nepalexpo's blog

Crond Demystified !!

In Linux Fedora Core 1, "crond" is started from the script "/etc/rc.d/init.d/crond". The manual `man cron` says that cron searches "/var/spool/cron" for crontab files which are named after accounts in "/etc/passwd"; crontabs found are loaded into memory. Cron also searches for "/etc/crontab" and the files in the "/etc/cron.d/" directory.

The "/etc/crontab" file contains the following.


# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly

Turbo-charge Mozilla Firefox !

Here's something for broadband people that will really speed Firefox up:

  1. Type "about:config" into the address bar and hit return. Scroll down and look for the following entries:

    Normally the browser will make one request to a web page at a time. When you enable pipelining it will make several at once, which really speeds up page loading.

  2. Alter the entries as follows:
    Set "network.http.pipelining" to "true"
    Set "network.http.proxy.pipelining" to "true"

Memory Usage with /proc/meminfo

The entries in the /proc/meminfo can help explain what's going on with your memory usage, if you know how to read it.

Example of `cat /proc/meminfo`:

        total:    used:    free:  shared: buffers:  cached:
Mem:  1050001408 1012899840 37101568        0 113672192 420950016
Swap: 2097434624 217985024 1879449600
MemTotal:      1025392 kB
MemFree:         36232 kB
MemShared:           0 kB
Buffers:        111008 kB
Cached:         279304 kB
SwapCached:     131780 kB
Active:         677908 kB
ActiveAnon:     487272 kB
ActiveCache:    190636 kB
Inact_dirty:    129164 kB
Inact_laundry:   23948 kB

Check your spellings on the Command Line Interface

The aspell command, from the aspell package is designed to spell check files. This package also provides the command spell, ispell, and run-with-aspell which call aspell in different ways.

For example, to check if the word "compatible" is correct, execute the command:

echo compatible|aspell -a

The output should be similar to:

@(#) International Ispell Version 3.1.20

This output shows that "compatible" is spelt correctly.

However, if you try to check the word "compatable," the output would be:

@(#) International Ispell Version 3.1.20

Rolling Back Your System with RPM

Configuring a rollback capability in your system is just a matter of taking a few well-planned steps. They involve creating some macros for the rpm system, choosing a "safe point" - the point in time before which you won't want to roll back your system - and creating an archive directory for rpms replaced by new versions.

Each time you update or uninstall an rpm package in a rollback-enabled system, the rpm is actually repackaged and archived. It's not deleted. That allows you the capability to re-install that package at any time.

Further, the system allows you to use an intuitive rollback criteria, such as `rpm -Uvh --rollback '3 days ago'`. This is done by the use of a field in the rpm database known as the Transaction ID (TID). The repackaged package is also noted with the TID. When rolling back, any package equal to or older than the requested TID (as interpreted from the plain-english time) is re-installed on the system. You can also rollback one particular package by `rpm -Uvh --rollback <package>`.

Redirection of Input and Output...

To read from an input file instead of typing the input in.

$ program < input

To send the output to an output file, this is hard to figure out if the program is interactive, since the prompts go to the output file.

$ program > output

To append the output to an output file

$ program >> output

Read the input from the input file, put the output to the output file.

$ program < input > output

Some programs output data to stderr not stdout ( the compiler is an example ). If you want to capture the compiler warnings in a text file try this.

vi Editor Basics...

Getting Started

The command "vi" without any file name will open a new file where you can enter the text and edit but while coming out you will be asked to enter a valid file name to save the text.
"vi" with a existing file name will open that file for editing.

Oracle SQL Plus Command Reference


At the SQL prompt, you can begin typing any SQL command. Upon hitting return (i.e., enter key) the SQL prompt will change to line number prompts. When you are finished typing a command, type / or RUN to execute the SQL command. Also, a semicolon at the end of the SQL command will execute the command immediately after hitting return. In addition to SQL commands, /, and RUN, other commands can be issued at the SQL prompt (a semicolon does not have to follow the nonSQL commands).


  • DESCRIBE tablename -- lists the attributes and their specifications of tablename