What is the "eval" command in bash?

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

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.

Say that you want to code an equivalent of VAR=value; echo $VAR. Note the difference in how the shell handles the writings of echo VAR=value:

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

   The shell tries to execute echo and VAR=value as two separate commands. It throws an error about the second string. The assignment remains ineffective.

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

   The shell merges (concatenates) the two strings echo and VAR=value, parses this single unit according to appropriate rules and executes it.

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.

The eval statement tells the shell to take eval’s arguments as command and run them through the command-line. It is useful in a situation like below:

In your script if you are defining a command into a variable and later on you want to use that command then you should use eval:

/home/user1 > a="ls | more"
/home/user1 > $a
bash: command not found: ls | more
/home/user1 > # Above command didn't work as ls tried to list file with name pipe (|) and more. But these files are not there
/home/user1 > eval $a
file.txt
mailids
remote_cmd.sh
sample.txt
tmp
/home/user1 >