How to convert a string to lower case in Bash?

tr:

a="$(tr [A-Z] [a-z] <<< "$a")"

AWK:

{ print tolower($0) }

sed:

y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/

In Bash 4:

To lowercase

$ string="A FEW WORDS"
$ echo "${string,}"
a FEW WORDS
$ echo "${string,,}"
a few words
$ echo "${string,,[AEIUO]}"
a FeW WoRDS

$ string="A Few Words"
$ declare -l string
$ string=$string; echo "$string"
a few words

To uppercase

$ string="a few words"
$ echo "${string^}"
A few words
$ echo "${string^^}"
A FEW WORDS
$ echo "${string^^[aeiou]}"
A fEw wOrds

$ string="A Few Words"
$ declare -u string
$ string=$string; echo "$string"
A FEW WORDS

Toggle (undocumented, but optionally configurable at compile time)

$ string="A Few Words"
$ echo "${string~~}"
a fEW wORDS
$ string="A FEW WORDS"
$ echo "${string~}"
a FEW WORDS
$ string="a few words"
$ echo "${string~}"
A few words

Capitalize (undocumented, but optionally configurable at compile time)

$ string="a few words"
$ declare -c string
$ string=$string
$ echo "$string"
A few words

Title case:

$ string="a few words"
$ string=($string)
$ string="${string[@]^}"
$ echo "$string"
A Few Words

$ declare -c string
$ string=(a few words)
$ echo "${string[@]}"
A Few Words

$ string="a FeW WOrdS"
$ string=${string,,}
$ string=${string~}
$ echo "$string"
A few words

To turn off a declare attribute, use +. For example, declare +c string. This affects subsequent assignments and not the current value.

The declare options change the attribute of the variable, but not the contents. The reassignments in my examples update the contents to show the changes.

Edit:

Added "toggle first character by word" (${var~}) as suggested by ghostdog74.

Edit: Corrected tilde behavior to match Bash 4.3.

echo "Hi All" | tr "[:upper:]" "[:lower:]"

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")! :-)