I periodically receive files that are somewhat cryptically named, and I like to rename them to something more understandable.

For example, I might receive several files whose names are something like this:

TS_412_96k.fil
TS_413_96k.fil
TS_414_32k.fil

And I want to rename them something like:

Total Entry #412.txt
Total Entry #413.txt
Total Entry #414.txt

After nearly tearing my hair out trying various combinations of awk and sed in my scripts, I discovered that the bash shell has its own string handling functions! This quickly allowed me create the following simple script:

for prog in *.fil; do
    progno=${prog:3:3}
    mv $prog 'Total Entry #'$progno'.txt'
done

Voila! Note that bash starts character numbering at zero rather than one, as do awk and sed. Thus, in the example above, the substring returned by {prog:3:3} would be the numeric values in the original filenames.