Implement $(< file)

[Crestwave]

This is basically equivalent to $(cat file) in Bash but with builtins.

[andychu]

Hm it looks like this case triggers a bug:

osh$ $(< INSTALL.txt)
Warning: SimpleCommand or Assignment has no location information

For some reason completion also doesn't work after <, but it does after cat <.

[jyn514]

I can reproduce this without command substitution: > f. This should create (and possibly overwrite) fexactly as if run with c > f where c has no output.

[andychu]

OK I fixed the location error.

I'm not sure about $(< file) being like cat though. dash doesn't implement it, but bash, mksh, and zsh do.

Is it commonly used in scripts, or does POSIX require it?

[Crestwave]

OK I fixed the location error.

I'm not sure about $(< file) being like cat though. dash doesn't implement it, but bash, mksh, and zsh do.

Is it commonly used in scripts, or does does POSIX require it?

No, it's not POSIX. It's used to read a file by people who know of it; e.g., Neofetch uses it.

[andychu]

Background: https://unix.stackexchange.com/questions/189749/understanding-bashs-read-a-file-command-substitution/368663#368663

[andychu]

An interesting thing about this is that zsh supports an extended version of it. I think bash is just literally special casing $(< file) to mean something different?

echo $(<file)  # works in both
echo $(< file; echo hi)      # zsh respects the meaning of < file, but bash doesn't

Also, zsh extends the meaning to process subs, like

cat < <(<file; echo hi)       # a confusing way of doing it, but it works

[andychu]

And it also doesn't support pipelines and subshells:

(< file)    # doesn't cat the file

< file | wc -l  # doesn't work, nothing goes to stdout

[bb010g]

And it also doesn't support pipelines and subshells:

See Zsh's NULLCMD options & section "Redirections with no command" of zshmisc(1).