Bash check array for existing directories

Question

Why does an empty variable test true if I check for existing directory?

I have two array's with a couple of dirs to process, which I loop / check like this:

for n in {1..20}
do
   if test -d ${dir[$n]}
   then
       echo "Execute ${dir[$n]}"
   fi
   if test -d ${other[$n]}
   then
       echo "Execute ${other[$n]}"
   fi 
done

However, after execution of the existing dirs the script also tries to execute the none existing array elements. It does not matter if I check for dir, file or even nonempty file.
Why?

Of course I can do a separate check if an element exists before checking if the dir exist, but can I not do this in one check?

user3392484 · Accepted Answer · 2014-03-14 20:18:53Z

1

Put your ${foo[$n]} in double quotes:

if test -d "${dir[$n]}"

Consider:

$ test -d '' || echo false
false
$ test -d || echo false
$

In general, always put variable dereferences into double quotes. If you don't and a variable is undefined then bad things happen, as you're seeing here.

answered Mar 14, 2014 at 20:18

user3392484

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2 Comments

Requist Over a year ago

Sometimes life can be so easy :) Thanks!

U. Windl Over a year ago

I'd consider it to be a bug in test: Actually it should output a "syntax error" and exit with some error code different from 1. Doing an strace showed that test did not test anything for test -d.

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