Parameter expansion seems to be missing a piece

Question

I'm currently learning how to write simple scripts and can't get my head around a very simple problem.

I have the following command in my script...

touch ${DIRECTORY}/${FILE}

This command appears several times in my script and works fine in all instances except for one. For some reason, at one point only, it decides to include the slash in the filename that it's trying to create and I get the error...

cannot touch `/myfile.dat': Permission denied

At first I thought it was just probably down to my inexperience (which it probably is) but after doing a bit of research, I've seen this line used in countless examples working correctly. I can't understand - the same line is working fine in several other locations in my script!

Anybody care to shed some light on this? It's the inconsistency that's really annoying me. Can the commands surrounding this particular command affect its outcome?

Answer 1

Looks like ${DIRECTORY} is empty. In that case touch ${DIRECTORY}/${FILE} expands to touch /${FILE}

Answer 2

Several things might be happening.

• Maybe you misspelled DIRECTORY at this point, so the script is running something like touch ${DRIECTORY}/${FILE}. Since DIRECTORY is not set, if $FILE is myfile.dat, this expands to touch /${FILE} (an unset variable is treated as if it was empty).
  You can make the shell stop with an error message if you every try to expand an unset variable. Put set -u at the top of the script (after the #! line). You can switch back and forth between unset-triggers-error and unset-is-empty with set -u and set +u.
• Maybe the variable DIRECTORY is unset or empty at this point. See previous point.
• Maybe you accidentally put an extra space: touch ${DIRECTORY} /${FILE}. Then the touch command receives two parameters, it operates on ${DIRECTORY} (which, since this is presumably the name of an existing directory, sets that directory's modification time to the current time), then it chokes on /${FILE}.
• Maybe there is an extra space at the end of the value of DIRECTORY. For example, if the value of the variable is /some/path with a space at the end, then touch receives two parameters: /some/path and /myfile.dat. See previous point.

Your script won't work if DIRECTORY or FILE contain any whitespace or globbing character \[?*. This is because when you write e.g. $FOO, this doesn't just expand to the value of the variable: that value is then split into words at each whitespace sequence, and each word is interpreted as a wildcard pattern and replaced by matching file names (if no file name matches, the pattern is left alone). Always use double quotes around variable substitutions (and likewise command substitutions). Inside double quotes, "$FOO" expands to just the value of FOO with no further mangling.

touch "$DIRECTORY/$FILE"

(See also $VAR vs ${VAR} and to quote or not to quote)

To get a better idea of what the script is doing, put set -x after the #! line. This enabled a debug mode where the shell prints each command before executing it. You can switch debug traces on and off with set -x and set +x.