moreleters.ext'
SUBSTRING=$(echo $INPUT| cut -d'_' -f 2)
echo $SUBSTRING
Generic solution where the number can be anywhere in the filename, using the first of such sequences:
number=$(echo $filename | egrep -o '[[:digit:]]{5}' | head -n1)
Another solution to extract exactly a part of a variable:
number=${filename:offset:length}
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:]]')