Softpanorama (slightly skeptical) Open Source Software Educational Society

May the source be with you, but remember the KISS principle ;-)

Linux Tips

• redhat.com Home

• The Linux Tips HOWTO v3.6, June 1998 by Paul Anderson

• Tips, Tricks, How-To from Fermilab.

• Tips for Linux Explorers

• Linux tips

• Linux Tips Think of this as the LSADP (LinuxSA Documentation Project).

• Ubuntu and Kubuntu Linux Tips

• O'Reilly -- Top 10 Tips for Linux Users

• Linux Tips collection from Mike Chirico

Old News ;-)

Tips & Tricks

• One tip, and one trick from the Red Hat Knowledgebase
• Changing timezone
• RPM
  • The RPM Packager Manager [RPM] -- Revisited
  • The RPM Package Manager [RPM]
  • Aiding and Abetting RPM
• Using Dynamic Host Configuration Protocol (DHCP)
• Kickstart Basics
• Using Rescue Mode
• The Newbie's Guide to the Command Line
• The Newbie's Guide to IP Networking
• /proc/meminfo explained
• Share your files and printer on a Windows network with samba
• Automatically Displaying System Console Messages on your Desktop
• Bash Scripting on the Command Line
• The Watch Command
• Getting a colored shell prompt
• Configuring Your Graphical Environment
• less Surprises

partprobe - inform the OS of partition table changes

One of the major benefits to using Red Hat Enterprise Linux is that once the operating system is up and running, it tends to stay that way. This also holds true when it comes to reconfiguring a system; mostly. One Achilles heel for Linux, until the past couple of years, has been the fact that the Linux kernel only reads partition table information at system initialization, necessitating a reboot any time you wish to add new disk partitions to a running system.

The good news, however, is that disk re-partitioning can now also be handled 'on-the-fly' thanks to the 'partprobe' command, which is part of the 'parted' package.

Using 'partprobe' couldn't be more simple. Any time you use 'fdisk', 'parted' or any other favorite partitioning utility you may have to modify the partition table for a drive, run 'partprobe' after you exit the partitioning utility and 'partprobe' will let the kernel know about the modified partition table information. If you have several disk drives and want to specify a specific drive for 'partprobe' to scan, you can run 'partprobe <device_node>'

Of course, given a particular hardware configuration, shutting down your system to add hardware may be unavoidable, it's still nice to be given the option of not having to do so and 'partprobe' fills that niche quite nicely.

partprobe [-d] [-s] [devices...]  

DESCRIPTION

This manual page documents briefly the partprobe command.

partprobe is a program that informs the operating system kernel of partition table changes, by requesting that the operating system re-read the partition table.  

OPTIONS

This program uses short UNIX style options.

-d

Don't update the kernel.

-s

Show a summary of devices and their partitions.

-h

Show summary of options.

-v

Show version of program.

CENTOS/RHEL 5 CONFIGURATION TIPS

Yum & Repositories
I noticed this issue with both CentOS 4 and 5 - Yum will often choose bad mirrors from the mirrorlist file - for example, choosing overseas servers, when an official NZ server exists. And in some cases, the servers it has chosen are horribly slow.

You will probably find that you get better download speeds by editing /etc/yum.repos.d/CentOS-Base.repo and commenting out the mirrorlist lines and setting the baseurl line to point to your preferred local mirror.

Yum-updatesd
CentOS 5 has a new daemon called yum-updatesd, which replaces the old cron job yum update scripts. This script will check frequently for updates, and can be configured to download and/or install them.

However, this daemon is bad for a server, since it doesn't run at a fixed time - I really don't want my server downloading and updating software during the busiest time of day thank-you-very-much!

So, it's bad for a server. Let's disable it with:
 

service yum-updatesd stop
chkconfig --level 2345 yum-updatesd off

Plus I don't like the idea of having a full blown daemon where a simple cronjob will do the trick perfectly fine - seems like overkill. (although it appears yum-updatesd has some useful features like dbus integration for desktop users)

So, I replace it with my favorite cronjob script approach, by running the following (as root of course):
 

cat << "EOF" > /etc/cron.daily/yumupdate
 #!/bin/sh
 # install any yum updates
/usr/bin/yum -R 10 -e 0 -d 1 -y update yum > /var/log/yum.cron.log 2>&1
/usr/bin/yum -R 120 -e 0 -d 1 -y update  >> /var/log/yum.cron.log 2>&1
if [ -s /var/log/yum.cron.log ]; then
        /bin/cat /var/log/yum.cron.log | mail root -s "Yum update information" 2>&1
fi
EOF

and if you want to clear up the package cache every week:
 

cat << "EOF" > /etc/cron.weekly/yumclean
 #!/bin/sh
 # remove downloaded packages
/usr/bin/yum -e 0 -d 0 clean packages
EOF

