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 $pipe wc  # Will give error because 
df: '|': No such file or directory
df: 'wc': No such file or directory
$ eval df $pipe wc  # eval executes it as shell command
12      73     705

Here, the df command shows a system disk's usage:

A shell script showing the use of eval is as follows:
#!/bin/bash
#Filename: eval.sh
#Description: Evaluating string as a command using eval

cmd="ls /usr"
echo "Output of command $cmd -"
eval $cmd   #eval will treat content of cmd as shell command and execute it
cmd1="ls /usr | wc -l"
echo "Line count of /usr -"
eval $cmd1

expression="expr 2 + 4 \* 6"
echo "Value of $expression"
eval $expression

Running the script will give you the following result:

Output of command ls /usr -
avr  bin  games  include  lib  lib64  libexec  local  sbin  share  src  tmp
Line count of /usr -
12
Value of expr 2 + 4 \* 6
26