Question

I am a shell script newbie. I want to know the difference between

${var%pattern}

and

${var%%pattern}

Thanks

Answer 1

From man bash:

${parameter%word}
${parameter%%word}

The word is expanded to produce a pattern just as in pathname expansion. If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the “%” case) or the longest matching pattern (the “%%” case) deleted.

Here’s an example of what the difference is:

$ VAR=abcdefabcdef
$ echo ${VAR%def*}
abcdefabc
$ echo ${VAR%%def*}
abc

Notice that there are two possible matches for def* at the end of $VAR: both “defabcdef” and just “def” match. With the “%” the shortest possible match for the pattern def* is deleted, so the trailing “def” is removed. With the “%%” the longest possible match is deleted, so “defabcdef” bites the dust.