[SOLVED] Effect of using eval to execute a command as opposed to writing it on the commandline

daudiam · 06-19-2010, 11:59 PM

Code:

cmd='date | wc' or cmd="date | wc"
$cmd

If this script is executed, an error is generated. The reason written was that "The execution fails because the pipe is not expanded and is passed to date as an argument". What is meant by expansion of pipe. When we execute date | wc on the command line, it goes fine. then | is not treated as an argument. Why?

If I write

Code:

cmd='date | wc' or cmd="date | wc"
eval $cmd

It works. How?

grail · 06-20-2010, 01:11 AM

Well your initial error is to do with quoting. Using full quoting, ie single quote, tells the shell not to expand special charachters.
Therefore, your pipe is now a character and no longer has its special meaning.

eval on the other hand takes a string as argument and now evaluates the entire string as if you were at the command line, hence the command now works
as the pipe is now a redirect.