Trap on DEBUG signal for the dash shell?

Question

With the bash shell, you can install a trap on the synthetic DEBUG signal. This is useful and will execute your function or bit of code on every shell command. For example:

$ trap 'logger -t shell "${BASH_COMMAND}"' DEBUG

The idea here is to install the trap in a file dropped in, for instance, /etc/profile.d/bash-logger and have any user shell session commands logged automatically to syslog whenever they happen. This is also useful to easily trace remote commands.

Looking at the dash shell source code, I seem to understand there's no such DEBUG signal available.

Are there any workarounds or alternatives to achieve the same result with the dash shell?

Answer 1

The most thorough approach would be to re-compile the shell with syslog support enabled. That also gets around shells that (for whatever reason) don't load ~/.whatever-profile or ~/.whatever-rc.

This is included as an option in the standard source for Bash, but for other shells you might need to find or write a patch.

I would hesitate to recommend reading and evaling line-by-line, because commands can span multiple lines and eval of half a command will either fail, or do the wrong thing. Affected command types include: pipelines and conditional commands (where each line ends with &&, || or |), compound commands (including bracketed groups), lines continued with backslashes, multi-line quoted strings, and << here-docs. (There are probably more than I haven't thought of.)

For limited cases you may find set -v or set -x useful, combined with redirecting stderr into a pipe to logger. However it's imperfect as also redirects stderr from the programs you run. (In Bash you can choose a different filedescriptor for set -x to write to, which solves that problem.)

I would point out that if you go ssh host some_command then there's no interactive or login shell involved, so your rc files don't get loaded, which would bypass your logging.