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I'm trying to use alias command to switch between two definitions (trying to use Rust's cargo and Crystal's compiler crystal from command line). I have the following in my ~/.bash_profile:

   alias c=crystal
   alias cargo="unalias c; alias c=cargo"
   alias crystal="unalias c; alias c=crystal"

However, when I try to run something like

> c macro.rs

in macOS terminal I get

bash: alias: macro.rs: not found

What may be the problem here?

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Let's see what's happening here:

$ set -x
$ c file
+ unalias c
+ alias c=crystal file
bash: alias: file: not found
$ set +x

So, c file is expanded to unalias c; alias c=crystal file (via an intermediate expansion to crystal file).

alias c=crystal file doesn't make sense. What I think you might have wanted is for crystal file to be executed.

You might want to rename the aliases that switches the meanings of c:

alias usecrystal="alias c=crystal"
alias usecargo="alias c=cargo"

Alternatively, you may protect the command from alias expansion:

alias c="\crystal"
alias crystal="alias c=\crystal"
alias cargo="alias c=\cargo"

Or use command (which IMHO looks better):

alias c="command crystal"
alias crystal='alias c="command crystal"'
alias cargo='alias c="command cargo"'

The command command also protects from alias expansion.

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