bash Cookbook

Taking It One Character at a Time

Problem

You have some parsing to do and for whatever reason nothing else will do—you need to take your strings apart one character at a time.

Solution

The substring function for variables will let you take things apart and another feature tells you how long a string is:

#!/usr/bin/env bash
# cookbook filename: onebyone
#
# parsing input one character at a time

while read ALINE
do
    for ((i=0; i < ${#ALINE}; i++))
    do
        ACHAR=${ALINE:i:1}
        # do something here, e.g. echo $ACHAR
        echo $ACHAR
    done
done

Discussion

The read will take input from standard in and put it, a line at a time, into the variable $ALINE. Since there are no other variables on the read command, it takes the entire line and doesn’t divvy it up.

The for loop will loop once for each character in the $ALINE variable. We can compute how many times to loop by using ${#ALINE}, which returns the length of the contents of $ALINE.

Each time through the loop we assign ACHAR the value of the one-character substring of ALINE that begins at the ith position. That’s simple enough. Now, what was it you needed to parse this way?

See Also

• Check out the other parsing techniques in this chapter to see if you can avoid working at this low level