eval in shell scripting

mobydick · #1 (**permalink**) 05-21-2008

what is the real usage of eval in shell scripting

whats the difference between
$ cmd

&&
eval cmd

era · #2 (**permalink**) 05-21-2008

eval is useful when cmd contains something which needs to be evaluated by the shell. The typical case would be when you have a variable containing the index of an argument you want to access.

Code:

n=1  # or whatever, just to make this self-contained
eval echo \${$n}

Search these forums for examples; some of them are rather elaborate.

mobydick · #3 (**permalink**) 05-21-2008

but whats the difference if i write

echo $($n) instead of

eval echo \${$n}

era · #4 (**permalink**) 05-21-2008

Experiment to see.

Code:

vnix$ n=1
vnix$ echo $($n)
bash: 1: command not found
vnix$ n="date"
vnix$ echo $($n)
Wed May 21 20:39:00 EEST 2008

So it uses the value of $n as a command. (It's the same as echo `$n` with backticks.)

Contrast:

Code:

vnix$ set -- one two three  # sets $1 $2 $3
vnix$ echo $1
one
vnix$ n=1
vnix$ echo ${$n}  # attempt to echo $1
bash: ${$n}: bad substitution
vnix$ eval echo \${$n}
one

See? Now if you assign n=2 it will echo the value of $2, etc. So you can change the name of the variable you are referring to programmatically, dynamically.

This is an advanced topic; if you don't have a use for it, don't bother. If you really want to understand this, I suggest you play around with set -x and try different things until you understand what's going on.

mobydick · #5 (**permalink**) 05-22-2008

thanks its convincing and clear ..

thanks for the help

mobydick · #6 (**permalink**) 05-22-2008

is there any way to include or import one script into other
like application programs

nua7 · #7 (**permalink**) 05-22-2008

Include as in..? You can call one script from the other. That can work.

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