Extract substring in Bash
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.
Solution 1:
You can use Parameter Expansion to do this.
If a 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=${tmp%_*} # remove suffix starting with "_"
If there are other underscores, it's probably feasible anyway, albeit more tricky. If anyone knows how to perform both expansions in a single expression, I'd like to know too.
Both solutions presented are pure bash, with no process spawning involved, hence very fast.
Solution 2:
Use cut:
echo 'someletters_12345_moreleters.ext' | cut -d'_' -f 2
More generic:
INPUT='someletters_12345_moreleters.ext'
SUBSTRING=$(echo $INPUT| cut -d'_' -f 2)
echo $SUBSTRING
Solution 3:
Generic solution where the number can be anywhere in the filename, using the first of such sequences:
number=$(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:]]')
Solution 4:
just try to use cut -c startIndx-stopIndx
Solution 5:
Here's how i'd do it:
FN=someletters_12345_moreleters.ext
[[ ${FN} =~ _([[:digit:]]{5})_ ]] && NUM=${BASH_REMATCH[1]}
Explanation:
Bash-specific:
-
[[ ]]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.