Question

I’m trying to understand how the eval statement works. Here’s a snippet of code I’m looking at that I don’t understand:

    eval `$SOME_DIR/util/dbget.pl \
          VARIABLE1 \
          VARIABLE2 \
          VARIABLE3 \
          VARIABLE4 `

Anyways my initial idea after some research is that eval just literally types in those strings in the command line, meaning it runs the .pl script, then… I’m not sure what it’s doing with those variables. The comment above this code said “get a few parameters” but I’m not sure what that means.

Does eval ‘VARIABLE’ initialize the variable? This seems to be the case because after the eval the code goes on to use these variables as if they’re now initialized (does if statement checks, etc).

I’m clearly new to this kind of scripting and just jumping into a large repository. Thanks for any help.

Answer 1

Ok, the backticks are replaced with the output of the command between them.

The eval takes that output and executes it as if typed.

In this case I assume the perl is printing out some variable assignments, like set VARIABLE1=value1 or setenv VARIABLE1 value1. In which case, after the above eval, you should have $VARIABLE1 available to later csh code.

Just running the .pl script (which I assume is Perl) without the eval would not affect the environment of the csh process. By using eval, you’re giving the Perl code permission to modify that environment.

This means the Perl has to know that its output is intended for csh, btw. In sh/bash the syntax of variable assignment is different.

ETA More attempted clarification:

If you do this:

$SOME_DIR/util/dbget.pl

You get a bunch of variable assignment statements printed to your screen. This is not useful, and has no effect whatsoever on the actual variables in your shell.

If you do this:

echo `$SOME_DIR/util/dbget.pl`

You get the same result in a more roundabout fashion: instead of the perl code printing directly to your screen, it’s printing into an expression that gets passed to the csh echo command, and then the echo is what does the actual printing to the screen. That distinction is subtle, but important: the backticks take control away from the perl and put it back in the hands of the shell. Once the shell has captured the output, it can store it in a variable, write it into a file, or do lots of other things with it that the Perl code doesn’t have to be rewritten to deal with. The Perl is still just printing.

If you do this:

eval `$SOME_DIR/util/dbget.pl`

Then what the shell does is execute the output of the Perl as if it were typed at the prompt. This is very powerful. It’s also dangerous, because the surrounding program winds up executing code that the person who wrote that program has never seen. So you have to trust the source of that code (in this case the perl script) not to produce anything damaging (intentinonally or otherwise).