Shell – What are some useful use-cases for eval in Shell

evalshell

You hear it a lot eval is evil, whether it's in Shell/POSIX world, or in other langs like python etc…

But I'm wondering, is it actually useless? or is it there some arcane, non-documented, interesting or just useful use-cases for it?

Would prefer if the answer is sh/bash centric, but it's fine if it's about other Shells too.

PS: I'm well aware why eval is considered evil.

Best Answer

I know of two ... common ... use cases for eval:

Argument processing with getopt:

[T]his implementation can generate quoted output which must once again be interpreted by the shell (usually by using the eval command).
Setting up an SSH agent:

[T]he agent prints the needed shell commands (either sh(1) or csh(1) syntax can be generated) which can be evaluated in the calling shell, eg eval `ssh-agent -s` for Bourne-type shells such as sh(1) or ksh(1) and eval `ssh-agent -c` for csh(1) and derivatives.

Both uses might have alternatives, but I wouldn't bat an eyelid on seeing either of them.

Related Solutions

Bash – What is the ‘eval’ Command?

eval is part of POSIX. Its an interface which can be a shell built-in.

Its described in the "POSIX Programmer's Manual": http://www.unix.com/man-page/posix/1posix/eval/

eval - construct command by concatenating arguments

It will take an argument and construct a command of it, which will be executed by the shell. This is the example of the manpage:

1) foo=10 x=foo
2) y='$'$x
3) echo $y
4) $foo
5) eval y='$'$x
6) echo $y
7) 10

In the first line you define $foo with the value '10' and $x with the value 'foo'.
Now define $y, which consists of the string '$foo'. The dollar sign must be escaped with '$'.
To check the result, echo $y.
The result will be the string '$foo'
Now we repeat the assignment with eval. It will first evaluate $x to the string 'foo'. Now we have the statement y=$foo which will get evaluated to y=10.
The result of echo $y is now the value '10'.

This is a common function in many languages, e.g. Perl and JavaScript. Have a look at perldoc eval for more examples: http://perldoc.perl.org/functions/eval.html

Shell – Keep exit codes when trapping SIGINT and similar

All you need to do is change the EXIT handler inside your cleanup handler. Here's an example:

#!/bin/bash
cleanup() {
    echo trapped exit
    trap 'exit 0' EXIT
}
trap cleanup EXIT
read -p 'If you ctrl-c me now my return code will be the default for SIGTERM. '

Best Answer

Related Solutions

Bash – What is the ‘eval’ Command?

Shell – Keep exit codes when trapping SIGINT and similar

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