[colug-432] Gracefully Doing Nothing

[jep200404 at columbus.rr.com]

How does one do nothing gracefully in a Bourne shell for loop?

A example that fails to do nothing gracefully follows.

    jep at colug:~ $ cat ~/bin/backup 
    #!/bin/sh

    dir=~/stuff
    prefix=`date --iso=ns`

    cd "$dir"
    for f in [a-z]*; do
        mv "$f" "$prefix"-"$f"
    done
    jep at colug:~ $ mkdir stuff
    jep at colug:~ $ cd stuff/
    jep at colug:~/stuff $ ll ~ >foo 
    jep at colug:~/stuff $ ll 
    total 4
    -rw-r--r-- 1 pi pi 1439 Jan 30 12:43 foo
    jep at colug:~/stuff $ backup 
    jep at colug:~/stuff $ ll 
    total 4
    -rw-r--r-- 1 pi pi 1439 Jan 30 12:43 2017-01-30T12:43:34,754775510-0500-foo
    jep at colug:~/stuff $ backup 
    mv: cannot stat ‘[a-z]*’: No such file or directory
    jep at colug:~/stuff $ 

In this example the complaint from mv causes no problem,
but it would be better for the loop to not execute
the body when there are no matching files.