Extract substring in Bash

string bash shell substring
Berek Bryan picture Berek Bryan · Jan 9, 2009 · Viewed 1.3M times · Source

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.

Answer

JB. picture JB. · Jan 9, 2009

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.

Related questions

How to check if a string contains a substring in Bash

I have a string in Bash: string="My string" How can I test if it contains another string? if [ $string ?? 'foo' ]; then echo "It's there!" fi Where ?? is my unknown operator. Do I use echo and grep? if echo "$string" | …

Read More
How to convert a string to lower case in Bash?

Is there a way in bash to convert a string into a lower case string? For example, if I have: a="Hi all" I want to convert it to: "hi all"

Read More
Multi-line string with extra space (preserved indentation)

I want to write some pre-defined texts to a file with the following: text="this is line one\n this is line two\n this is line three" echo -e $text > filename I'm expecting something like this: this is …

Read More