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Top Forums UNIX for Dummies Questions & Answers difference between source, exec and ./script Post 2037 by mib on Sunday 15th of April 2001 09:24:44 AM
Old 04-15-2001
./script creates a new shell and executes each command in the script within the new shell. When the end of the script file is encountered, the new shell exits. Any changes in the new shell caused by the script are lost when the shell exits.

For example, if the file
/home/user/sample/script/test contains...

cd /usr/games
pwd
echo hi

Then the command sequence would yield the following output:

prompt>cd /home/user/sample/script
prompt>pwd
/home/user/sample/script
prompt>chmod +x test
prompt>./test
/usr/games
hi
prompt>pwd
/home/user/sample/script

source Execution
source execute a shell script within the context of the current shell. Since execution takes place within the context of the current shell, any changes in the shell are retained following the completion of the shell.
Example:

prompt>cd /home/user/sample/script
prompt>pwd
/home/user/sample/script
prompt>source test
/usr/games
hi
prompt>pwd
/usr/games


Execing a <command> (ie. shell script or executable) means give exec <command> on the shell prompt.

The exec command will execute a command in place of the current shell, that is, it terminates the current shell and starts a new process in its place.

exec was often used to execute the last command of a shell script. This would kill the shell slightly earlier; otherwise, the shell would wait until the last command was finished. This practice saves a process and some memory.

try exec ls. you will be logged out from your login shell.

exec also manipulates file descriptors in the Bourne shell.
$exec >>logoutput
after issuing this command you will not see output of any command in your console. all output goes into logoutput file.

use exec >/dev/console to return.

$exec 2>errs.out
means that from then on, stderr goes into errs.out file


hope this will help you

[Edited by mib on 04-15-2001 at 12:14 PM]
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scotty(1)							 Tnm Tcl Extension							 scotty(1)

__________________________________________________________________________________________________________________________________________________

NAME
scotty - A Tcl shell including the Tnm extensions. SYNOPSIS
scotty ?fileName arg arg ...? _________________________________________________________________ DESCRIPTION
scotty is a Tcl interpreter with extensions to obtain status and configuration information about TCP/IP networks. After startup, scotty evaluates the commands stored in .scottyrc and .tclshrc in the home directory of the user. SCRIPT FILES
If scotty is invoked with arguments then the first argument is the name of a script file and any additional arguments are made available to the script as variables (see below). Instead of reading commands from standard input scotty will read Tcl commands from the named file; scotty will exit when it reaches the end of the file. If you create a Tcl script in a file whose first line is #!/usr/local/bin/scotty2.1.11 then you can invoke the script file directly from your shell if you mark the file as executable. This assumes that scotty has been installed in the default location in /usr/local/bin; if it's installed somewhere else then you'll have to modify the above line to match. Many UNIX systems do not allow the #! line to exceed about 30 characters in length, so be sure that the scotty executable can be accessed with a short file name. An even better approach is to start your script files with the following three lines: #!/bin/sh # the next line restarts using scotty exec scotty2.1.11 "$0" "$@" This approach has three advantages over the approach in the previous paragraph. First, the location of the scotty binary doesn't have to be hard-wired into the script: it can be anywhere in your shell search path. Second, it gets around the 30-character file name limit in the previous approach. Third, this approach will work even if scotty is itself a shell script (this is done on some systems in order to handle multiple architectures or operating systems: the scotty script selects one of several binaries to run). The three lines cause both sh and scotty to process the script, but the exec is only executed by sh. sh processes the script first; it treats the second line as a comment and executes the third line. The exec statement cause the shell to stop processing and instead to start up scotty to reprocess the entire script. When scotty starts up, it treats all three lines as comments, since the backslash at the end of the second line causes the third line to be treated as part of the comment on the second line. VARIABLES
Scotty sets the following Tcl variables: argc Contains a count of the number of arg arguments (0 if none), not including the name of the script file. argv Contains a Tcl list whose elements are the arg arguments, in order, or an empty string if there are no arg arguments. argv0 Contains fileName if it was specified. Otherwise, contains the name by which scotty was invoked. tcl_interactive Contains 1 if scotty is running interactively (no fileName was specified and standard input is a terminal-like device), 0 otherwise. PROMPTS
When scotty is invoked interactively it normally prompts for each command with ``% ''. You can change the prompt by setting the variables tcl_prompt1 and tcl_prompt2. If variable tcl_prompt1 exists then it must consist of a Tcl script to output a prompt; instead of out- putting a prompt scotty will evaluate the script in tcl_prompt1. The variable tcl_prompt2 is used in a similar way when a newline is typed but the current command isn't yet complete; if tcl_prompt2 isn't set then no prompt is output for incomplete commands. SEE ALSO
Tnm(n), Tcl(n) AUTHORS
Juergen Schoenwaelder <schoenw@cs.utwente.nl> Tnm scotty(1)
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