Tuesday, September 20, 2011

Linux device naming

10:33 AM

SADBAD

Linux device naming

This topic provides information to help you understand naming patterns for devices on Linux.

Special device files are used by kernel drivers to control a device. There can be more than one special device file that maps to the same physical device. For example, in a multipath environment with four paths to the same device, four different device files will map to the same physical device.

The device files are located in the /dev directory and are addressed by a major and minor number pair. Fibre Channel attached devices are managed as SCSI disk devices by the sd driver. Thus, each of the attached storage unit LUNs has a special device file in the Linux directory /dev.

SCSI disk devices have special device files that start with an ‘sd’ prefix, and have the following naming format:

/dev/sd[a-z][a-z][1-15]

Names without a trailing digit refer to the whole disk, while names with a trailing digit refer to a partition of that whole disk. By convention, SCSI disks have a maximum of 16 minor numbers mapped to a single disk. Thus, for each whole disk, there is a maximum of 15 partitions per disk because one minor number is used to describe the entire disk (for example /dev/sda), and the other 15 minor numbers are used to refer to partitions for that disk (for example /dev/sda1, /dev/sda2, etc). The following example displays the device files for the whole disk /dev/sda, which has a major number of 8 and a minor number of 0, and its 15 partitions.

# ls -l /dev/sda*

brw-rw---- 1 root disk 8, 0 May 24 08:09 /dev/sda

brw-rw---- 1 root disk 8, 1 May 24 08:09 /dev/sda1

brw-rw---- 1 root disk 8, 10 May 24 08:09 /dev/sda10

brw-rw---- 1 root disk 8, 11 May 24 08:09 /dev/sda11

brw-rw---- 1 root disk 8, 12 May 24 08:09 /dev/sda12

brw-rw---- 1 root disk 8, 13 May 24 08:09 /dev/sda13

brw-rw---- 1 root disk 8, 14 May 24 08:09 /dev/sda14

brw-rw---- 1 root disk 8, 15 May 24 08:09 /dev/sda15

brw-rw---- 1 root disk 8, 2 May 24 08:09 /dev/sda2

brw-rw---- 1 root disk 8, 3 May 24 08:09 /dev/sda3

brw-rw---- 1 root disk 8, 4 May 24 08:09 /dev/sda4

brw-rw---- 1 root disk 8, 5 May 24 08:09 /dev/sda5

brw-rw---- 1 root disk 8, 6 May 24 08:09 /dev/sda6

brw-rw---- 1 root disk 8, 7 May 24 08:09 /dev/sda7

brw-rw---- 1 root disk 8, 8 May 24 08:09 /dev/sda8

brw-rw---- 1 root disk 8, 9 May 24 08:09 /dev/sda9

For Red Hat, the kernel automatically creates the device files for 128 devices. For SUSE, there are only special device files for the first 16 disks. You must create the device files for additional disks by using the mknod command. For 2.6 kernels, the special device files are only created when they are detected and recognized by the kernel. The /proc/partitions file lists all the ‘sd’ devices that are recognized by the SCSI disk driver, including the sd name, major number, minor number, and size of each disk device.

The following is an example /proc/partitions file:

