Looping

In a bash loop, we are looping over the indices of the characters. Bash strings and arrays use zero-based indexing.

reversed=''
for ((i = 0; i < ${#string}; i++)); do
    reversed="${string:i:1}$reversed"
done

• We loop from zero up to (but not including) the string length (${#string}).
• Extracting the character at index i is done with the ${var:offset:length} parameter expansion.
• We prepend the character to the accumulating variable to reverse the string.

Finding the string length is a surprisingly expensive operation in bash (more details in the Performance article). We don't have to re-calculate it for every loop iteration, just do it once.

reversed=''
len=${#string}
for ((i = 0; i < len; i++)); do
    reversed="${string:i:1}$reversed"
done

An alternate way to calculate it just once is to loop backwards.

reversed=''
for ((i = ${#string} - 1; i >= 0; i--)); do
    reversed+="${string:i:1}"
done

• Here, we start the loop at one less than the string length, which is the index of the last character, and we loop down to (and including) zero.
• Since we're accessing the characters in the reverse order, we'll append to the accumulating variable.

Another performance note: accessing each character with this parameter expansion is still slow: bash has to walk the string until reaching the desired index. The most efficient solution is discussed in the Performance article).