June 14, 2020
I’m currently working on a somewhat minimalistic Fish Shell implementation of jrnl, a popular CLI journal tool written in Python. My Fish version is still a work in progress but there’s one insight I’d like to share: how to correctly implement subcommands in Fish so that CLI are options are passed through to subcommands.
My first attempt looked something like this. At the bottom of this snippet (which you can paste into a file and run!) I’m defining the “main” export, a function called
foo. This function has a single subcommand, to which it passes its arguments. That function, called
__hello, defines a single option,
--info). Additionally, there’s some shared functionality implemented in
__shared. And to make matters a bit more complicated,
__shared also accepts an option.
#!/usr/bin/env fish function __shared set -l options (fish_opt -s t -l tag) argparse $options -- $argv echo $_flag_t echo $argv end function __hello set -l options (fish_opt -s i -l info) argparse $options -- $argv echo $_flag_i __shared $argv end function foo -a cmd switch $cmd case hello __hello $argv end end
Running this with
source foo.fish; foo hello -t -i results in an error:
__hello: Unknown option “-t” foo.fish (line 12): argparse $options -- $argv ^ in function '__hello' with arguments 'hello -t -i' called on line 22 of file foo.fish in function 'foo' with arguments 'hello -t -i' __shared: Unknown option “-i” foo.fish (line 5): argparse $options -- $argv ^ in function '__shared' with arguments 'hello -t -i' called on line 16 of file foo.fish in function '__hello' with arguments 'hello -t -i' called on line 22 of file foo.fish in function 'foo' with arguments 'hello -t -i'
Line 12 refers to the
argparse call in
__hello. As the error message implies, the option
-t is indeed unknown to the options I’ve specified in
__hello. Luckily Fish added an
--ignore-unknown option in 3.1b1, which solves this issue. Before that release you had to redirect stderr to
/dev/null which wasn’t ideal. By changing that one line to
argparse -i $options -- $argv the script works again, hooray! Now let’s see what happens if you pass positional parameters to
__shared. Change the call to
__shared foo bar $argv and run the script again:
$ source foo.fish; foo hello -t -i -i -t foo bar hello
No errors, but the name of the subcommand shows up in the arguments list, which is not what I want. Luckily that’s easy to fix. Simply use
set -e argv to remove the first element (the subcommand name) from the original argument list. Note that I’m not using a dollar sign here which is intentional:
case hello set -e argv __hello $argv ``` Let's recap what happens to `$argv`, when the script is called with `foo param -t -i`, as the arguments flow through the different function calls. - `param -t -i` - Remove first element `param` -> `-t -i` - Any option that `argparse` can succesfully parse is removed, in this case `-i`, but we're adding `foo bar` -> `foo bar -t` - Parse `-t` option -> `foo bar` The final `$argv` at the very end of `__shared` is therefore just `foo bar`, which is exactly what I need. `argparse` and `fish_opt` are some of the coolest helpers that Fish makes available to you as a script author and evertime I write POSIX sh or Bash scripts I miss their utility. It's therefore absolutely worth it to check out the [documentation](https://fishshell.com/docs/current/cmds/argparse.html)! Happy Fish Scripting!
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