Lesson 5 - Process Management
Required Reading
-
Learning UNIX: Chapter 7 (Multitasking)
- WWW: Doing
things periodically - Using cron
- WWW: Cron
(event scheduler)
Additional Notes
Wondering what a daemon
is?
Sometimes is is useful to see processes running on the entire
system, not just for your user ID. The command to do this
various for different UNIX varieties, but 'ps -ef' is common.
Check 'man ps' to see a full list of options.
The concept of an interpreted program was brought up in the
text discussion of the 'ps' command. An interpreted program
is one that is not pre-compiled (like a C or C++ program),
but rather is stored in it's "raw" format and is
not independently executable. For a C program, once it's compiled
you can execute it independently anywhere on the system. A
perl program, however, is not self-sufficient. The perl interpreter
(aka 'perl') is actually the command you run, and it reads
the perl program and "interprets" it at run-time.
This has a couple of implications for webmasters:
- every time a user runs a perl program, a new process will
be started for the perl interpreter; you will see this if
you monitor background processes
- the perl interpreter must be on the system in a publicly-available
space in order to run perl scripts (ie 'perl' must be readable
and executable to all users in the system)
Cron can be very useful for webmasters if your server provides
it. Any UNIX command can be automated via cron, such as copying
your HTML files to a backup directory every night or sending
an email to the webmaster with the day's usage statistics.
Just take the exact command you type at the command line (including
parameters and/or I/O redirection) and put it in the crontab
file with the desired schedule.
To sum up the crontab commands:
crontab -l
=> list your current cron table
crontab -r
=> delete your current cron table
crontab -e
=> edit your cron table
crontab filename
=> import the file "filename" into your cron
table
If you want to change your default editor (so that crontab
-e uses something like pico instead of vi)
you can follow
these directions. ACC uses the bash shell.
And finally, read the message returned by crontab -l
carefully when checking for cron access. The following message
means you do not have access:
You are not allowed
to use this program (crontab) See crontab(1) for more information
whereas this message means you do have access but do not
yet have entries in your cron table:
no crontab for unixweb01
Next Step
Take the self-quiz
Complete project 4
Continue to lesson 6
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