Bash – How to see what command was actually run in the shell, through an alias or function

bashdebuggingshell

I have a bash function (or alias), for instance function install() {sudo apt-get install $@}. When running the command install dicelab, what I expect will actually be run is sudo apt-get install dicelab. Where can I see what was actually run by the shell? I would like to make sure that my more complicated aliases are working as expected.

Best Answer

Use set -x in the shell.

$ alias hello='echo hello world!'
$ hello
hello world!

$ set -x
$ hello
+ echo hello world!
hello world!

Using set -x turns on the xtrace shell option (set +x turns it off) and should work in all Bourne-like shells, like bash, dash ksh93, pdksh and zsh. This prompts the shell to display the command that gets executed after alias expansions and variable expansions etc. has been performed.

The output will be on the standard error stream of the shell (just like the ordinary prompt) so it will not interfere with redirections of standard output, and it will be preceded by a prompt as defined by the PS4 shell variable (often +␣ by default).

Example with a few functions:

$ world () { echo "world"; }
$ hello () { echo "hello"; }
$ helloworld () { printf '%s %s!\n' "$(hello)" "$(world)"; }

$ helloworld
hello world!

$ set -x
$ helloworld
+ helloworld
++ hello
++ echo hello
++ world
++ echo world
+ printf '%s %s!\n' hello world
hello world!

With your specific example (with syntax corrected and added quotes):

$ install () { sudo apt-get install "$@"; }
$ set -x
$ install dicelab
+ install dicelab
+ sudo apt-get install dicelab
bash: sudo: command not found

(I don't use or have sudo on my system, so that error is expected.)

Note that there is a common utility already called install, so naming your function something else (aptin?) may be needed if you at some point want to use that utility.

Note that the trace output is debugging output. It is a representation of what the shell is doing while executing your command. The output that you see on the screen may not be suitable for shell input.

Also note that I was using bash above. Other shells may have another default trace prompt (zsh incorporates the string zsh and the current command's history number, for example) or may not "stack" the prompts up like bash does for nested calls.

I'm running I used to run with set -x in all my interactive shells by default. It's nice to see what actually got executed... but I've noticed that programmable tab completion etc. may cause unwanted trace output in some shells, and some shells are a bit verbose in their default trace output (e.g. zsh).

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