(please excuse the leading space infront of the comments ( #) - it is to work around a limitation in my site, which I will fix shortly. Just copy the lines into a text editor and remove the space, before pasting into the terminal)

This will install 2 scripts that get run around 4:00am (as set in /etc/crontab) which will check for updates and download and install any automatically. If there were any updates, it will send out an email, if there were none, it doesn't send anything.

(of course, you need sendmail/whatever_fucking_email_server_you_like configured correctly to get the alerts!)

You can change yum to just download and not install the updates (just RTFM), but I've never had a update break anything - update compatibility and quality is always very high - so I use automatic updates.

CentOS 4 had something very similar to this, with the addition of a bootscript to turn the cronjobs on and off.

* Please check out the update at the bottom of this page for futher information on this.


Apache Quirks
If you are using indexing in apache (indexing is when you can browse folders/files), you may find that the browsing page looks small and nasty.

The fix is to edit /etc/httpd/conf/httpd.conf and change the following line:
 

IndexOptions FancyIndexing VersionSort NameWidth=* HTMLTable

to
 

IndexOptions FancyIndexing VersionSort NameWidth=*

This should make the index full screen again. I'm not sure if this is an apache bug, a distro bug or some other weird issue, because I'm sure HTMLTable isn't supposed to be all small like that.

(FYI: CentOS 4 did not have the HTMLTable option active)



SSL Certificates
Redhat have moved things around with SSL certificates a lot. What it seems like happened (I have only had a quick look into this), is that they were going to provide a new tool to generate SSL certificates called "genkey" but pulled it out before release.

To make things more fun, they also removed the good old Makefile that was in /etc/httpd/conf/ that allowed you to generate SSL certificates & keys.

However, I found the same Makefile again in /etc/pki/tls/certs/


Vi vs. Vim
If you use vi/vim, you should check this posting out.

That's all the issues that I've come across for now - if I find any more things to note, I'll update this page with the information and put a note on my blog.

/etc/sysconfig/network

Note the NETMASK should be defined in /etc/sysconfig/network-scripts/ifcfg-eth0

/etc/sysconfig/network

The /etc/sysconfig/network file is used to specify information about the desired network configuration. The following values may be used:

• NETWORKING=<value>, where <value> is one of the following boolean values:
  • yes — Networking should be configured.
  • no — Networking should not be configured.
• HOSTNAME=<value>, where <value> should be the Fully Qualified Domain Name (FQDN), such as hostname.expample.com, but can be whatever hostname is necessary.

  Note

  For compatibility with older software that some users may need to install, such as trn, the /etc/HOSTNAME file should contain the same value as set here.

• GATEWAY=<value>, where <value> is the IP address of the network's gateway.
• GATEWAYDEV=<value>, where <value> is the gateway device, such as eth0.
• NISDOMAIN=<value>, where <value> is the NIS domain name.

How to see parameter of ext3 filesystem

# tune2fs -l /dev/mapper/vg00-lv06
tune2fs 1.38 (30-Jun-2005)
Filesystem volume name: <none>
Last mounted on: <not available>
Filesystem UUID: c0615eba-5bb6-443d-81c7-7f3c1eb829b2
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal filetype needs_recovery sparse_super
Default mount options: (none)
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 1310720
Block count: 2621440
Reserved block count: 131072
Free blocks: 2365370
Free inodes: 1309130
First block: 0
Block size: 4096
Fragment size: 4096
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 16384
Inode blocks per group: 512
Filesystem created: Mon May 21 11:16:17 2007
Last mount time: Tue May 6 17:40:40 2008
Last write time: Tue May 6 17:40:40 2008
Mount count: 3
Maximum mount count: 500
Last checked: Thu Apr 3 11:51:39 2008
Check interval: 5184000 (2 months)
Next check after: Mon Jun 2 11:51:39 2008
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 128
Journal inode: 8
Default directory hash: tea
Directory Hash Seed: 36a54cdf-3f8e-482b-9e2c-a48b6ac1d27e
Journal backup: inode blocks

[Nov 30, 2007] freshmeat.net Project details for Expect-lite

About:
Expect-lite is a wrapper for expect, created to make expect programming even easier. The wrapper permits the creation of expect script command files by using special character(s) at the beginning of each line to indicate the expect-lite action. Basic expect-lite scripts can be created by simply cutting and pasting text from a terminal window into a script, and adding '>' '

Release focus: Major feature enhancements

Changes:
The entire command script read subsystem has changed. The previous system read directly from the script file. The new system reads the script file into a buffer, which can be randomly accessed. This permits looping (realistically only repeat loops). Infinite loop protection has been added. Variable increment and decrement have been added to support looping.

