What is the “eval” command in bash?

Question

What can a man do with the "eval" command, why is it usefull? It's some kind of built-in function in bash? There is no "man" page for it..

Answer 1

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

1. In the first line you define $foo with the value '10' and $x with the value 'foo'.
2. Now define $y, which consists of the string '$foo'. The dollar sign must be escaped with '$'.
3. To check the result, echo $y.
4. The result will be the string '$foo'
5. 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.
6. 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

Answer 2

Yes, eval is a bash internal command so it is described in bash man page.

eval [arg ...]
    The  args  are read and concatenated together into a single com-
    mand.  This command is then read and executed by the shell,  and
    its  exit status is returned as the value of eval.  If there are
    no args, or only null arguments, eval returns 0.

Usually it is used in combination with a Command Substitution. Without an explicit eval, the shell tries to execute the result of a command substitution, not to evaluate it.

Note the difference:

1. The shell tries to execute VAR=value as a command:

   andcoz@...:~> $( echo VAR=value )
   bash: VAR=value: command not found
   

2. The shell evaluates the command:

   andcoz@...:~> eval $( echo VAR=value )
   andcoz@...:~> echo $VAR
   value
   

Last but not least, eval can be a very dangerous command. Any input to an eval command must be carefully checked to avoid security problems.

Answer 3

eval has no man page because it is not a separate external command, but rather a shell built-in, meaning a command internal to and known only by the shell (bash). The relevant part of the bash man page says:

eval [arg ...]
    The args are read and concatenated together into a single command.  
    This command is then  read  and executed by the shell, and its exit 
    status is returned as the value of eval.  If there are no args, or only 
    null arguments, eval returns 0

In addition, the output if help eval is:

eval: eval [arg ...]
    Execute arguments as a shell command.

    Combine ARGs into a single string, use the result as input to the shell,
    and execute the resulting commands.

    Exit Status:
    Returns exit status of command or success if command is null.

eval is a powerful command and if you intend to use it you should be very careful to head off the possible security risks that come with it.