How can I 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 uppercase to lowercase. All the search results show approaches of using the tr command.

The problem with the tr command is that I am able to get the result only when I use the command with the 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

One 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 built-in string manipulation:

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

Answer 3

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.

Answer 4

Execute in backticks:

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

This assigns the result of the command in backticks to the variable x. (I.e., it's not particular to tr, but it is a common pattern/solution for shell scripting.)

You can use $(..) instead of the backticks. See here for more info.

Answer 5

I'm on Ubuntu 14.04 (Trusty Tahr), with Bash version 4.3.11. However, I still don't have the fun built-in string manipulation ${y,,}

This is what I used in my script to force capitalization:

CAPITALIZED=`echo "${y}" | tr '[a-z]' '[A-Z]'`

Answer 6

If you define your variable using declare (old: typeset) then you can state the case of the value throughout the variable's use.

declare -u FOO=AbCxxx
echo $FOO

Output:

ABCXXX

Option -l to declare does lowercase:

When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.

Answer 7

Building on Rody's answer, this worked for me.

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

One small modification: if you are using underscore next to the variable, you need to encapsulate the variable name in {}.

string="${val}_world"