I am trying to perform case modification with bash/zsh parameter expansion on macOS (11.4) and making some mistakes. Specifically, I want to take a variable that contains a string and turn it to snakecase (i.e.: from This is a STRING to this_is_a_string). I am taking baby steps and so far I am just trying to turn everything to lowercase and, as far as I understand it, the theory should work like this:
$ VAR="StRING"
$ echo "${VAR,,}" # turn the string characters to lowercase
string
This did not work at first, because macOS bash is the very much outdated 3.2.57. I installed the current version (5.1.8) with homebrew and it worked.
Still, this does not work with zsh (most recent version). I guess this happens because parameter expansion is different in zsh, am I right? Still, I cannot find any resourceful reference. I believe that zsh works a bit differently, more like sed. Sure, I could use tr and even sed itself, but I wanted to use parameter expansion.
In Zsh, you can use expansion modifiers:
echo ${VAR:l}
## => string
To turn the string to upper case, you can use
echo ${VAR:u}
## => STRING
See an online Zsh demo.
Or, you may use expansion flags:
echo ${(L)VAR}
## => string
echo ${(U)VAR}
## => STRING