# cat /proc/partitions

major minor #blocks name

8 0 17774160 sda

8 1 1052226 sda1

8 2 208845 sda2

8 3 10490445 sda3

8 16 976576 sdb

8 32 976576 sdc

8 48 976576 sdd

8 64 976576 sde

8 80 976576 sdf

8 96 976576 sdg

8 112 976576 sdh

8 128 976576 sdi

8 144 976576 sdj

8 160 976576 sdk

8 176 976576 sdl

8 192 976576 sdm

8 208 976576 sdn

8 224 976576 sdo

8 240 976576 sdp

65 0 976576 sdq

65 16 1048576 sdr

65 32 1048576 sds

65 48 1048576 sdt

65 64 1048576 sdu

65 80 1048576 sdv

65 96 1048576 sdw

65 112 1048576 sdx

65 128 1048576 sdy

65 144 1048576 sdz

65 160 1048576 sdaa

65 176 1048576 sdab

65 192 1048576 sdac

65 208 1048576 sdad

65 224 1048576 sdae

65 240 1048576 sdaf

66 0 1048576 sdag

66 16 1048576 sdah

66 32 1048576 sdai

66 48 1048576 sdaj

66 64 1048576 sdak

66 80 1048576 sdal

66 96 1048576 sdam

66 112 1048576 sdan

66 128 1048576 sdao

66 144 1048576 sdap

66 160 1048576 sdaq

66 176 1048576 sdar

66 192 1048576 sdas

66 208 1048576 sdat

66 224 1048576 sdau

66 240 1048576 sdav

LUN detection procedures

10:26 AM

SADBAD

LUN detection procedures

This topic describes how to manually configure LUNs on your Linux host system.

If you have a Linux driver that does not automatically configure any LUNs other than LUN 0, you can manually configure the other LUNs, depending on the parameters and settings that are used for the SCSI mid-layer driver. Figure 1 shows an example of the /proc/scsi/scsi file for a Linux host that only configures the first LUN (LUN 0) on each host adapter port.

Figure 1. Example of a /proc/scsi/scsi file from a Linux host that only configures LUN 0

# cat proc/scsi/scsi
...
Host: scsi0 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM-PSG  Model: DPSS-318350M  F  Rev: S9HA
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi0 Channel: 00 Id: 15 Lun: 00
  Vendor: IBM      Model: TP4.6 V41b3      Rev: 4.1b
  Type:   Processor                        ANSI SCSI revision: 02
Host: scsi2 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 01 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03

There are two ways to work around the issue of only having LUN 0 configured:

Create a script to manually add devices into /proc/scsi/scsi.
Detect LUNs automatically when the system is started by modifying the initial ram-disk (initrd).

Creating a script to manually add devices into /proc/scsi/scsi

Use the scsi add-single-device command to consecutively configure all of the LUNs that are assigned to your host system. Write a script that repeats the scsi add-single-device command for each LUN on each ID for each host adapter. The script must scan all host adapter ports and identify all of the LUNs that are assigned to each port.

After you run the script, you can view all of the assigned LUNs in the /proc/scsi/scsi file.

Figure 2 shows part of an example /proc/scsi/scsi file for a Linux host after a script has configured every LUN.

Figure 2. Example of a /proc/scsi/scsi file for a Linux host with configured LUNs

# cat proc/scsi/scsi
...
Host: scsi3 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 01 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 01
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 02
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 03
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 04
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
...

Detecting LUNs automatically when the system is started by modifying the initial ram-disk (initrd)

The second method of configuring LUNs for a Linux system with only LUN 0 configured involves setting the parameter for the SCSI mid-layer driver that controls how many LUNs are scanned during a SCSI bus scan. The following procedure works for both 2.4 and 2.6 kernels, but it assumes the SCSI mid-layer driver is compiled as a scsi_mod module that is loaded automatically when the system is started.

To set the maximum number of disk devices under Linux to properly detect all volumes in Linux 2.4 kernels, set the max_scsi_luns option for the SCSI mid-layer driver. For example, if max_scsi_luns is set to 1, this limits SCSI bus scans to only LUN 0. Set this value to the respective maximum number of disks the kernel can support, for example, 128 or 256. The same procedure applies to Linux 2.6 kernels except that the parameter is max_luns.

Perform the following steps to set the maximum number of disk devices:

Open the /etc/modules.conf file.
For Linux 2.4 kernels, add the following line:
- options scsi_mod max_scsi_luns=n
For Linux 2.6 kernels, add the following line:
- options scsi_mod max_luns=n
n is the total number of LUNs to probe.
Save the file.
To rebuild the ram-disk associated with the current kernel, issue the mkinitrd command for your operating system. In the following examples, kernel refers to the uname –r output that shows the currently running kernel level, for example, 2.4.21-292-smp.
For SUSE distributions, issue the following command:
```
cd /boot
mkinitrd –k vmlinuz-kernel -i initrd-kernel
```
For Red Hat distributions, issue the following command:
```
cd /boot
mkinitrd –v initrd-kernel.img kernel
```
Restart the host.
Verify that the boot files are correctly configured for the new initrd image in the /boot/grub/menu.lst file.

