Linux Shell Scripting Essentials

Construct commands using eval

The eval command is a shell builtin command used to construct a command by concatenating arguments passed to eval. A concatenated command is further executed by shell and returns a result. If no arguments are given to eval, it returns 0.

The syntax of the eval command is as follows:

eval [arg …]

The following example shows the expansion of a variable to the name of another variable using eval:

$ name=foo
$ foo="Welcome to foo world"
$ echo $name
foo
$ new_name='$'$name    #new_name just stores string value $foo
$ echo $new_name
$foo
$ eval new_name='$'$name  # eval processes $foo string into variable and  prints                 # foo variable value
Welcome to foo world

Another example where eval can be useful is as follows:

$ pipe="|" $ df ...