Shell – Fish shell testing for existence of file in $PATH

fishshell

I'm trying to test in a fish shell script for the existence of the figlet binary. Since I use Linux and OS X I cannot rely on the file being in the same location and need to resolve it dynamically. I'm used to doing this with $(which) in bash, which works.

With fish though this does not work properly. Why?

function print_hostname --description 'print hostname'
  if test -x (which figlet)
    hostname | figlet
  end
end

Best Answer

Use type in fish like in Bourne-like shell:

if type -q figlet
  hostname | figlet
end

Or to limit to executables in $PATH (ignoring functions, builtins):

if command -s figlet > /dev/null
  hostname | figlet
end

Related Solutions

Bash – command substitution handling in fish vs. other shells (sh, bash, zsh)

Ok, I think I have a clue what is going on.

The behaviour you are observing in zsh and bash when executing echo $(mc) is caused by mc.

When you run mc normally, it doesn't react when Ctrl+c is pressed, since it ignores SIGINT. The way to end mc is by pressing F10 and Enter.

When you run echo $(mc) the input goes to the mc process, so it is no wonder that when you press Ctrl+c nothing will happen, because mc ignores SIGINT. But when you run echo $(mc) and press F10 and Enter it will react. (When I then press F10 and Enter again it actually quits; it should quit at the first try but I have no idea why it does not.)

From this fact I deduce that Midnight Commander is running normally, but the output from it is put into the shell buffer, so it can later be used by echo. Something we should consider normal.

Also you think that bash and zsh are hanging, but I would say that they are not hanging but waiting for the echo command to return. But echo can only return when mc returns, which it only does when F10 and Enter are pressed. With this we can also explain what happens when you press Ctrl+z. By pressing the combination mc goes to sleep like it would if you had run mc normally and echo is still waiting for the sleeping mc.

So everything is normal behavior. Except for what fish does. fish seems to start commands run with echo ($command) in the background. It kind of makes sense, since such a command normally does not need any input. For the why and how I have no answer. But you can see that it runs in the background when you enter

echo (mc)
jobs

Shell – hook-like system for shell

Things that may help you:

The preexec hook function is called before every command line is executed (not before every command).

preexec() echo will execute: $1

precmd before each prompt.

precmd() echo executed: $history[$[HISTCMD-1]]

You can extend a function like this:

functions[git]='
  (){ '$functions[git]'; } "$@"; local myret=$?
  do-extra-stuff
  return $myret'

Or:

functions[git-old]=$functions[git]
git() {
  git-old "$@"
  local myret=$?
  do-extra-stuff || return $myret
}

The DEBUG trap is executed after each command:

trap 'echo $ZSH_DEBUG_CMD returned with $?' DEBUG

You can redefine the accept-line widget (mapped to Enter) to do stuff in there:

accept-line() {
  zle -R "going to run: $BUFFER"
  sleep 1
  zle .$WIDGET
}

zle -N accept-line

Best Answer

Related Solutions

Bash – command substitution handling in fish vs. other shells (sh, bash, zsh)

Shell – hook-like system for shell

Related Question