DIY (Fish) Shell Templating

May 07, 2023 May 11, 2023 (IST)

If you’ve seen my note on creating a Static Site Build Tool, you might’ve seen the link to pp, and that I lament the syntax and the fact that it uses ash/dash. Turns out it’s quite easy to implement in fish, along with a syntax I prefer!

It started with me just going about my day, when I remembered the eval command and I started thinking of the pieces I might need to create a parser/state machine of sorts in fish. I tinkered a bit with some of the commands I was thinking of, and I managed to get it working quite well!

I haven’t made a way of putting in variables or reading and writing from a file, and I haven’t thought of too many edge cases, but the basic functionality is there.

If I run ./parser.fish "the date is {{ date -I }}" it will give me the output the date is 2023-05-07

I’ve pasted in the whole thing, as it currently is, at the end, if you’re curious.

How it Works

The basic way it works, is it uses a variable to figure out if it’s reading normal text or command text. If it’s reading normal text, it simply passes it along to stdout, and if it’s reading command text, it gathers that into a string, then runs eval on it, which spits out its output to stdout.

Future Work

I want to be able to parse files instead of just strings. The answer might be somewhere in string join \n $fileContents.

When parsing files, it would be useful to provide variables for substitution. Perhaps it could be done by specifying environment variables in the same line as the command. Something like VAR=val fish-template str "{{ echo $VAR }}. Another way would be a syntax for keys and values. Perhaps --variables "key1: val1; key2: val2".

A short syntax for variable expansion might also be nice. Something like {{ $var }} or {$ var $}

It would be cool to have a way to limit its scope, depending on where it’s being called from. Include a list to allow certain commands, as well as one to block some. If the first the command isn’t in the allow list, or is present in the block list, then raise an error instead of evaluating it.

In a similar vein, providing additional commands would be great. A simple way is to create commands in a bin/ directory and then run them with {{ ./bin/my-command }}, but a shorter syntax would be quite useful. Perhaps use the bin/ directory with a syntax like {{ @my-command }}? Or simply {{ my-command }}, such that if my-command is not installed, it would look in the bin directory.

It is ideal that the template evaluation takes place from a specific directory. Especially if relying on file operations or something like a ./bin/ directory. When calling the command on a template file, ensure the current working directory is the same as the file’s parent directory, unless specified otherwise. Changing directories in the script sounds like a potential footgun, since it’s implicit behaviour that people will not be aware of. An easy flag, then, to execute in the context of the template file’s folder. So --dir < current | file-dir | <path> >, where is relative (starting with ./) or absolute.

It would also be great to have all commands in the file/template evaluate in the same context. This way a variable declared at the top could be used in a command further down. Currently, it spawns subshells using the eval command. Maybe it could use source or a flag in eval? Another approach might be to gather content as an echo $content command along with the other commands in the given sequence, then calling all the echo and other commands using a single eval command at the end. Use string escape to prevent unwanted expansions.

Code

set parserMode normal
set command ''

echo $argv[1] | while read -n 1 char
  switch "$parserMode $char"

    case 'normal {'
      set parserMode open-brace

    case 'open-brace {'
      set parserMode command-mode
      set command ''

    case 'open-brace *'
      set parserMode normal
      printf '{%s' $char

    case 'command-mode }'
      set parserMode close-brace

    case 'close-brace }'
      set parserMode normal
      # echo \'$command\'
      eval $command

    case 'close-brace *'
      set parserMode command
      set command "$command\}$char"

    case 'command-mode '
      # newline creates '' when read
      set command "$command ; "

    case 'command-mode #'
      set parserMode comment

    case 'comment '
      # newline creates '' when read
      set parserMode command-mode
      set command "$command ; "

    case 'command-mode *'
      set command "$command$char"

    case 'normal '
      # newline creates '' when read
      printf \n

    case 'normal *'
      printf %s $char
  end
end

References to this page:

• Static Site Build Tool

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