bash shell: the difference among 'exec', 'eval', 'source'

 

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

Code:

for i in 1 2 3do   eval myvar="$i"   echo "$myvar"done# this gives 123# why? because there is no metavalue or special meaning to 1 or 2 or 3

Code:

for i in ls dfdo    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 temporally when you do this kind of thing until the new process is finished

Code:

#!/bin/bashexec echo "leaving this script forever  $0"   # Exit from script here.# ----------------------------------# The following line never happensecho "This echo will never echo."

moreover, a "&" tail means that run the new process in backgroud.

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:

# script1MYVAR="HI there!"# end script1

Code:

# script2# first, run script 1script1# now is the MYVAR variable set?  No -why? it got set in the child, when the child exited it GOT LOSTecho "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 THINGsource script1# now is the MYVAR variable set?  This time we see that MYVAR has a valueecho "MYVAR= $MYVAR"