UNIX World

What is a shell script?

2011-03-17T18:56:00.000+05:30

There will be many occasions, where the system administrators need to execute the commands in series. For example, you may require providing some input and based on the input you have provided, some command should be executed. In this case, a few commands will be executed in sequence. Those commands can be saved in the same sequence in a plain text file. This sequence of commands kept in a plain text file is called as a shell script.

As we know, there is no importance for the extension of a file (like .xls, .doc). However, administrators usually keep the extension of a shell script file as .sh. For example, file will be named as hello.sh. This is only for the understanding that the file is script file.

A simple example for shell script is given below.

=============================

echo “Enter Your name”

read name

echo “Good Morning, $name”

=============================

When we execute the script,

==============================

Enter Your name

Renjith

Good Morning, Renjith

==============================

What is a Shell

2011-03-12T02:06:00.000+05:30

The user who uses a UNIX OS may know English. However the kernel, the heart of the OS, which processes the user request could not understand the normal English, and will know only machine language. So there should a mediator between the user and kernel to translate the language between them. In UNIX related Operating systems, shell is the utility which will do this task. That is, when the user enters the commands to shell prompt, the shell will accept the same and will interpret to kernel, the meaning of the same. The kernel will understand the same and will process the data and will revert to shell with the results. The same results will be conveyed to user by the shell in human readable form. In other words, Shell the interpreter between User and Kernel (The Usual definition).

Below are the common shells available in UNIX.

1. Bourne Shell – sh

2. Bourne again shell – bash

3. Korn Shell – ksh

4. C Shell – csh

1. How to know your current shell?

You can issue the below command to find out the current working shell.

#echo $SHELL

Here, SHELL is an environment variable.

2. How to change your current shell?

You can change the current shell by executing the below command.

#chsh

You can change the login shell in /etc/passwd also.

Notes on Inode

2011-03-11T22:54:00.000+05:30

What is Inode:

Inode can be defined as a data structure which stores information about the file, directory, devices, filesystem object etc. Any file does have an inode number. All the files can be identified by its inode number. Let us see how to find out inode number.

Below are the contents of inode:

1. Ownership

2. File permissions

3. Type of file

4. File length

5. No. of links to file

6. Access privileges

7. Last modification time

8. Time of last inode change

9. Pointers to the files data

10. File allocation information

11. Direct and indirect extent information

How to find out the inode number of the file:

By using the option i along with ls command, we can find out inode of the file or directories

[root@MAYURA ~]# ls -ilrt

total 124

1056239 drwxrwxr-x 4 distcache 141 4096 Aug 4 2005 xen-2.0-install

1056721 drwxrwxr-x 6 root root 4096 Jan 8 2008 bridge-utils-1.4

1056003 -rw-r--r-- 1 root root 5177 Nov 2 04:22 install.log.syslog

inode number is highlighted in blue color.

Hard Link and Soft Link

2011-03-11T20:23:00.000+05:30

All the files have a data structure called i-node which stores the information about the file. We use the name of the file to refer to the i-node of the file. Link is a way to refer the data in a file. There are 2 types of links which are mentioned below.

Hard Links:

1. Hard link is a pointer to files inode.

2. Original file and hard link will have the same i-node number.

3. Hard link can be used only to link the files and not the directory.

4. If we remove the original file, then also hard link file will be there.

5. Hard link can span only within the filesystem.

6. The command to create hard link is,

#ln sourcefile destinationfile

Soft Links:

1. Also known as symbolic link is a file containing the name of another file.

2. Original file and the soft link file will have different i-node number.

3. Soft link can be used to link both files and directories.

4. If we remove the original file, then there will not be any use with the link.

5. Soft link can span across different file systems.

6. The command to create the soft link is,

#ln –s sourcefile destinationfile

Root Password Recovery

2011-03-09T01:21:00.001+05:30

It is the responsibility of the system administrator to keep the password safe. However, there are chances that you may forget to update the root password in your password sheet when working with multiple servers (sometimes, even servers of multiple customers). Below are the steps to recover the forgotten root password, which will cause the downtime of at least 15 minutes.

1. Insert a recovery media (AIX OS CD/Tape containing the OS image) and reboot the system. (Your boot list should be set in such a way that the system will boot from the inserted media)

2. Press F1 or 1 on hearing the beep sound.

3. Go to boot options

4. Configure Boot device order

5. Set the proper boot device

6. On saving the same, system will reboot automatically from the new boot device

7. Define the system console and select the language

8. You will get a menu from where you need to select the option “Start the maintenance mode for System Recovery”

9. Then, “Access a Root Volume Group”

10. This will list the all the VGs. Select the rootvg. You can list the VG information and confirm which one is rootvg.

11. Select “Access this Volume Group and start Shell”

12. Change the password using the passwd command

# passwd

13. Restart the system and login with the new password

Changing the corrupted Boot Order

2011-03-09T00:55:00.001+05:30

If the boot order is corrupted, or the boot order is wrong, then the system will not boot. We need to correct the boot order from maintenance mode. Below are the steps to change the corrupted boot order.

1. Insert a recovery media (AIX OS CD/Tape containing the OS image) and reboot the system. (Your boot list should be set in such a way that the system will boot from the inserted media)

2. Press F1 or 1 on hearing the beep sound.

3. Go to boot options

4. Configure Boot device order

5. Set the proper boot device

6. On saving the same, system will reboot automatically from the new boot device

7. Define the system console and select the language

8. You will get a menu from where you need to select the option “Start the maintenance mode for System Recovery”

9. Then, “Access a Root Volume Group”

10. This will list the all the VGs. Select the rootvg. You can list the VG information and confirm which one is rootvg.

11. Select “Access this Volume Group and start Shell”

12. Change the boot order using the command

# bootlist –m normal –o hdisk0 hdisk1 cd0

13. Restart the system to boot with the new order

Recovering from a corrupted BLV

2011-03-09T00:45:00.002+05:30

Impact of corrupted BLV and its recovery:

As we know, Boot Logical Volume (BLV) have AIX kernel, reduced ODM, boot commands, rc.boot scripts which are required to boot the AIX system. There are chances that BLV can get corrupted, due to bad blocks in the disk. Possibility is there the LV hd5 which corresponds to the BLV is deleted by mistake. Suppose, we are trying to boot a system which lacks the BLV, the system will not boot up.