Dynamic SAN fabric reconfiguration

10:24 AM

SADBAD

Dynamic SAN fabric reconfiguration

There are several methods that you can use to force the Linux operating system to recognize disks that are added or removed from the fabric.

When you add or remove disks from the fabric, you can use any of the following methods to force the Linux host to recognize these changes:

Rescan the SCSI bus by restarting the host.
Rescan the SCSI bus by unloading and reloading the host adapter driver.
Rescan the bus by echoing the /sys filesystem (Linux 2.6 kernels only).
Manually add and remove SCSI disks by echoing the /proc or /sys filesystem.

Because devices are discovered by scanning the SCSI bus, it is typically easiest to rescan the SCSI bus to detect any SAN fabric changes.

Rescan the bus by restarting the host or by unloading and reloading the host adapter driver

A bus rescan is automatically triggered by restarting the system or reloading the host adapter driver.

Reloading the host adapter driver assumes that the host adapter driver is built as a module. Restarting the system works whether the driver is compiled in the kernel or as a module.

Before performing these tasks, you must:

Stop all I/O.
Unmount all file systems.
If SDD is being used, unload the SDD driver with the sdd stop command before reloading the host adapter driver. After the host adapter driver is reloaded, reload SDD with the sdd start command.

Rescan the bus by echoing the /sys filesystem

For Linux 2.6 kernels only, a rescan can be triggered through the /sys interface without having to unload the host adapter driver or restart the system. The following command scans all channels, targets, and LUNs on host H.

echo “- - -” > /sys/class/scsi_host/hostH/scan

Manually add and remove SCSI disks by echoing the /proc or /sys filesystem

You can use the following commands to manually add and remove SCSI disk.

Note: In the following command examples, H, B, T, L, are the host, bus, target, and LUN IDs for the device.

You can unconfigure and remove an unused SCSI disk with the following command:

echo "scsi remove-single-device H B T L" > /proc/scsi/scsi

If the driver cannot be unloaded and loaded again, and you know the host, bus, target and LUN IDs for the new devices, you can add them through the /proc/scsi/scsi file using the following command:
echo "scsi add-single-device H B T L" > /proc/scsi/scsi

For Linux 2.6 kernels, devices can also be added and removed through the /sys filesystem. Use the following command to remove a disk from the kernel’s recognition:

echo “1” > /sys/class/scsi_host/hostH/device/H:B:T:L/delete
or, as a possible variant on other 2.6 kernels, you can use the command:
echo “1” > /sys/class/scsi_host/hostH/device/targetH:B:T/H:B:T:L/delete

To reregister the disk with the kernel use the command:

echo “B T L” > /sys/class/scsi_host/hostH/scan

Note: The Linux kernel does not assign permanent names for the fabric devices in the /dev directory. Device file names are assigned in the order in which devices are discovered during the bus scanning. For example, a LUN might be /dev/sda. After a driver reload, the same LUN might become /dev/sdce. A fabric reconfiguration might also result in a shift in the host, bus, target and LUN IDs, which makes it unreliable to add specific devices through the /proc/scsi/scsi file.

LUN identification for the Linux host system

10:22 AM

SADBAD

LUN identification for the Linux host system

This topic describes LUN identification for the Linux host system.

In Linux 2.4 kernels, SCSI devices are discovered by scanning the SCSI bus when the host adapter driver is loaded. A list of devices which have been discovered and are recognized by the SCSI subsystem are listed in /proc/scsi/scsi. It is good practice to use the cat command to display the output of /proc/scsi/scsi to verify that the correct number of LUNs has been recognized by the kernel.

The following is example output:

# cat proc/scsi/scsi
# cat /proc/scsi/scsi | less
Attached devices:
Host: scsi1 Channel: 00 Id: 02 Lun: 00
Vendor: IBM Model: 2107900 Rev: .248
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi1 Channel: 00 Id: 02 Lun: 01
Vendor: IBM Model: 2107900 Rev: .248
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi1 Channel: 00 Id: 03 Lun: 00
Vendor: IBM Model: 2107900 Rev: 5.53
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi1 Channel: 00 Id: 03 Lun: 01
Vendor: IBM Model: 2107900 Rev: 5.53
Type: Direct-Access ANSI SCSI revision: 05

Note: The /proc/scsi/scsi list of devices is not dynamic and does not reflect state changes caused by fabric changes. Please see the section on “Dynamic SAN fabric reconfiguration' for more information on how to deal with fabric changes.

