Why isn't the diff command working as I expect in this Bash script?

I'm not understanding why the diff command is not working correctly in my script. Can anybody clarify it for me?

Here is the bash script:

#!/bin/bash

SRC="src-folder"
DST="dst-folder"
FILES=( a/ b/ c/ )

for f in ${FILES[@]}; do
  cmd="diff -q  $SRC/$f $DST/$f"
  echo $cmd  # row-9
  $cmd       # row-10
done

Script output seems like this:

diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/a/ dst-folder/a/
(until buffer overflow)

When I disable/remove/comment row-10 I can see correct output (when I run these commands in an interactive shell rather than in the script I get correct results):

diff -q src-folder/a/ dst-folder/a/
diff -q src-folder/b/ dst-folder/b/
diff -q src-folder/c/ dst-folder/c/

When I change diff to rsync, everything is working correctly (call from script).

So how can I use diff in scripts? What's the reason that diff called by my script is not working as expected?

I tried also a very simple script:

#!/bin/bash

diff -q  src-folder/a dst-folder/a
diff -q  src-folder/b dst-folder/b

But it doesn't work. When I change diff command to any other (e.g. rsync), everything is working.

tl;dr: Works for me.

Long answer: When I run the script from your question on one of my test machine, it behaves exactly as expected:

a-schmidt@s-laugz2repo01:~$ ./show-diffs
diff -q src-folder/a/ dst-folder/a/
diff: src-folder/a/: No such file or directory
diff: dst-folder/a/: No such file or directory
diff -q src-folder/b/ dst-folder/b/
diff: src-folder/b/: No such file or directory
diff: dst-folder/b/: No such file or directory
diff -q src-folder/c/ dst-folder/c/
diff: src-folder/c/: No such file or directory
diff: dst-folder/c/: No such file or directory
a-schmidt@s-laugz2repo01:~$

So apparently the diff command on your system is not the same as on mine. There are several possibilities how this could happen, for example through shell aliases or local installation of a program of that name. In order to find out what it is, enter the shell command:

type diff

On an unmodified Ubuntu system this will output:

diff is /usr/bin/diff

If it outputs something else, such as "diff is aliased to ..." or "diff is a function ..." then you need to look into your shell profile to find out where that change is made, and possibly correct it.

I think the problem is that within your command cmd="diff -q $SRC/$f $DST/$f" there are options and after the expansion at row-10 the string contained in $cmd is not evaluated properly as a command by Bash.

When you are using the exec command the string contained in $cmd is evaluated properly. But what exec does is that it replaces the current shell with new one where the command is executed, so your loop no longer exists after the first iteration.

I've highlighted evaluated few times because in Bash have eval command which does exactly that - evaluates a string as a command. So I think the proper solution is to modify the script like this:

eval $cmd       # row-10

As a final note, if it is the entire script, why you need to assign the command to a variable which is evaluated immediately (at the next line)? I mean in this particular case you can simplify the loop in a way as this:

for f in ${FILES[@]}; do
  diff -q  "$SRC/$f" "$DST/$f"
done