How to convert a string to lower case in Bash?

Question

Is there a way in bash to convert a string into a lower case string?

For example, if I have:

a=\"Hi all\"

I want to convert it to:<

Answer 1

Many answers using external programs, which is not really using Bash.

If you know you will have Bash4 available you should really just use the ${VAR,,} notation (it is easy and cool). For Bash before 4 (My Mac still uses Bash 3.2 for example). I used the corrected version of @ghostdog74 's answer to create a more portable version.

One you can call lowercase 'my STRING' and get a lowercase version. I read comments about setting the result to a var, but that is not really portable in Bash, since we can't return strings. Printing it is the best solution. Easy to capture with something like var="$(lowercase $str)".

How this works

The way this works is by getting the ASCII integer representation of each char with printf and then adding 32 if upper-to->lower, or subtracting 32 if lower-to->upper. Then use printf again to convert the number back to a char. From 'A' -to-> 'a' we have a difference of 32 chars.

Using printf to explain:

$ printf "%d\n" "'a"
97
$ printf "%d\n" "'A"
65

97 - 65 = 32

And this is the working version with examples.
Please note the comments in the code, as they explain a lot of stuff:

#!/bin/bash

# lowerupper.sh

# Prints the lowercase version of a char
lowercaseChar(){
    case "$1" in
        [A-Z])
            n=$(printf "%d" "'$1")
            n=$((n+32))
            printf \\$(printf "%o" "$n")
            ;;
        *)
            printf "%s" "$1"
            ;;
    esac
}

# Prints the lowercase version of a sequence of strings
lowercase() {
    word="$@"
    for((i=0;i<${#word};i++)); do
        ch="${word:$i:1}"
        lowercaseChar "$ch"
    done
}

# Prints the uppercase version of a char
uppercaseChar(){
    case "$1" in
        [a-z])
            n=$(printf "%d" "'$1")
            n=$((n-32))
            printf \\$(printf "%o" "$n")
            ;;
        *)
            printf "%s" "$1"
            ;;
    esac
}

# Prints the uppercase version of a sequence of strings
uppercase() {
    word="$@"
    for((i=0;i<${#word};i++)); do
        ch="${word:$i:1}"
        uppercaseChar "$ch"
    done
}

# The functions will not add a new line, so use echo or
# append it if you want a new line after printing

# Printing stuff directly
lowercase "I AM the Walrus!"$'\n'
uppercase "I AM the Walrus!"$'\n'

echo "----------"

# Printing a var
str="A StRing WITH mixed sTUFF!"
lowercase "$str"$'\n'
uppercase "$str"$'\n'

echo "----------"

# Not quoting the var should also work, 
# since we use "$@" inside the functions
lowercase $str$'\n'
uppercase $str$'\n'

echo "----------"

# Assigning to a var
myLowerVar="$(lowercase $str)"
myUpperVar="$(uppercase $str)"
echo "myLowerVar: $myLowerVar"
echo "myUpperVar: $myUpperVar"

echo "----------"

# You can even do stuff like
if [[ 'option 2' = "$(lowercase 'OPTION 2')" ]]; then
    echo "Fine! All the same!"
else
    echo "Ops! Not the same!"
fi

exit 0

And the results after running this:

$ ./lowerupper.sh 
i am the walrus!
I AM THE WALRUS!
----------
a string with mixed stuff!
A STRING WITH MIXED STUFF!
----------
a string with mixed stuff!
A STRING WITH MIXED STUFF!
----------
myLowerVar: a string with mixed stuff!
myUpperVar: A STRING WITH MIXED STUFF!
----------
Fine! All the same!

This should only work for ASCII characters though.

For me it is fine, since I know I will only pass ASCII chars to it.
I am using this for some case-insensitive CLI options, for example.