Author:
Craig Miller [contact developer]

[Nov 30, 2007] Got more than a gig of RAM and 32-bit Linux Here's how to use it By Bruce Byfield

[Nov 20, 2007] Games with discolors

eval `dircolors ~/.dir_colors`
alias ls="ls --color=auto"

The command 'dircolors' takes its data from the file ~/.dir_colors and
creates an environment variable LS_COLORS.  The command 'ls --color' takes
its colors from the environmental variable LS_COLORS.

So, write a suitable ~/.dir_colors file, and execute the command
'dircolors'.  To get a starting file for editing, do this:

dircolors -p > ~/.dir_colors

The ~/.dir_colors file so created includes directions on coding the colors
for different kinds of files.

See man dircolors.

 

[Nov 1, 2007] Changing Gnome behavior to standard UNIX CDE style (application displayed on a particular desktop are visible only this desktop toolbar.

You need to unclick:

/apps/panel/applets/windows_list_screen/pref/display_in_all_workspaces

Controlling runaway processes on Linux • Rudd-O.com

... Have you heard of the magic SysRq key?

No?

Well, it’s magic. It’s directly shunted to the Linux kernel. You press ALT, press the PrintScreen (SysRq) key, and while holding them both down, press one of the letters (each letter has a different function assigned to it).

It’s not normally enabled, but you can enable it by putting

kernel.sysrq = 1

in your machine’s /etc/sysctl.conf file. Oh, and then rebooting.

Here’s why it’s useful.

So, what does SysRq do, really?

Hit Alt+SysRq+K — the windowing system will restart. More effective than Ctrl+Alt+Backspace.

Suppose a GUI application you just opened is starting to swallow massive amounts of RAM. Like, one gigabyte, perhaps? Your machine is locking up, and you feel the mouse start to stutter at first, then freeze completely — while the hard disk light in your computer’s front panel is lighting up frantically, gasping for airmemory.

You now have three choices:

1. Sit it out and let the Linux kernel detect this situation and kill the abusive application. This can take way more than 15 minutes.
2. Press the computer’s power off button for 5 seconds. This shuts your machine down uncleanly and leads to data loss.
3. Hit the magic SysRq combo: Alt+SysRq+K.

Should you choose option 3, the graphical subsystem dies immediately. That’s because Alt+SysRq+K kills any application that holds the keyboard open — and, you guessed it, the graphical subsystem is holding it open. This premature death of the GUI causes all GUI applications to die in a cascade, including the abusive application.

Two to ten seconds later, you will be presented with a login prompt.

Sure, you lost changes to all files you haven’t saved, and all the tabs in your Web browser… but at least you didn’t have to reboot uncleanly, did you?

But, Ctrl+Alt+Backspace?

Once the machine is in a critically heavy memory crunch, Ctrl+Alt+Backspace will take too much time to work, because the windowing system will be pressed for memory to even execute. The magic SysRq key has the luxury of not having that problem — if Ctrl+Alt+Backspace were an IV drip, SysRq would be like a central line.

Why this key combination exists

The reason this key combo exists is simple. Alt+SysRq+K is called SAK (System Attention Key). It was designed back in the days of, um, yore, to kill all applications snooping on the keyboard — so administrators wishing to log in could safely do so without anyone sniffing their passwords.

As a preventative security measure, it sure works against keyloggers and other malware that may be snooping on your keyboard, may I say. And it most definitely works against your run-of-the-mill temporary memory shortage ;-).

Advantages/disadvantages

Well, the major disadvantages are:

• Anyone with keyboard access can reboot or hang your machine using a SysRq key combination.
• Once you hit it, since the GUI dies, all of your open applications close, forcibly.

But, on a memory crunch, this beats rebooting hands-down. And that’s the biggest advantage.

Last updated: June 06, 2008