Filename Expansion

Now that we have our files renamed with the prefix “original-”, let’s reverse course and take off the prefix. By placing the variable’s name within curly braces, e.g., ${filename}, you gain new powers—like the ability to modify the variable’s value when you extract it.

for filename in *.dat
do
    echo mv $filename ${filename#original-}
done

The # notation removes text from the beginning of a variable’s value. So this loop would print:

mv original-basilisk.dat basilisk.dat
mv original-unicorn.dat unicorn.dat

And using % instead of # removes text from the end:

for filename in *.dat
do
    echo mv $filename ${filename%.dat}
done

prints:

mv original-basilisk.dat original-basilisk
mv original-unicorn.dat original-unicorn

Avoid confusing variable names and text

Sometimes you may want to add something to the end of a variable’s value. For example, you might add “backup” to the end of your files’ names:

for filename in *.dat
do
    echo mv $filenamebackup
done

Oops: each time through the loop the shell looks for a variable named filenamebackup, which doesn’t exist. To avoid confusing our $filename variable with the text “backup”, we can use curly braces:

for filename in *.dat
do
    echo mv $filename ${filename}backup
done

which prints:

mv original-basilisk.dat original-basilisk.datbackup
mv original-unicorn.dat original-unicorn.datbackup