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

In spite of how old this question is and similar to this answer by technosaurus. I had a hard time finding a solution that was portable across most platforms (That I Use) as well as older versions of bash. I have also been frustrated with arrays, functions and use of prints, echos and temporary files to retrieve trivial variables. This works very well for me so far I thought I would share. My main testing environments are:

1. GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
2. GNU bash, version 3.2.57(1)-release (sparc-sun-solaris2.10)

lcs="abcdefghijklmnopqrstuvwxyz"
ucs="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
input="Change Me To All Capitals"
for (( i=0; i<"${#input}"; i++ )) ; do :
    for (( j=0; j<"${#lcs}"; j++ )) ; do :
        if [[ "${input:$i:1}" == "${lcs:$j:1}" ]] ; then
            input="${input/${input:$i:1}/${ucs:$j:1}}" 
        fi
    done
done

Simple C-style for loop to iterate through the strings. For the line below if you have not seen anything like this before this is where I learned this. In this case the line checks if the char ${input:$i:1} (lower case) exists in input and if so replaces it with the given char ${ucs:$j:1} (upper case) and stores it back into input.

input="${input/${input:$i:1}/${ucs:$j:1}}"