What does this expr do in a shell script?

Question

What does the following do exactly?

newProg=`expr "${newProg}" : ".* -> \(.*\)$"`   
if expr "x${newProg}" : 'x/' >/dev/null; then    
   prog="${newProg}"     
else    
    progdir=`dirname "${prog}"`   
    prog="${progdir}/${newProg}"    
fi   

Answer 1

The first line uses expr to:

• Match the value of $newProg against the basic regular expression .* -> \(.*\)$ (anchored on both ends). That's anything with -> in it.
• Set newProg to the part of the original value matched by the capturing subexpression written in parentheses (the part after ->).

The greedy matching semantics are such that this always captures and outputs the last such match, so there is never a -> in the output.

This would, for example, parse the output of ls -l on a symlink to find the target (given well-behaved filenames, etc, usual caveats).

The second line uses expr again to check whether the new value of newProg begins with a /, i.e., it represents an absolute path.

In the case where it does, prog is just set to that value. Otherwise, prog is set to a computed relative path using dirname on $prog, a variable not defined in this snippet. The effect will presumably be to construct a path relative to that original location.

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If you are only targeting a more capable shell or environment than pure POSIX, there is likely to be a more reliable way of doing this than expr. readlink is a very commonly-available command also, which would save on parsing ls.