"eval" Command

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eval - construct command by concatenating arguments

The eval utility shall construct a command by concatenating arguments together, separating each with a character. The constructed command shall be read and executed by the shell.

root:~# man eval

root:~# info eval

Example-1

1.1 Execute Command stored in string

root@:~# ll="ls -ltrh"
root@:~# eval $ll
drwxr-xr-x 2 root root 4.0K Dec  7 05:57 segments_24Mbps
drwxr-xr-x 2 root root 4.0K Dec  7 06:33 segments_1Mbps
-rw-r--r-- 1 root root   58 Dec 28 18:01 dns_name.txt
-rw-r--r-- 1 root root   14 Dec 28 20:06 temp_file
-rw-r--r-- 1 root root   18 Jan  3 06:44 test.txt
root@:~# eval ll
-bash: ll: command not found

As shown, eval command first evaluates the argument and then runs the command stored in the argument.

Example-2

2.1 Evaluate Command

root@:~# foo=100
root@:~# x=foo
root@:~# y='$'$x
root@:~# echo $y
$foo
root@:~# eval y='$'$x
root@:~# echo $y
100

Line by Line explanation:
=> In 1^st line, we have defined foo variable. => In 2^nd line, we have defined x variable. => In 3^rd line, we have defined y variable which consists of string '$foo'. The dollar sign must be escaped with '$'. => In 4^th line, we have print the result of y variable. => In 5^th line, we have re-define y variable with eval. Here, it will first evaluate $x to the string 'foo'. Now the statement y=$foo will be evaluated to y=100. => In 6^th line, we have print the result of y variable.