How to Dynamically Build Command and Execute

In Bash we can dynamically build and execute the commands. Below we can find two examples of building command stored in variable and then execute the commands to get result:

(1) eval - Build command and execute

cmd="echo Hello World!"
cmd_file="> out.txt"
eval ${cmd} ${cmd_file}

We build command: echo Hello World! > out.txt from two variables and then execute it with eval
The result is creating a new file out.txt and writing text: Hello World!

(2) $($cmd) - Execute command

cmd="ls -a"
res=$($cmd)
echo $res

Alternative solution is to use $($cmd) to run a command which is stored in a variable. The result will be:

create variable with ls -a command
execute the command
print out the result

(3) Dynamically Build Command in script

#!/bin/bash
cmd="ls -a"
res=$($cmd)
echo $res

Saving the above as test.sh. Next we can run the script by ./test.sh.