What do the commas in "${var,,}" mean?

Question

read -p "Enter yes/no   " SOMEVAR

SOMEVAR=`"echo ${SOMEVAR,,}"`

The code above gives me a ${SOMEVAR,,}: bad substitution error.

Answer 1

The parameter expansion ${variable,,} would expand to the value of $variable with all character in lower case in the bash shell. Given that you get a "bad substitution" error when this code runs suggests that you are in fact either

• not using that shell but possibly /bin/sh (which is not always bash). But not getting an error for read -p suggests that it's more likely that you are
• using an older release of bash which does not support this expansion (introduced in release 4 of bash).

The generic form of the expansion is ${variable,,pattern} in which all characters in $variable that matches pattern would be converted to lower case (use ^^ to convert to upper case):

$ str="HELLO"
$ printf '%s\n' "${str,,[HEO]}"
heLLo

See also the bash manual on your system.

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For older releases of bash, you could instead do the following to lowercase the value of a variable:

variable=$( tr 'A-Z' 'a-z' <<<"$variable" )

This passes the value of the variable through tr using a "here-string". The tr utility transliterates all characters in the A to Z ASCII range (assuming the C/POSIX locale) to the corresponding character in the a to z range.

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Note also that

SOMEVAR=`"echo ${SOMEVAR,,}"`

is better written as

SOMEVAR=${SOMEVAR,,}

In fact, what you wrote would give you a "command not found" error in bash release 4+, unless you have a command called echo string, including the space (where string was what the user inputted). This is due to the command substitution trying to execute the double quoted string.