Starting with the Linux 2.6 kernels, the /proc filesystem is migrating over to the improved /sys filesystem. The /sys filesystem supports dynamic changes such as adding and removing LUNs without requiring a host adapter driver reload or restarting the host system. Typically, more accurate information about what SCSI devices are recognized by the host system is attained by checking the contents of the directory /sys/class/scsi_host/hostN, where N is host adapter ID number. lsscsi is a user-space tool that uses the information in /sys to display a summarized list of all the recognized devices. This tool may not currently be available on all Linux 2.6 distribution kernels.

Friday, June 10, 2011

50 Most Frequently Used UNIX / Linux Commands (With Examples)

9:41 AM

SADBAD

50 Most Frequently Used UNIX / Linux Commands (With Examples)

　
1. tar command examples
Create a new tar archive.
$ tar cvf archive_name.tar dirname/
Extract from an existing tar archive.
$ tar xvf archive_name.tar
View an existing tar archive.
$ tar tvf archive_name.tar
　
2. grep command examples
Search for a given string in a file (case in-sensitive search).
$ grep -i "the" demo_file
Print the matched line, along with the 3 lines after it.
$ grep -A 3 -i "example" demo_text
Search for a given string in all files recursively
$ grep -r "ramesh" *
　
3. find command examples
Find files using file-name ( case in-sensitve find)
# find -iname "MyCProgram.c"
Execute commands on files found by the find command
$ find -iname "MyCProgram.c" -exec md5sum {} \;
Find all empty files in home directory
# find ~ -empty
More find examples: Mommy, I found it! — 15 Practical Linux Find Command Examples
4. ssh command examples
Login to remote host
ssh -l jsmith remotehost.example.com
Debug ssh client
ssh -v -l jsmith remotehost.example.com
Display ssh client version
$ ssh -V
OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003
More ssh examples: 5 Basic Linux SSH Client Commands
5. sed command examples
When you copy a DOS file to Unix, you could find \r\n in the end of each line. This example converts the DOS file format to Unix file format using sed command.
$sed 's/.$//' filename
Print file content in reverse order
$ sed -n '1!G;h;$p' thegeekstuff.txt
Add line number for all non-empty-lines in a file
$ sed '/./=' thegeekstuff.txt sed 'N; s/\n/ /'
More sed examples: Advanced Sed Substitution Examples
6. awk command examplesRemove duplicate lines using awk
$ awk '!($0 in array) { array[$0]; print }' temp
Print all lines from /etc/passwd that has the same uid and gid
$awk -F ':' '$3==$4' passwd.txt
Print only specific field from a file.
$ awk '{print $2,$5;}' employee.txt
More awk examples: 8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR
7. vim command examples
Go to the 143rd line of file
$ vim +143 filename.txt
Go to the first match of the specified
$ vim +/search-term filename.txt
Open the file in read only mode.
$ vim -R /etc/passwd
More vim examples: How To Record and Play in Vim Editor
8. diff command examples
Ignore white space while comparing.
# diff -w name_list.txt name_list_new.txt
2c2,3
< John Doe --- > John M Doe
> Jason Bourne
More diff examples: Top 4 File Difference Tools on UNIX / Linux – Diff, Colordiff, Wdiff, Vimdiff
9. sort command examples
Sort a file in ascending order
$ sort names.txt
Sort a file in descending order
$ sort -r names.txt
Sort passwd file by 3rd field.
$ sort -t: -k 3n /etc/passwd more
10. export command examples
To view oracle related environment variables.
$ export grep ORACLE
declare -x ORACLE_BASE="/u01/app/oracle"
declare -x ORACLE_HOME="/u01/app/oracle/product/10.2.0"
declare -x ORACLE_SID="med"
declare -x ORACLE_TERM="xterm"
To export an environment variable:
$ export ORACLE_HOME=/u01/app/oracle/product/10.2.0
11. xargs command examples
Copy all images to external hard-drive
# ls *.jpg xargs -n1 -i cp {} /external-hard-drive/directory
Search all jpg images in the system and archive it.
# find / -name *.jpg -type f -print xargs tar -cvzf images.tar.gz
Download all the URLs mentioned in the url-list.txt file
# cat url-list.txt xargs wget –c
12. ls command examples
