Understand eval, exec and source in linux

eval “translates” a value buried inside a variable, and then runs the command that was buried in there

Code:

for i in 1 2 3
do
   eval myvar="$i"
   echo "$myvar"
done
# this gives 
1
2
3
# why? because there is no metavalue or special meaning to 1 or 2 or 3

Code:

for i in ls df
do
    eval myvar="$i"
    echo "$myvar"
done
# here you get output from the ls command and the df command

exec starts another process – BUT – it exits the current process when you do this kind of thing

Code:

#!/bin/bash

exec echo "leaving this script forever  $0"   # Exit from script here.

# ----------------------------------
# The following line never happens

echo "This echo will never echo."

source

When you run a command in the shell – like another script or a command like ls –
the shell creates a subprocess (called child process). Any environment variable that got defined or changed down in the child is LOST FOREVER to the parent process.

However if you source a script (there are two ways) you force the script to run in the current process. That means environment variables in the script you ran are NOT LOST.

Code:

# script1
MYVAR="HI there!"
# end script1

Code:

# script2
# first, run script 1
script1
# now is the MYVAR variable set?  No -why? it got set in the child, when the child exited it GOT LOST
echo "MYVAR= $MYVAR"
# now source script1 :: putting a lonesome dot or the command  "source" 
#  in front of the script name have the same effect
. script1
#  . script1 == source script1  these two are the SAME THING
source script1
# now is the MYVAR variable set?  This time we see that MYVAR has a value
echo "MYVAR= $MYVAR"