Stupid Bash Expansion Trick

I got asked a question regarding filename expansion in bash the other day, and was stumped. It turns out to be something I should have considered a long time ago, and will always keep in mind when writing a script.

Question 1:

What does the following script do if there is a file abc in the current directory?


#!/bin/bash
for i in a*
do
  echo $i
done

Answer:

This a* matches abc and expands to abc, and the script outputs:
abc

Question 2:

What if you run the same script in a directory without any files?

Answer:

The script outputs:

a*

Why?

According to The Bash Reference Manual:

Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found, and the shell option nullglob is disabled, the word is left unchanged.

So bash will output ‘a*’, because that is how filename expansion works.

Question 3:

What if you run the following script and in a directory with no filename beginning with a:


#!/bin/bash
for i in a*
do
  echo /usr/bin/$i
done

Answer:

The script outputs:

/usr/bin/a2p /usr/bin/a2p5.10.0 /usr/bin/a2p5.8.9 /usr/bin/aaf_install /usr/bin/aclocal /usr/bin/aclocal-1.10 /usr/bin/addftinfo /usr/bin/afconvert /usr/bin/afinfo /usr/bin/afmtodit /usr/bin/afplay /usr/bin/afscexpand /usr/bin/agvtool /usr/bin/alias /usr/bin/allmemory /usr/bin/amavisd /usr/bin/amavisd-agent /usr/bin/amavisd-nanny /usr/bin/amavisd-release /usr/bin/amlint /usr/bin/ant /usr/bin/applesingle /usr/bin/appletviewer /usr/bin/apply /usr/bin/apr-1-config /usr/bin/apropos /usr/bin/apt /usr/bin/apu-1-config /usr/bin/ar /usr/bin/arch /usr/bin/as /usr/bin/asa /usr/bin/at /usr/bin/atos /usr/bin/atq /usr/bin/atrm /usr/bin/atsutil /usr/bin/autoconf /usr/bin/autoheader /usr/bin/autom4te /usr/bin/automake /usr/bin/automake-1.10 /usr/bin/automator /usr/bin/autoreconf /usr/bin/autoscan /usr/bin/autospec /usr/bin/autoupdate /usr/bin/auval /usr/bin/auvaltool /usr/bin/awk

Why?

Because you’re re-evaluating ‘/usr/bin/$i’ which is now ‘/usr/bin/a*’, which expands to the order list above due to shell filename expansion rules. If you want to avoid this you need to protect your variables using quotes. Here is the safe version of the script:


#!/bin/bash
for i in a*
do
  echo /usr/bin/"$i"
done

Just something simple to think about when writing your bash scripts. Expect to enter loops on globs that don't match anything, always protect your variables, and consider setting the failglob option in your scripts.