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

Question

I have been searching to find a way to convert a string value from upper case to lower case. All the search results show approaches of using tr command.

The problem with the tr command is that I am able to get the result only when I use the command with echo statement. For example:

y="HELLO" echo $y| tr '[:upper:]' '[:lower:]' 

The above works and results in 'hello', but I need to assign the result to a variable as below:

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

When assigning the value like above it gives me an empty result.

PS: My Bash version is 3.1.17

Answer 1

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.

Answer 2

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"