10.10 Simulated Bourne Shell Functions and Aliases

If you have a Bourne shell with no functions (10.9) or aliases (10.2), you can do a lot of the same things with shell variables and the eval (8.10) command.

Let's look at an example. First, here's a shell function named scp (safe copy). If the destination file exists and isn't empty, the function prints an error message instead of copying:

test	scp() { if test ! -s "$2" then cp "$1" "$2" else echo "scp: cannot copy $1: $2 exists" fi }

If you use the same scp twice, the first time you'll make bfile. The second time you try, you see the error:

$ scp afile bfile
   ...
$ scp afile bfile
scp: cannot copy afile: bfile exists

Here's the same scp-stored in a shell variable instead of a function:

scp='
if test ! -s "$2"
then cp "$1" "$2"

else echo "scp: cannot copy $1: $2 exists"
fi
'

Because this fake function uses shell parameters, you have to add an extra step: setting the parameters. Simpler functions are easier to use:

set	$ set afile bfile $ eval "$scp" ... $ eval "$scp" scp: cannot copy afile: bfile exists