How to convert a string from uppercase to lowercase in Bash? [duplicate]

If you are using bash 4 you can use the following approach:

x="HELLO"
echo $x  # HELLO

y=${x,,}
echo $y  # hello

z=${y^^}
echo $z  # HELLO

Use only one , or ^ to make the first letter lowercase or uppercase.

The correct way to implement your code is

y="HELLO"
val=$(echo "$y" | tr '[:upper:]' '[:lower:]')
string="$val world"

This uses $(...) notation to capture the output of the command in a variable. Note also the quotation marks around the string variable -- you need them there to indicate that $val and world are a single thing to be assigned to string.

If you have bash 4.0 or higher, a more efficient & elegant way to do it is to use bash builtin string manipulation:

y="HELLO"
string="${y,,} world"

Note that tr can only handle plain ASCII, making any tr-based solution fail when facing international characters.

Same goes for the bash 4 based ${x,,} solution.

The awk tool, on the other hand, properly supports even UTF-8 / multibyte input.

y="HELLO"
val=$(echo "$y" | awk '{print tolower($0)}')
string="$val world"

Answer courtesy of liborw.