Below are the steps to recreate the BLV:

1. Insert a recovery media (AIX OS CD/Tape containing the OS image) and start the system.

2. Press F1 or 1 on hearing the beep sound.

3. Go to boot options

4. Go to Configure Boot device order

5. Go to Set the proper boot device

6. On saving the same, system will reboot automatically from the new boot device

7. Define the system console and select the language

8. You will get a menu from where you need to select the option “Start the maintenance mode for System Recovery”

9. Then, “Access a Root Volume Group”

10. This will list the all the VGs. Select the rootvg. You can list the VG information and confirm which one is rootvg.

11. Select “Access this Volume Group and start Shell”

12. Recreate the BLV using the command

# bosboot –ad /dev/hdiskX

13. You can set the boot order with the proper disks using the command

# bootlist –m normal –o hdisk0 hdisk1

14. Reboot the system.

BLV and Its contents

2011-03-09T00:32:00.002+05:30

BLV stands for Boot Logical Volume (hd5). When an AIX system is powered on, system firmware will check the bootlist to find out the device from where the system has to boot up. Here bootlist is a part of system firmware. Once the boot device is found out from bootlist, system firmware will load the first 512 bytes of the boot device which will have the information about the BLV. This first 512 block is known as boot strap image. Boot strap code will locate the BLV from the boot device and the same will be loaded to the primary memory.

Below are the contents of the BLV:

1. AIX Kernel: AIX kernel is the core of AIX OS which will do the functions like process management, memory management, device management etc. There will be copy of AIX kernel under hd4 with name of the file as /unix, it will always be loaded from the BLV (hd5). This copy of the kernel will be used for rebuilding the kernel in BLV

2. Boot Commands: There will be many programs which will be called during the boot process. Those are known as boot commands. The commands bootinfo, cfgmgr etc are some examples.

3. Reduced ODM: In order to activate the rootvg, we should have many devices to be activated. Corresponding devices’ ODM files should be available in BLV.

4. rc.boot: Once the kernel is started, rc.boot scripts will be controlling the entire boot process. The same will be stored in BLV. The rc.boot scripts are rc.boot1, rc.boot2 and rc.boot3.

Boot Process In AIX

2011-03-09T00:20:00.003+05:30

AIX Boot Process

AIX Booting Process explained below.

Phases of the Boot Process:

I. Read Only Storage Kernel Init Phase

1. Motherboard is Checked - Power On Self Test

2. Bootlist is found - System Read Only Storage detects the first boot device in the bootlist spacified.

3. Boot image is read into memory - First 512 bytes - the bootstrap code - of the Hdisk is loaded into the RAM.

4. Initialization starts - Bootstrap code locates the BLV (hd5) from hdisk.

- BLV contains AIX Kernel, rc.boot scripts, reduced ODM, and boot commands.

- BLV get uncompressed in RAM and kernel gets released from it.

- Then AIX kernel gets the control

- AIX Kernel creates a RAMFS

- Kernel Starts the init process from BLV

- init executes rc.boot script from the BLV in the RAM

II. Base Device Configuration Phase

1. All devices are configured with cfgmgr command - init process from RAMFS executes rc.boot 1

- restbase command copies ODM from BLV to RAMFS

- cfgmgr will run and configure all the base devices

III. System Boot Phase

1. Logical volumes are varied on - rc.boot 2 is executed.

- ipl_varyon will varyon the rootvg

- run fsck on / and mount the same to RAMFS

- run fsck on /usr and mount the same to RAMFS

- run fsck on /var and mount the same to RAMFS

2. Paging is started - copycore command checks the occurrunce of the dump and copies the same from primary dump device to /var/ras/adm

- unmount /var and activate the paging space

- mount /var

- Now /, /usr and /var are mounted in rootvg in hdisk

3. /etc/inittab is processed - Kernal remove RAMFS

- init process is started from / in rootvg

- in rc.boot3, init processes /etc/inittab and remaining devices are configured.

- /etc/init starts /etc/inittab and runs /sbin/rc.boot 3

- run fsck on /tmp and mount the same.

- syncvg for rootvg and reports the stale PPs

4. Relevant Services according to run level starts. srcmstr daemon controls this - starts the deamons like errdaemon, syncd etc..

What is AIX?

2010-06-23T02:04:00.003+05:30

SOME FACTS ABOUT AIX:

AIX stands for Advanced Interactive eXecutive
It is an IBM flavour of UNIX
Initial version was released in 1986
First Stable AIX OS was launched in 1990 for RS/6000
It was released for IBM 6150 RISC workstation initially
Now it supports RS/6000 series and later IBM POWER and PowerPC - based systems, IBM system i, System/370 mainframes, PS/2 personal computers and the Apple Network Server
AIX is based on Unix Systtem V
Latest stable AIX version is 7.1

Purpose of the Site

2010-06-17T17:05:00.001+05:30

Dear All,
Hearty Welcome to "UNIX World"!!!!!
This site is meant for those who are interested to learn on different flavour of UNIX.
Currently, we have the categories Basic UNIX, IBM AIX, Shell Scripting.
We will be introducing more topics one by one.
Your suggestions are invited!!!!!