Converting string to lower case in Bash shell scripting

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)

$ 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)

$ 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

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

Edit:

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

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

tr:

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

AWK:

{ print tolower($0) }

sed:

y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/

Regular expression

I would like to take credit for the command I wish to share but the truth is I obtained it for my own use from http://commandlinefu.com. It has the advantage that if you cd to any directory within your own home folder that is it will change all files and folders to lower case recursively please use with caution. It is a brilliant command line fix and especially useful for those multitudes of albums you have stored on your drive.

find . -depth -exec rename 's/(.*)\/([^\/]*)/$1\/\L$2/' {} \;

You can specify a directory in place of the dot(.) after the find which denotes current directory or full path.

I hope this solution proves useful the one thing this command does not do is replace spaces with underscores - oh well another time perhaps.

For a standard shell (without bashisms) using only builtins:

uppers=ABCDEFGHIJKLMNOPQRSTUVWXYZ
lowers=abcdefghijklmnopqrstuvwxyz

lc(){ #usage: lc "SOME STRING" -> "some string"
    i=0
    while ([ $i -lt ${#1} ]) do
        CUR=${1:$i:1}
        case $uppers in
            *$CUR*)CUR=${uppers%$CUR*};OUTPUT="${OUTPUT}${lowers:${#CUR}:1}";;
            *)OUTPUT="${OUTPUT}$CUR";;
        esac
        i=$((i+1))
    done
    echo "${OUTPUT}"
}

And for upper case:

uc(){ #usage: uc "some string" -> "SOME STRING"
    i=0
    while ([ $i -lt ${#1} ]) do
        CUR=${1:$i:1}
        case $lowers in
            *$CUR*)CUR=${lowers%$CUR*};OUTPUT="${OUTPUT}${uppers:${#CUR}:1}";;
            *)OUTPUT="${OUTPUT}$CUR";;
        esac
        i=$((i+1))
    done
    echo "${OUTPUT}"
}

Using Korn shell (ksh) typeset:

$ echo $SHELL
/usr/bin/ksh
$ typeset -l test_var
$ test_var='HELLO theRE'
$ echo $test_var
hello there

Pre Bash 4.0

Bash Lower the Case of a string and assign to variable

VARIABLE=$(echo "$VARIABLE" | tr '[:upper:]' '[:lower:]') 

echo $VARIABLE

If using v4, this is baked-in. If not, here is a simple, widely applicable solution. Other answers (and comments) on this thread were quite helpful in creating the code below.

# Like echo, but converts to lowercase
echolcase () {
    tr [:upper:] [:lower:] <<< "${*}"
}

# Takes one arg by reference (var name) and makes it lowercase
lcase () { 
    eval "${1}"=\'$(echo ${!1//\'/"'\''"} | tr [:upper:] [:lower:] )\'
}

Notes:

• Doing: a="Hi All" and then: lcase a will do the same thing as: a=$( echolcase "Hi All" )
• In the lcase function, using ${!1//\'/"'\''"} instead of ${!1} allows this to work even when the string has quotes.

For Bash versions earlier than 4.0, this version should be fastest (as it doesn't fork/exec any commands):

function string.monolithic.tolower
{
   local __word=$1
   local __len=${#__word}
   local __char
   local __octal
   local __decimal
   local __result

   for (( i=0; i<__len; i++ ))
   do
      __char=${__word:$i:1}
      case "$__char" in
         [A-Z] )
            printf -v __decimal '%d' "'$__char"
            printf -v __octal '%03o' $(( $__decimal ^ 0x20 ))
            printf -v __char \\$__octal
            ;;
      esac
      __result+="$__char"
   done
   REPLY="$__result"
}

technosaurus's answer had potential too, although it did run properly for mee.

In bash 4 you can use typeset

Example: A="HELLO WORLD" typeset -l A=$A