pattern matching in Bash

Question

Hi,

Here is an example to get different parts of a filename

bash-3.2$ pathandfile=/tmp/ff.txt  
bash-3.2$ filename=$(basename $pathandfile)  
bash-3.2$ echo $filename    
ff.txt    
bash-3.2$ echo ${filename##*.}     
txt    
bash-3.2$ echo ${filename%.*}    
ff    

I was wondering what does ## and % mean in the patterns. How is the patten matching working?

Thanks and regards!

Answer 1

See http://tldp.org/LDP/abs/html/string-manipulation.html.

• ${string##substring}
  • Deletes longest match of $substring from front of $string.
• ${string%substring}
  • Deletes shortest match of $substring from back of $string.

Answer 2

From http://tldp.org/LDP/abs/html/string-manipulation.html:

${string##substring}

Deletes longest match of $substring from front of $string.

and

${string%substring}

Deletes shortest match of $substring from back of $string.

Answer 3

The manpage for bash says:

${parameter#word}
${parameter##word}

Remove matching prefix pattern. The word is expanded to produce a pattern just as in pathname expansion. If the pattern matches the beginning of the 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. If parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

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

Remove matching suffix pattern. 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. If parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.