Login or Register to Ask a Question and Join Our Community

Sponsored Content
Full Discussion: trap command
Top Forums UNIX for Dummies Questions & Answers trap command Post 302537011 by Corona688 on Wednesday 6th of July 2011 11:02:25 PM
Old 07-07-2011
Quote:
Originally Posted by Straitsfan
I'm sorry to be so dense corona, but can you tell me what is meant by external command, foreground process, etc. I'm not sure the book is clear on this. Smilie
An external command is one that's not built into the shell. It exists as an actual file somewhere, and when you run it, it creates a whole new process.
Code:
$ whereis -b cat
cat: /bin/cat
$ /bin/cat file.txt
contents of file
$ whereis read
read:
$ read VAR
qwertyuiop
$ echo $VAR
qwertyuiop
$

builtins, on the other hand, are commands the shell just automatically knows how to do. There's no 'read' program on the system, but 'read' works anyway, putting the qwertyuiop typed on the keyboard into the variable VAR.

So when you run an external command, it's run seperately, and it has control of the terminal until it's done. When you run an internal command, no programs are run, the shell just does what you tell it internally.

Another important builtin is one you probably already know -- 'cd'. Consider how external programs are run -- they have a separate process, their own environment. If cd wasn't a builtin it couldn't possibly work, because it'd create a new process, change that process' directory, quit, and return to your shell, leaving your shell unchanged.
 

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Using TRAP command

I'm using the trap command to capture any signals received whilst my script is running. How's the best way of writing the signal and any other error messages to a file/error log' without having to type '2>$1' on the command line after the script name? Cheers (3 Replies)
Discussion started by: dbrundrett
3 Replies

2. UNIX for Dummies Questions & Answers

trap command

i have the following script that displays the current time until the user presses CTR + c.... but it does not work properly.... Something is not right with the trap command... Help plz... :confused: # script to continuously display current time. # if script is terminated trap signal... (3 Replies)
Discussion started by: onlyc
3 Replies

3. UNIX for Dummies Questions & Answers

trap command

Dear All could you please explain me what does the trap command do and how I can write a program which can work as a trap command(in C Language). (1 Reply)
Discussion started by: mobile01
1 Replies

4. Programming

trap command in Unix

Could anybody tell me what the trap command does and how it performs the action it does. I had read the trap manual page but it is too concise that nothing is clear about it. Please tell how it works. (1 Reply)
Discussion started by: mobile01
1 Replies

5. Shell Programming and Scripting

Use of TRAP Command

Hi, I would like to know the use of TRAP command. I am very new to the UNIX environment. I have just started learning the basic. So please teach me in a very simple way to understand. Also i would like to know the use of following command: trap 'dialog --msgbox "Script Aborted1" 6 50 ;... (2 Replies)
Discussion started by: Deepakh
2 Replies

6. Shell Programming and Scripting

Cntl+z Trap is not detecting ??? Help required to add a trap detection ???

Hi folks, I have tried to add some trap detection in the below script....this script is used to monitor database activities...in a rather awkward way :rolleyes:.... The idea behind adding trap is that....this script creates lots of temporary files in the running folder to store the count... (1 Reply)
Discussion started by: frozensmilz
1 Replies

7. UNIX for Advanced & Expert Users

trap command

Hello experts! I need to know the use of trap command please In one of our program we have trap "rm -f temp1 ; exit 1" 1 2 15 0 and program always exit with 1 there is a rm -f temp1 as well at the end of the program as rm -f temp1 exit 0 when I test a probram with set... (4 Replies)
Discussion started by: ramshree01
4 Replies

8. Shell Programming and Scripting

trap command

dear all; I can't under stand what does "trap" command do: for example see below: trap "echo; echo no interrupts >&2; sleep 3" 2 3 15 Plz , can any body explain the action of this command? BR (3 Replies)
Discussion started by: ahmad.diab
3 Replies

9. Homework & Coursework Questions

VM trap may work differently than a pure install trap.

Use and complete the template provided. The entire template must be completed. If you don't, your post may be deleted! 1. The problem statement, all variables and given/known data: That is the last reply I received from my instructor, and I'm looking for some alternatives. When using... (2 Replies)
Discussion started by: newuser45
2 Replies

10. Shell Programming and Scripting

Trap command not working

Hi Folks - For some reason, my trap command is not working. It's placed just prior to a normal exit: #:: ------------------------------------------------------------------------ #::-- Script Name: LCM_Backup.sh #:: #::-- Description: This script leverages Utility.sh to perform LCM... (16 Replies)
Discussion started by: SIMMS7400
16 Replies

LEARN ABOUT OSX

platform::shell

platform::shell(n)					       Tcl Bundled Packages						platform::shell(n)

__________________________________________________________________________________________________________________________________________________

NAME

       platform::shell - System identification support code and utilities

SYNOPSIS

       package require platform::shell ?1.1.4?

       platform::shell::generic shell
       platform::shell::identify shell
       platform::shell::platform shell
_________________________________________________________________

DESCRIPTION

       The platform::shell package provides several utility commands useful for the identification of the architecture of a specific Tcl shell.

       This  package  allows the identification of the architecture of a specific Tcl shell different from the shell running the package. The only
       requirement is that the other shell (identified by its path), is actually executable on the current machine.

       While for most platform this means that the architecture of the interrogated shell is identical to the architecture of  the  running  shell
       this  is  not generally true. A counter example are all platforms which have 32 and 64 bit variants and where a 64bit system is able to run
       32bit code. For these running and interrogated shell may have different 32/64 bit settings and thus different identifiers.

       For applications like a code repository it is important to identify the architecture of the shell which will  actually  run  the  installed
       packages, versus the architecture of the shell running the repository software.

COMMANDS

       platform::shell::identify shell
	      This command does the same identification as platform::identify, for the specified Tcl shell, in contrast to the running shell.

       platform::shell::generic shell
	      This command does the same identification as platform::generic, for the specified Tcl shell, in contrast to the running shell.

       platform::shell::platform shell
	      This command returns the contents of tcl_platform(platform) for the specified Tcl shell.

KEYWORDS

       operating system, cpu architecture, platform, architecture

platform::shell 						       1.1.4							platform::shell(n)
All times are GMT -4. The time now is 06:36 PM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy