Ggtcf

I find myself often doing silly things like:

cmd $( echo foo1,2,3 | tr ' ' , )

when I want to invoke cmd foo1,foo2,foo3. I would like instead to be able to do something along the lines of:

OFS=, ; cmd foo1,2,3

to force the brace expansion to give me a comma separated list. Is there any such functionality available?

I don't think it's possible with a simple variable or setting.

However, you can capture the brace expansion in an array, and then use IFS and parameter substitution to achieve that effect:

$ ( a=( foo1,2,3 ); IFS=,; printf -- "%sn" "$a[*]" )
foo1,foo2,foo3

There, I'm using a subshell so I don't alter IFS in the current shell.

Here's a short
function that's much tidier:

$ join() local IFS=$1; shift; printf -- "%sn" "$*"; 
$ join , foo1,2,3
foo1,foo2,foo3

You can create a function that will echo a comma-separated list of its args:

commas() local IFS=,; echo "$*"; 

Then you can use it in your calls to cmd:

cmd "$(commas foo1,2,3)"

Edit: Note that echo is somewhat broken, in that if you pass a single arg that looks like a valid echo options (e.g., -e, -n, etc), echo will consume it and interpret it as an option instead of printing it.

$ commas -e #prints nothing
$ commas -n #prints nothing
$ commas -e -n
-e,-n

The last one works because "$*" expands to -e,-n, which echo sees as a single argument.

And echo doesn't respect the -- argument as an "end of options" indicator like printf does. So you may want to use printf -- "%sn" "$*", as in @glennjackman's answer. Then again, the whole point of the commas function is to combine multiple args into a comma-separated list, as a single string. So it's probably unlikely that you'd run into any issues with echo. But hey, it only costs you a few more characters to type printf -- "%s" instead of echo.