LinuxBabu ………..

This morning I was doing some work with one of my website transfer and to see the changes on my
MacBook laptop I knew I would need to flush the DNS cache so I wouldn’t have to wait for the cache to expire.

So for anyone else who needs to know the commands here they are:

OS X < = 10.5.1 (Mac OSX versions 10.5.1 and before)

lookupd -flushcache

OS X >= 10.5.2 (Mac OSX Leopard)

dscacheutil -flushcache

In Linux, the nscd daemon manages the DNS cache. To flush the DNS cache, restart the nscd daemon.

To restart the nscd daemon, use the command

/etc/rc.d/init.d/nscd restart

In Microsoft Windows, you can use the command to flush the DNS resolver cache:

C:\>ipconfig /flushdns

Windows IP Configuration

Successfully flushed the DNS Resolver Cache.

You can also use the command to view the DNS resolver cache.

ipconfig /displaydns

Hope that will help anyone out there who needs to flush their dns cache.

System and Network Configuration

* linuxconf – A GUI interactive interface available on Redhat 6.0 or later which includes netconf configuration.
* netconf – A GUI interactive interface available on Redhat 6.0 and later.
* kbdconf – A Redhat Linux tool which configures the /etc/sysconfig/keyboard file which specifies the location of the keyboard map file. This is a GUI based tool.
* mouseconfig – A Redhat Linux tool used to configure the /etc/sysconfig.mouse file. This is a GUI tool.
* timeconfig – A Redhat Linux tool used to configure the /etc/sysconfig/clock file. This is a GUI tool used to set timezone and whether or not the clock is set to GMT time.
* kernelcfg – A Redhat kernel configuration utility to be started from X.
* stty – Used to configure and print the console devices.
* setterm – Set terminal attributes.
* vmstat – Report statistics on virtual memory.

X Configuration

* XF86Setup – A newer X configuration program with a GUI interface which modifies the “/etc/X11/XF86Config” configuration file.
* xf86config – An older X configuration program with a text based interface. It also modifies the “/etc/X11/XF86Config” configuration file.
* Xconfigurator – The Redhat tool used during system setup to configure X.
* SuperProbe – A program that probes the video card to determine its type for use with setting up X.
* xvidtune – This program will test video modes on the fly without modification to your X configuration. Read the usr/X11R6/lib/X11/doc/VideoModes.doc file before running this program.

Library and kernel Dependency Management

Library management:
* ldd – Used to determine shared libraries used by binary files. Type “ldd /bin/ls” to see the shared libraries used by the “ls” command.
* ldconfig – Used to update links and cache for system use of the most recent runtime shared libraries.

Kernel Management:

* lsmod – List currently installed kernel modules.
* depmod – Creates a dependency file, “modules.dep” in the directory “/lib/modules/x.x.x”, later used by modprobe to automatically load the relevant modules.
* insmod – Installs a loadable kernel module into the running kernel.
* rmmod – Unloads modules, Ex: rmmod ftape
* modprobe – Used to load a module or set of modules. Loads all modules specified in the file “modules.dep”.

General Diagnostic
System resources

* free – Show system memory availability and usage
* df – Show the amount of disk free space on each mounted filesystem.
* du – Show disk usage
* lspci – List PCI devices
* pnpdump – Lists ISA PNP device resource information.
* vmstat – Reports virtual memory statistics.

Other:

* env – List the current environment variables.
* printenv – Print a copy of the environment.
* set – Shows how the environment is set up. This command can be very useful when debugging the environment.
* runlevel – List the current and previous runlevel.
* uname – Print system information. In my case, it prints “Linux”.
* dmesg – Show the last kernel messages printed during the last boot.

We geeks all have some sort of bluetooth device on us like your Nokia N80 or your new iPhone (which I will be getting and LOVE) so why not take advantage of one of these as a beacon to your computer. With a small application you can set your computer to detect the proximity of your device and perform actions based on when it comes in or goes out of range.

I’m using the free utility Proximity to do the detection. You could even unlock the computer when you come back in range, but I just want it to lock when I walk away since I always forget. With a small AppleScript that’s easy:

tell application “ScreenSaverEngine” to activate

That’s all there is to it and it works great.

I needed a quick way to maintain a current symlink to the most recent file by time stamp. Here’s one way to do it:

ls -t1 *.jpg | head -1 | xargs -i ln -sf {} current.jpg

I think the number one skill a sysadmin should have is a solid understanding of shells and shell scripting. The Advanced Bash-Scripting Guide is a good place to start or take a refresher. From the introduction:

A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it.

