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Full Discussion: bash shell: 'exec', 'eval', 'source' - looking for help to understand
Top Forums Shell Programming and Scripting bash shell: 'exec', 'eval', 'source' - looking for help to understand Post 302169844 by jim mcnamara on Friday 22nd of February 2008 02:20:38 PM
Old 02-22-2008
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"

This User Gave Thanks to jim mcnamara For This Post:
 

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LEARN ABOUT OPENDARWIN

script

SCRIPT(1)						    BSD General Commands Manual 						 SCRIPT(1)

NAME

     script -- make typescript of terminal session

SYNOPSIS

     script [-a] [-k] [-q] [-t time] [file [command ...]]

DESCRIPTION

     The script utility makes a typescript of everything printed on your terminal.  It is useful for students who need a hardcopy record of an
     interactive session as proof of an assignment, as the typescript file can be printed out later with lpr(1).

     If the argument file is given, script saves all dialogue in file.	If no file name is given, the typescript is saved in the file typescript.

     If the argument command ... is given, script will run the specified command with an optional argument vector instead of an interactive shell.

     Options:

     -a       Append the output to file or typescript, retaining the prior contents.

     -k       Log keys sent to program as well as output.

     -q       Run in quiet mode, omit the start and stop status messages.

     -t time  Specify time interval between flushing script output file.  A value of 0 causes script to flush for every character I/O event.  The
	      default interval is 30 seconds.

     The script ends when the forked shell (or command) exits (a control-D to exit the Bourne shell (sh(1)), and exit, logout or control-d (if
     ignoreeof is not set) for the C-shell, csh(1)).

     Certain interactive commands, such as vi(1), create garbage in the typescript file.  The script utility works best with commands that do not
     manipulate the screen.  The results are meant to emulate a hardcopy terminal, not an addressable one.

ENVIRONMENT

     The following environment variable is utilized by script:

     SHELL  If the variable SHELL exists, the shell forked by script will be that shell.  If SHELL is not set, the Bourne shell is assumed.  (Most
	    shells set this variable automatically).

SEE ALSO

     csh(1) (for the history mechanism).

HISTORY

     The script command appeared in 3.0BSD.

BUGS

     The script utility places everything in the log file, including linefeeds and backspaces.	This is not what the naive user expects.

     It is not possible to specify a command without also naming the script file because of argument parsing compatibility issues.

     When running in -k mode, echo cancelling is far from ideal.  The slave terminal mode is checked for ECHO mode to check when to avoid manual
     echo logging.  This does not work when in a raw mode where the program being run is doing manual echo.

BSD
								   June 6, 1993 							       BSD
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