How to convert a string to lower case in Bash?

Question

Is there a way in bash to convert a string into a lower case string?

For example, if I have:

a="Hi all" 

I want to convert it to:

"hi all" 

Answer 1

The are various ways:

POSIX standard

tr

$ echo "$a" | tr '[:upper:]' '[:lower:]' hi all 

AWK

$ echo "$a" | awk '{print tolower($0)}' hi all 

Non-POSIX

You may run into portability issues with the following examples:

Bash 4.0

$ echo "${a,,}" hi all 

sed

$ echo "$a" | sed -e 's/\(.*\)/\L\1/' hi all # this also works: $ sed -e 's/\(.*\)/\L\1/' <<< "$a" hi all 

Perl

$ echo "$a" | perl -ne 'print lc' hi all 

Bash

lc(){     case "$1" in         [A-Z])         n=$(printf "%d" "'$1")         n=$((n+32))         printf \\$(printf "%o" "$n")         ;;         *)         printf "%s" "$1"         ;;     esac } word="I Love Bash" for((i=0;i<${#word};i++)) do     ch="${word:$i:1}"     lc "$ch" done 

Note: YMMV on this one. Doesn't work for me (GNU bash version 4.2.46 and 4.0.33 (and same behaviour 2.05b.0 but nocasematch is not implemented)) even with using shopt -u nocasematch;. Unsetting that nocasematch causes [[ "fooBaR" == "FOObar" ]] to match OK BUT inside case weirdly [b-z] are incorrectly matched by [A-Z]. Bash is confused by the double-negative ("unsetting nocasematch")! :-)