Not to mention I’ll bet every sysadmin is doing some menial tasks over and over again instead of automating or simplifying them with a script. Sometimes you just don’t realize it. For instance, I can’t tell you how many times I’ve typed a loooong find command to recursively set AFS permissions and I just wrote a wrapper to do it with just two parameters the other day. That one script will literally save me minutes each week. Keep at it and you’ll save hours a week.

restarting/stopping/starting a service

On any init.d based linux distro you can restart a service with the following…

/etc/init.d/serviceName restart

You may replace ‘restart’ with ’stop’ or ’start’ (and in some cases ’status’).
Forcefully stopping a service

killall processName

Killing on instance of a service

kill pid

The pid can be gathered by either top or ps

Disabling/adding/listing services

chkconfig –list

displays all the services and if they are set to run in different runlevels
use the –del daemonName to remove a service or –add daemonName to add one

 chkconfig [--level <levels>] <name> <on|off|reset>

 eg.  chkconfig –level 3 httpd on

This will set the httpd to ON on run level 3.

setting a program to run at startup

Add a line executing the command at the end of /etc/rc.local

File Manipulation
Editing Text Files

vi is by far the best text editor but has a learning curve to it. If you want simplicity use nano
display a text file from the command line

cat filename

or

more filename

Display the last few lines of a text file

tail filename

or you can display the last 50 lines of a file with…

tail -50 filename

or you can display lines as they are written to a file (or follow) with the following: (UBER useful for log files)

tail -f filename

copy a file

cp filename destination

move a file

mv filename destination

delete a file

rm -f filename : removes the file. -f makes it so it doesn’t ask you if you are sure

Displaying the differences between two files

diff file1 file2

Installing crap

On redhat derived systems (RedHat, Fedora, CentOS, Rocks, Mandrake, etc) yum is your package manager.

yum install appname : installs the application from the remote yum repository

yum search appname : does a search on the repository for a given program

yum remove appname : uninstalls an app

use ‘man yum’ for a complete list

Archives

tar.gz or .tgz is the most common compression found in the linux world. that is tared (Tape ARchive) and gziped. Sometimes called “tar balls”.

tar -xzf file.tgz : will X’trackt a tar/gzip file.

tar -czf myfile.tgz someDir : will create a tar and gziped archive of the given directory

gunzip : un gzips a file

unzip : unzips a .zip file

useradd

useradd userName

then run “passwd userName” to set the new users pw

passwd

passwd username

will ask for the new pw twice

Listing Permissions

ls -al will display all files in a list with their owners and permissions

-rw-r–r– 1 irq13 irq13 1006 Jan 24 10:16 .bashrc

Now to break down the above example…

-rw-r–r– is the permissions area.

The first – would be d if the item is directory, otherwise it will be -.

The second 3 dashes indicate read/write/execute for the owner,
the second is r/w/x for the group and third is r/w/x for everyone else.

The next number is the inodes associated with the file. This isn’t important for you to know the basics

Next when it says irq13 irq13 that indicates the owner of the files name group
Changing ownership of a file

chown username:groupname file
Changing permissions of a file

chmod XXX filename

chmod uses a numeric system for assigning ownership.
XXX represents 3 numbers.
The first is the permissions applied to the owning user, 2nd is group, 3rd is everyone else.

1: execute 2: write 3: write & execute 4: read 5: read & execute 6: read & write 7: read, write & execute

Remember that 777 is only to be used as a trouble shooting step to rule fs permissions out.

NEVER leave a dir as 777. Its useful to do ‘ls -alh * > perm_capture.txt’ before messing with a file.

That way you can restore its original permissions.
Attributes

Files also have attributes, similar to the ones found in the windows world.

lsattr filename : Lists the attributes of a file or directory

chattr +-=[ASacDdIijsTtu] filename

to add an attribute use + to remove use -

File Attributes

append only (a)
compressed (c)
no dump (d)
immutable (i)
data journaling (j)
secure deletion (s)
no tail-merging (t)
undeletable (u)
no atime updates (A)
synchronous directory updates (D)
syn-chronous updates (S)
top of directory hierarchy (T)

Use man chattr for an explanation of each attribute
launching scripts and bins

* If an executable file is in your path you may simply type its name from anywhere on the system and it will execute.
* To see what your path is type ‘path’
* To execute a file in the current directory type “./filename
* To execute a file it must have execute permissions for either your username or a group you belong to.

ps | grep ssh — only display lines that contain ssh

; used to “stack commands” or issue multiple commands on 1 line. cd ..; ls

& puts a command in the background. Will let you know when the command is finished

> write what is displayed on the screen from a given command to a text file ls -alh /root > /root/myRoot.txt

>>

appends screen output to an existing file