bash Cookbook

Reversing the Words on Each Line

problem

You want to print the input lines with words in the reverse order.

Solution

  $ awk '{
>      for (i=NF; i>0; i--) {
>          printf "%s ", $i;
>    }
>    printf "\n"
> }'

You don’t type the > characters; the shell will print those as a prompt to say that you haven’t ended your command yet (it is looking for the matching single-quote mark). Because the awk program is enclosed in single quotes, the bash shell lets us type multiple lines, prompting us with the secondary prompt > until we supply the matching end quote. We spaced out the program for readability, even though we could have stuffed it all onto one line like this:

$ awk '{for (i=NF; i>0; i--) {printf "%s ", $i;} printf "\n" }'

Discussion

The awk program has syntax for a for loop, very much in the C language style. It even supports a printf mechanism for formatted output, again modeled after the C language version (or the bash version, too). We use the for loop to count down from the last to the first field, and print each field as we go. We deliberately don’t put a \n on that first printf because we want to keep the several fields on the same line of output. When the loop is done, we add a newline to terminate the line of output.

The reference to $i is very different in awk compared to bash. In bash, when we write $i we are getting at the value stored in the variable named i. But in awk, as with most programming languages, we simply reference the value in i by naming it—that is by just writing i. So what ...