Bash/Zsh: Catching all commands that start with a `/`

Question

Is there a way to get this example catchall function to be called whenever any command starts with the / character in an interactive shell (instead of searching in $PATH and executing something else, etc)?

For example, I would want

/arg1 arg2 arg3

to call

function catchall() {
    /* $1 == "arg1"
       $2 == "arg2"
       $3 == "arg3" */
    ...
}

(1) Should it work in an interactive shell? or in a script? Both? (2) Please make sure there is no [XY problem](https://meta.stackexchange.com/a/66378/355310) here. [Edit] the question if there is. What *actual problem* are you trying to solve? — Kamil Maciorowski, May 31 '22 at 02:35
@KamilMaciorowski Edited to include "interactive shell". There is no XY problem here. The actual problem I am trying to solve is a function call can generalize commands by a prefix. The `/` is an example. The `catchall` function is also only an example. — Aaron Esau, May 31 '22 at 02:47
(3) Do you want `catchall` to run instead of the typed command? or along? (before or after?). — Kamil Maciorowski, May 31 '22 at 02:48
That's a good clarification. I mean instead of the typed command. — Aaron Esau, May 31 '22 at 02:53
I think it can be done in zsh using the _precmd_ feature. See the answer to [this](https://stackoverflow.com/questions/12580675/zsh-preexec-command-modification) question. However, I would consider it highly disturbing and error prone, if under the hood a different command is executed than the one I have entered. — user1934428, Jun 27 '22 at 13:01

score 0 · Answer 1 · answered May 31 '22 at 06:33

A limited (imperfect) solution for Bash:

shopt -s extdebug

_generic() {
   case "$BASH_COMMAND" in
      /* )
         eval "_specific $BASH_COMMAND"
         return 1
      ;;
   esac
}

_specific() {
   printf "Got arguments: "
   printf '<%s> ' "$@"
   printf '\n'
   printf 'The command was: %s\n' "$BASH_COMMAND"
}

trap '_generic' DEBUG

Based upon:

Example usage:

$ /foo "a         b" /dev/[nf]ul* ; /bar <(cat)
Got arguments: </foo> <a         b> </dev/full> </dev/null> 
The command was: /foo "a         b" /dev/[nf]ul*
Got arguments: </bar> </dev/fd/63> 
The command was: /bar <(cat)
$

Notes:

I don't know Zsh. This answer is for Bash.
eval can be evil. AFAIK evaluating $BASH_COMMAND is not wrong because $BASH_COMMAND is equivalent to the string you typed and it hasn't been evaluated yet. Our eval does what the shell would normally do anyway.
Still $BASH_COMMAND is not the full string you typed. E.g. /foo; /bar will call _generic (and thus _specific) separately for /foo and /bar. This is how the DEBUG trap works. In the context of your question this seems fortunate.
_generic operates on $BASH_COMMAND with is a string. _specific works with an array of arguments from evaluation of $BASH_COMMAND, nevertheless $BASH_COMMAND is still available. In principle you should use the array (or elements of it), i.e. "$@" (or "$1", "$2", …). I can imagine $BASH_COMMAND may be used for some wizardry though.
You could use eval "_specific ${BASH_COMMAND#/}" instead of eval "_specific $BASH_COMMAND" to strip the leading / when calling _specific. Note if you do this and if the command is / whatever … then the first argument to _specific will be whatever, not an empty string. For this reason you may want to strip the leading / only inside _specific (e.g. argument1="${1#/}").
Expanding the code to cover more prefixes is simple: build any number of specific functions and add more cases for case.
The core of the solution is the DEBUG trap which is for debugging and as such it tries not to disturb stdin, stdout or the exit status. Our _specific cannot fully replace /foo in cases like <input /foo >output, /foo | wc -c or /foo || whatever. A solution with command_not_found_handle would be better in these matters, but it would only kick in if /foo wasn't a valid command; so it would probably work for /arg1 or /bin/nonexistent, but not for /bin/echo. (Compare this answer: How do I create a command prefix for bash with an alias?)

Bash/Zsh: Catching all commands that start with a `/`

1 Answers1