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Full Discussion: What does this do in bash: ${pid:-}
Top Forums Shell Programming and Scripting What does this do in bash: ${pid:-} Post 302245955 by mindOnMy$ on Sunday 12th of October 2008 01:49:16 AM
Old 10-12-2008
Question What does this do in bash: ${pid:-}
As i understand it, this means
if pid is
a) unset, or
b) set to null
then replace with the value on the right of the minus sign--which is null

This confuses me because
a) I thought if a variable isn't set to anything it's automatically null. If not then what is an uninitiated variable set to?
b) if the variable is already set to null, then why replace it with another null?

I see this on line 74 in the file /lib/lsb/init-functions (I'm using hardy heron ubuntu)
Here's an excerpt from the file:
Code:
if [ -z "${pidfile:-}" ]; then 
    pidfile=/var/run/${1##*/}.pid 
fi

 

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LEARN ABOUT REDHAT

kill

KILL(1) 						     Linux Programmer's Manual							   KILL(1)

NAME

       kill - terminate a process

SYNOPSIS

       kill [ -s signal | -p ] [ -a ] [ -- ] pid ...
       kill -l [ signal ]

DESCRIPTION

       The command kill sends the specified signal to the specified process or process group.  If no signal is specified, the TERM signal is sent.
       The TERM signal will kill processes which do not catch this signal.  For other processes, it may be necessary to use the KILL  (9)  signal,
       since this signal cannot be caught.

       Most  modern  shells  have  a  builtin  kill function, with a usage rather similar to that of the command described here. The `-a' and `-p'
       options, and the possibility to specify pids by command name is a local extension.

OPTIONS

       pid... Specify the list of processes that kill should signal.  Each pid can be one of five things:

	      n      where n is larger than 0.	The process with pid n will be signaled.

	      0      All processes in the current process group are signaled.

	      -1     All processes with pid larger than 1 will be signaled.

	      -n     where n is larger than 1.	All processes in process group n are signaled.	When an argument of the form `-n' is given, and it
		     is  meant	to  denote  a process group, either the signal must be specified first, or the argument must be preceded by a `--'
		     option, otherwise it will be taken as the signal to send.

	      commandname
		     All processes invoked using that name will be signaled.

       -s signal
	      Specify the signal to send.  The signal may be given as a signal name or number.

       -l     Print a list of signal names.  These are found in /usr/include/linux/signal.h

       -a     Do not restrict the commandname-to-pid conversion to processes with the same uid as the present process.

       -p     Specify that kill should only print the process id (pid) of the named processes, and not send any signals.

SEE ALSO

       bash(1), tcsh(1), kill(2), sigvec(2), signal(7)

AUTHOR

       Taken from BSD 4.4.  The ability to translate process names to process ids was added by Salvatore Valente <svalente@mit.edu>.

Linux Utilities 						  14 October 1994							   KILL(1)
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