What is with the '&'. Post: 17264

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Top Forums UNIX for Dummies Questions & Answers What is with the '&'. Post 17264 by TioTony on Tuesday 12th of March 2002 10:31:19 PM

03-12-2002

Registered User

Hello,
The & has several modes of significance on the Unix command line. The 2 most common forms I usually encounter are for running jobs in the background and for redirecting STDERR.

When & appears at the end of a command string, this usually signifies you want that command to run in the background. This returns control of your session back to you. If you submit a command that normally takes 5 minutes with no & on the end of it, you have to wait 5 minutes until the prompt returns. With & on the end, you get the command prompt back immediatly and your commmand continues to run.

The command that follows is actually interpretted by the shell as 2 commands.

rose_pa--root::/data/connect>mkdir &Q2

ksh thinks you are running a command called mkdir and a second command called Q2. mkdir is being put in the background. That is why you get this message:

[1] 29264

This is the process ID of mkdir.

Then the shell tries to run Q2 thinking it is another command. Then you get:

ksh: Q2: not found.

mkdir then completes and gives you the error:

rose_pa--root::/data/connect>Usage: mkdir [-p] [-m mode]
Directory ...

This is because no directory name appeared prior to the &.

I hope this makes sense. You may want to check the man pages for fg as well. This brings background jobs to the foreground.

TioTony

TioTony

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tcl_allowexceptions

Tcl_AllowExceptions(3tcl)				      Tcl Library Procedures					 Tcl_AllowExceptions(3tcl)

__________________________________________________________________________________________________________________________________________________

NAME

       Tcl_AllowExceptions - allow all exceptions in next script evaluation

SYNOPSIS

       #include <tcl.h>

       Tcl_AllowExceptions(interp)

ARGUMENTS

       Tcl_Interp *interp (in)		Interpreter in which script will be evaluated.
_________________________________________________________________

DESCRIPTION

       If  a  script is evaluated at top-level (i.e. no other scripts are pending evaluation when the script is invoked), and if the script termi-
       nates with a completion code other than TCL_OK, TCL_ERROR or TCL_RETURN, then Tcl normally converts this into a TCL_ERROR  return  with	an
       appropriate  message.  The particular script evaluation procedures of Tcl that act in the manner are Tcl_EvalObjEx, Tcl_EvalObjv, Tcl_Eval,
       Tcl_EvalEx, Tcl_GlobalEval, Tcl_GlobalEvalObj, Tcl_VarEval and Tcl_VarEvalVA.

       However, if Tcl_AllowExceptions is invoked immediately before calling one of those a procedures, then arbitrary completion codes  are  per-
       mitted from the script, and they are returned without modification.  This is useful in cases where the caller can deal with exceptions such
       as TCL_BREAK or TCL_CONTINUE in a meaningful way.

KEYWORDS

       continue, break, exception, interpreter

Tcl									7.4						 Tcl_AllowExceptions(3tcl)

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tcl_allowexceptions