bash function to "wrap" 7z

Question

It appears that 7z can't write to a pipe, so I wrote the following bash function to emulate this functionality.

I know this is a simple problem, but this is my first bash function and I feel like it's really easy to write faulty code in bash, so any feedback is appreciated.

function 7z2pipe () (
 local out_dir=$(mktemp -d)
 local out_file="${out_dir}"/temp.7z
 local log_file="${out_dir}"/"outz.txt"
 7z -p"weak_password" -mhe=on a "${out_file}" "$@" &> "${log_file}"
 
 if [ $? -ne 0 ]; then
 cat "${log_file}"
 return $?
 fi
 cat "${out_file}"
)

Answer 1

Do you want to make persistent temporary files?

Note that while the default settings for mktemp put things in /tmp so that it will eventually get garbage collected, it doesn't immediately get garbage collected. Things can persist in /tmp for days.

cat has a return code

I'm not sure why are returning the return code from cat. Wouldn't it make more sense to return the return code from 7z?

Alternative code

 local return_code=$?
 if [ ${return_code} -ne 0]; then
 cat "${log_file}"
 else
 cat "${out_file}"
 fi
 rm -rf ${out_dir}
 return ${return_code}

That will leave things as you found them, with no extra temporary directories or files.

It will always return the return code from 7z.

It will output either the log file or the output file, as you originally did it.

Redirect to stderr

I'm not sure that your approach is what I would do. An alternative to creating the log file would be to redirect stdout to stderr.

 local out_file=$(mktemp --suffix .7z)
 7z -p"weak_password" -mhe=on a "${out_file}" "$@" 1>&2
 local return_code=$?
 cat "${out_file}"
 rm -f ${out_file}
 return ${return_code}

Now stdout will contain the compressed file and stderr will have all the output from the command.

Note: I do not normally use Bash functions, so don't take anything I say as validation of function syntax.

• Toby Speight: Actually, mktemp will put its file into $TMPDIR if that's set (this is how pam_tmpdir implements per-user temporary directories). The point still stands that it's good practice to add a trap to clean up, of course.