Editing Shell script (section)

===Generalization===
Simple batch jobs are not unusual for isolated tasks, but using shell loops, tests, and variables provides much more flexibility to users. A POSIX sh script to convert JPEG images to PNG images, where the image names are provided on the command-line—possibly via wildcards—instead of each being listed within the script, can be created with this file, typically saved in a file like <code>/home/''username''/bin/jpg2png</code>

<syntaxhighlight lang="sh">
#!/bin/sh
for jpg; do                                  # use $jpg in place of each filename given, in turn
    png=${jpg%.jpg}.png                      # construct the PNG version of the filename by replacing .jpg with .png
    printf 'converting "%s" ...\n' "$jpg"    # output status info to the user running the script
    if convert "$jpg" jpg.to.png; then       # use convert (provided by ImageMagick) to create the PNG in a temp file
        mv jpg.to.png "$png"                 # if it worked, rename the temporary PNG image to the correct name
    else                                     # ...otherwise complain and exit from the script
        printf >&2 'jpg2png: error: failed output saved in "jpg.to.png".\n'
        exit 1
    fi                                       # the end of the "if" test construct
done                                         # the end of the "for" loop
printf 'all conversions successful\n'        # tell the user the good news
</syntaxhighlight>

The <code>jpg2png</code> command can then be run on an entire directory full of JPEG images with just <code>/home/''username''/bin/jpg2png *.jpg</code>