An example of using eval

When learning (or teaching) eval, the first - and very valuable - example of it is to see eval's ability to evaluate the value of the last positional parameter:

$ set a b c d

$ eval print -- \$$#

d

We use eval it to force the shell to perform a double scan, when we need to find a value of a value. First scan will evaluate the $# to the number of available parameters (in this case: 4); the second scan will check the value of it (d). This is 'bread and butter' of eval application.

Other cases of using eval are a bit more obscure, and for that reason eval is often forgotten...

Here is an example that is not 'that' obscure - and possible quite useful. A delegate wanted to rename all files in a given directory, to change the character case from lower to upper. What he devised, successfully, is as follows:

for file in *

do

mv $file $(echo $file | tr '[a-z]' '[A-Z]')

done


This worked, until he found that some of the files contained 'unfriendly' characters: spaces. The script failed to cope with this.

This is where eval stepped in:

for file in *

do

eval mv '"$file"' \"$(echo "$file" | tr '[a-z]' '[A-Z]' )\"

done

In this case eval allows to insert an additional set of double quotes, which protected spaces embedded in file names.