Display filesize in human readable format (e.g. KB, MB etc.,)
$ ls -lh
-rw-r----- 1 ramesh team-dev 8.9M Jun 12 15:27 arch-linux.txt.gz
Order Files Based on Last Modified Time (In Reverse Order) Using ls -ltr
$ ls -ltr
Visual Classification of Files With Special Characters Using ls -F
$ ls -F
More ls examples: Unix LS Command: 15 Practical Examples
13. pwd command
pwd is Print working directory. What else can be said about the good old pwd who has been printing the current directory name for ages.
14. cd command examples
Use "cd -" to toggle between the last two directories
Use "shopt -s cdspell" to automatically correct mistyped directory names on cd
More cd examples: 6 Awesome Linux cd command Hacks
15. gzip command examples
To create a *.gz compressed file:
$ gzip test.txt
To uncompress a *.gz file:
$ gzip -d test.txt.gz
Display compression ratio of the compressed file using gzip -l
$ gzip -l *.gz
compressed uncompressed ratio uncompressed_name
23709 97975 75.8% asp-patch-rpms.txt
16. bzip2 command examples
To create a *.bz2 compressed file:
$ bzip2 test.txt
To uncompress a *.bz2 file:
bzip2 -d test.txt.bz2
More bzip2 examples: BZ is Eazy! bzip2, bzgrep, bzcmp, bzdiff, bzcat, bzless, bzmore examples
17. unzip command examplesTo extract a *.zip compressed file:
$ unzip test.zip
View the contents of *.zip file (Without unzipping it):
$ unzip -l jasper.zip
Archive: jasper.zip
Length Date Time Name
-------- ---- ---- ----
40995 11-30-98 23:50 META-INF/MANIFEST.MF
32169 08-25-98 21:07 classes_
15964 08-25-98 21:07 classes_names
10542 08-25-98 21:07 classes_ncomp
18. shutdown command examples
Shutdown the system and turn the power off immediately.
# shutdown -h now
Shutdown the system after 10 minutes.
# shutdown -h +10
Reboot the system using shutdown command.
# shutdown -r now
Force the filesystem check during reboot.
# shutdown -Fr now
19. ftp command examples
Both ftp and secure ftp (sftp) has similar commands. To connect to a remote server and download multiple files, do the following.
$ ftp IP/hostname
ftp> mget *.html
To view the file names located on the remote server before downloading, mls ftp command as shown below.
ftp> mls *.html -
/ftptest/features.html
/ftptest/index.html
/ftptest/othertools.html
/ftptest/samplereport.html
/ftptest/usage.html
More ftp examples: FTP and SFTP Beginners Guide with 10 Examples
20. crontab command examples
View crontab entry for a specific user
# crontab -u john -l
Schedule a cron job every 10 minutes.
*/10 * * * * /home/ramesh/check-disk-space
More crontab examples: Linux Crontab: 15 Awesome Cron Job Examples
21. service command examples
Service command is used to run the system V init scripts. i.e Instead of calling the scripts located in the /etc/init.d/ directory with their full path, you can use the service command.
Check the status of a service:
# service ssh status
Check the steatus of all the services.
service --status-all
Restart a service.
# service ssh restart
22. ps command examples
ps command is used to display information about the processes that are running in the system.
While there are lot of arguments that could be passed to a ps command, following are some of the common ones.
To view current running processes.
$ ps -ef more
To view current running processes in a tree structure. H option stands for process hierarchy.
$ ps -efH more
23. free command examples
This command is used to display the free, used, swap memory available in the system.
Typical free command output. The output is displayed in bytes.
$ free
total used free shared buffers cached
Mem: 3566408 1580220 1986188 0 203988 902960
-/+ buffers/cache: 473272 3093136
Swap: 4000176 0 4000176
If you want to quickly check how many GB of RAM your system has use the -g option. -b option displays in bytes, -k in kilo bytes, -m in mega bytes.
$ free -g
total used free shared buffers cached
Mem: 3 1 1 0 0 0
-/+ buffers/cache: 0 2
Swap: 3 0 3
If you want to see a total memory ( including the swap), use the -t switch, which will display a total line as shown below.
ramesh@ramesh-laptop:~$ free -t
total used free shared buffers cached
