## 29.14. Simulated Bourne Shell Functions and Aliases

Until System V Release 2 (circa 1984), the Bourne shell had no way for users to set up their own built-in commands. If you have a Bourne shell with no functions (Section 29.11) or aliases (Section 29.2) and haven't yet turned the host machine into a wet bar, CD/DVD storage case, or some other pragmatic but fun use for a 30-year-old computer, you can do a lot of the same things with shell variables and the eval (Section 27.8) command.

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

`test` Section 35.26

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

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

>     $ cps afile bfile
>        ...
>     $ cps afile bfile
>     cps: cannot copy afile: bfile exists

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

>     cps='
>     if test ! -s "$2"
>     then cp "$1" "$2"
>     else echo "cps: 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` Section 35.25

>     $ set afile bfile
>     $ eval "$cps"
>        ...
>     $ eval "$cps"
>     cps: cannot copy afile: bfile exists

-- JP

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