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Top Forums Shell Programming and Scripting Problem with looping construct Post 302087920 by blowtorch on Thursday 7th of September 2006 07:01:49 AM
Old 09-07-2006
Ok, you have a couple of misunderstandings there.

1. You are trying to define HTTPD variable as a command, but you are using the ` (backtick) character to do that. Using backticks actually causes the command to execute and the results to be returned and (in this case) stored in the HTTPD variable. Use the ' (single quote) character to do what you want.
2. You want to run the command line contained under the HTTPD variable (forget that you aren't getting the command line for a moment), but you are using ${} instead of $(). The $() is used to fork a subshell and execute processes. ${} is used to address shell variables.

By the way, I tried writing and running the script the way you want to, but it is giving me an error.
Code:
# cat test.sh
#!/usr/bin/bash

HTTPD='/usr/ucb/ps -axcu | /usr/bin/grep httpd > /dev/null; echo $?'
while [ $($HTTPD) -eq 0 ]; do
        echo "running"; sleep 10; 
done
# bash -x test.sh
+ HTTPD=/usr/ucb/ps -axcu | /usr/bin/grep httpd > /dev/null; echo $?
++ /usr/ucb/ps -axcu '|' /usr/bin/grep httpd '>' '/dev/null;' echo '$?'
ps: too many arguments
usage: ps [ -aceglnrSuUvwx ] [ -t term ] [ num ]
+ '[' -eq 0 ']'
test.sh: [: -eq: unary operator expected

I don't understand what the shell is passing as arguments to the ps command in this case. I tried escaping the |, the > and the $ in $?, but that didn't work. Maybe someone else would like to have a go.
 

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Session::Lock::MySQL(3) 				User Contributed Perl Documentation				   Session::Lock::MySQL(3)

NAME
Apache::Session::Lock::MySQL - Provides mutual exclusion using MySQL SYNOPSIS
use Apache::Session::Lock::MySQL; my $locker = Apache::Session::Lock::MySQL->new(); $locker->acquire_read_lock($ref); $locker->acquire_write_lock($ref); $locker->release_read_lock($ref); $locker->release_write_lock($ref); $locker->release_all_locks($ref); DESCRIPTION
Apache::Session::Lock::MySQL fulfills the locking interface of Apache::Session. Mutual exclusion is achieved through the use of MySQL's GET_LOCK and RELEASE_LOCK functions. MySQL does not support the notion of read and write locks, so this module only supports exclusive locks. When you request a shared read lock, it is instead promoted to an exclusive write lock. CONFIGURATION
The module must know how to connect to your MySQL database to acquire locks. You must provide a datasource name, a user name, and a password. These options are passed in the usual Apache::Session style, and are very similar to the options for Apache::Session::Store::MySQL. Example: tie %hash, 'Apache::Session::MySQL', $id, { LockDataSource => 'dbi:mysql:database', LockUserName => 'database_user', LockPassword => 'K00l' }; Instead, you may pass in an already opened DBI handle to your database. tie %hash, 'Apache::Session::MySQL', $id, { LockHandle => $dbh }; AUTHOR
This module was written by Jeffrey William Baker <jwbaker@acm.org>. SEE ALSO
Apache::Session perl v5.12.1 2008-01-08 Session::Lock::MySQL(3)
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