Running command within test statement in bash

Question

I have a piece of code to find the first file in a directory:

bash~> $( echo eval "ls | head -1" )
arguprog.sh

This snippet was then added to an if statement, to run a different set of commands if that file was arguprog.sh:

bash~>  if [[ $( echo eval "ls | head -1" ) == "arguprog.sh" ]]; then echo "TRUE"; else echo "FALSE"; fi;
FALSE

However this is not doing what I want. It returns FALSE even though the first file is arguprog.sh!

Is there a way to resolve this while still doing the string comparison entirely within the test block?

Answer 1

First, eval is evil, especially when it's not needed. In your case, eval is not needed!

Replace the coding horror you showed with just:

ls | head -1

and to include it in your test statement:

if [[ $(ls | head -1) = "arguprog.sh" ]]; then echo "TRUE"; else echo "FALSE"; fi

But this is wrong and broken (see below).

Now something more general: do not parse the output of ls. If you want to find the first file (or directory or...) in your current dir, use globs and this method:

shopt -s nullglob
files=( * )
# The array files contains the names of all the files (and directories...)
# in the current directory, sorted by name.
# The first one is given by the expansion of "${files[0]}". So:
if [[ "${files[0]}" = "arguprog.sh" ]]; then echo "TRUE"; else echo "FALSE"; fi

Notice that your method, parsing ls is wrong. Look:

$ # Create a new scratch dir
$ mkdir myscratchdir
$ # Go in there
$ cd myscratchdir
$ # touch a few files:
$ touch $'arguprog.sh\nwith a newline' "some other file"
$ # I created 2 files, none of them is exactly arguprog.sh. Now look:
$ if [[ $(ls | head -1) = "arguprog.sh" ]]; then echo "TRUE"; else echo "FALSE"; fi
TRUE
$ # HORROR!

There are twisted work-arounds for this, but really, the best method is the one I just gave you.

Done!