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I'm debugging a shell script (which I didn't write) that contains this loop:

read line < "$pid_file"
for p in $line ; do
  [ -z "${p//[0-9]/}" -a -d "/proc/$p" ] && pid="$pid $p"
done

Can someone point me to documentation on using (what I'm assuming) is a regular expression within a string variable construct, i.e. the "${p//[0-9]/}". This topic isn't easy to narrow down in Google.

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Try running that after cd /proc with a $pid_file that contains * 1\ 2=2 3 –  Stéphane Chazelas Jan 21 '14 at 7:35

1 Answer 1

up vote 1 down vote accepted

This is a form of parameter expansion (i.e. variable expansion) with a text transformation of the value of the variable. The value of the variable p undergoes the replacement of the pattern [0-9] to the empty string wherever it occurs — in other words, "${p//[0-9]/}" is the value of p without its digit characters.

In the bash documentation, you'll find it under ${parameter/pattern/string}. This form replaces the first occurrence of the specified pattern with the specified string. If the first slash is doubled, all occurrences are replaced. The pattern is a glob, i.e. the same wildcard patterns as in filename matching.

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Perfect, that link is exactly what I needed. THANK YOU! –  Alan Jan 21 '14 at 0:04
    
So basically, they're testing that p is numeric, and that its /proc/ directory exists. Correct? –  Alan Jan 21 '14 at 0:09
    
@Alan Yes, that's it. –  Gilles Jan 21 '14 at 0:10

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