Is it possible to do two different variable expansions in a single expansion?

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I have a variable, in array, $CLI_MODE. I fill it with a simple read:

read -a CLI_MODE

I would like to take the value of the first tiny element and, if possible, swap - for _. To get everything tiny I can use the expansion ,,:

$ variavel=aBcDe--FG

$ echo ${variavel,,}
abcde--fg

To replace the trace by the line below, I would do so:

$ variavel=aBcDe--FG

$ echo ${variavel//-/_}
aBcDe__FG

How do I get the equivalent result of these two expansions? As if it were ,, next to //-/_? It is possible to do this in a single expansion?

bash

  • I believe two expansions aren’t possible, but maybe they’re nested, something like echo ${${variavel//-/_},,}. Have you tried something like this?

  • @Woss, I think an invalid construction, but it doesn’t hurt to try...

  • bash: ${${variavel//-/_},,}: bad substitution =(

  • In ZSH I managed to make a similar construction; echo ${${variavel//a/T}//-/_} resulted in TBcDe__FG, but the ,, also gave the bad substitution.

  • Yeah, but I’m stuck at Bourne Again... at least I can access the position [0] and do the downcase... as if the positional expansion did not interfere with other expansions

  • 1

    From what I’ve been doing, bash can’t seem to stand nested expansions. Then one way to do it on a single line would be to use one of the expansions and replace the second with some other command: echo ${variavel//-/_} | tr '[:upper:]' '[:lower:]' or echo ${variavel,,} | sed s/-/_/g

  • 1

    @hkotsubo Put life. As unpleasant as it is to me, it is still an answer. Anyway, invoking two expansions seems cheaper than making a pipeline and invoking an executable (echo usually is the built-in, hardly the executable /bin/echo, when at the level of shell script, so I’m not counting it)

  • 1

    Well, I put an answer (I was just looking for references in the documentation). Even if it is not the most pleasant solution, it is there as an option :-)

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According to the bash documentation on expansions:

The basic form of Parameter Expansion is ${parameter}. The value of Parameter is substituted. The Parameter is a shell Parameter as described above (see Shell Parameters) or an array Reference (see Arrays).

That is, the parameter that is passed to the expansion must be a shell Parameter. And if we check the respective documentation, there is described that this parameter must be a variable, a positional parameter (as $1, $2, etc), or one of the special parameters (like the $@ and the $#, for example).

But when trying to make an expansion within the other (as for example ${${variavel//-/_},,}), the result of the first (the innermost) is any text, which is not a valid parameter, and so the external expansion fails.

Therefore, an alternative to doing everything in a single line would be to do just one of the expansions, and treat the other with some other command. In your case, it could be one of the options below:

echo ${variavel//-/_} | tr '[:upper:]' '[:lower:]'

echo ${variavel,,} | sed s/-/_/g

Other options for bash can be consulted here and here (I haven’t tested them all).

And another alternative, if possible, is to change shell. zsh, for example, supports nested expansions.

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