How to convert a string to lowercase in Bash

This note will show you how to convert a string to lowercase (small letters) on the Linux command line.

To convert a string to lower case regardless of its current case, use one of the following commands.

tr

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

Attention! If you want to change the case of any letters other than Latin (national alphabets, letters with diacritics), then do not use tr, but use any other solution suggested below. This is because the classic Unix tr operates on single-byte characters and is not compatible with Unicode.

AWK

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

Bash

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

Starting with Bash 5.1, there is a conversion option “L”, which is intended to convert a string to lowercase:

${var@L}

Example:

v="heLLo"
echo "${v@L}"
hello

sed

echo "Hi all" | sed -e 's/\(.*\)/\L\1/'
hi all

Or this solution:

echo "Hi all" | sed -e 's/\(.*\)/\L\1/' <<< "$a"
hi all

Another solution:

echo "Hi all" | sed 's/./\L&/g'

Perl

echo "Hi all" | perl -ne 'print lc'
hi all

Python

a="Hi all"
b=`echo "print ('$a'.lower())" | python`; echo $b

Ruby

a="Hi all"
b=`echo "print '$a'.downcase" | ruby`; echo $b

PHP

b=`php -r "print strtolower('$a');"`; echo $b

NodeJS

b=`node -p "\"$a\".toLowerCase()"`; echo $b

In zsh

a="Hi all"
echo $a:l