Mem: 3566408 1592148 1974260 0 204260 912556
-/+ buffers/cache: 475332 3091076
Swap: 4000176 0 4000176
Total: 7566584 1592148 5974436
24. top command examples
top command displays the top processes in the system ( by default sorted by cpu usage ). To sort top output by any column, Press O (upper-case O) , which will display all the possible columns that you can sort by as shown below.
Current Sort Field: P for window 1:Def
Select sort field via field letter, type any other key to return
a: PID = Process Id v: nDRT = Dirty Pages count
d: UID = User Id y: WCHAN = Sleeping in Function
e: USER = User Name z: Flags = Task Flags
........
To displays only the processes that belong to a particular user use -u option. The following will show only the top processes that belongs to oracle user.
$ top -u oracle
More top examples: Can You Top This? 15 Practical Linux Top Command Examples
25. df command examples
Displays the file system disk space usage. By default df -k displays output in bytes.
$ df -k
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda1 29530400 3233104 24797232 12% /
/dev/sda2 120367992 50171596 64082060 44% /home
df -h displays output in human readable form. i.e size will be displayed in GB’s.
ramesh@ramesh-laptop:~$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 29G 3.1G 24G 12% /
/dev/sda2 115G 48G 62G 44% /home
Use -T option to display what type of file system.
ramesh@ramesh-laptop:~$ df -T
Filesystem Type 1K-blocks Used Available Use% Mounted on
/dev/sda1 ext4 29530400 3233120 24797216 12% /
/dev/sda2 ext4 120367992 50171596 64082060 44% /home
26. kill command examples
Use kill command to terminate a process. First get the process id using ps -ef command, then use kill -9 to kill the running Linux process as shown below. You can also use killall, pkill, xkill to terminate a unix process.
$ ps -ef grep vim
ramesh 7243 7222 9 22:43 pts/2 00:00:00 vim
$ kill -9 7243
More kill examples: 4 Ways to Kill a Process – kill, killall, pkill, xkill
27. rm command examples
Get confirmation before removing the file.
$ rm -i filename.txt
It is very useful while giving shell metacharacters in the file name argument.
Print the filename and get confirmation before removing the file.
$ rm -i file*
Following example recursively removes all files and directories under the example directory. This also removes the example directory itself.
$ rm -r example
28. cp command examples
Copy file1 to file2 preserving the mode, ownership and timestamp.
$ cp -p file1 file2
Copy file1 to file2. if file2 exists prompt for confirmation before overwritting it.
$ cp -i file1 file2
29. mv command examples
Rename file1 to file2. if file2 exists prompt for confirmation before overwritting it.
$ mv -i file1 file2
Note: mv -f is just the opposite, which will overwrite file2 without prompting.
mv -v will print what is happening during file rename, which is useful while specifying shell metacharacters in the file name argument.
$ mv -v file1 file2
30. cat command examples
You can view multiple files at the same time. Following example prints the content of file1 followed by file2 to stdout.
$ cat file1 file2
While displaying the file, following cat -n command will prepend the line number to each line of the output.
$ cat -n /etc/logrotate.conf
1 /var/log/btmp {
2 missingok
3 monthly
4 create 0660 root utmp
5 rotate 1
6 }
31. mount command examples
To mount a file system, you should first create a directory and mount it as shown below.
# mkdir /u01
# mount /dev/sdb1 /u01
You can also add this to the fstab for automatic mounting. i.e Anytime system is restarted, the filesystem will be mounted.
/dev/sdb1 /u01 ext2 defaults 0 2
32. chmod command exampleschmod command is used to change the permissions for a file or directory.
Give full access to user and group (i.e read, write and execute ) on a specific file.
$ chmod ug+rwx file.txt
Revoke all access for the group (i.e read, write and execute ) on a specific file.
$ chmod g-rwx file.txt
Apply the file permissions recursively to all the files in the sub-directories.
$ chmod -R ug+rwx file.txt
More chmod examples: 7 Chmod Command Examples for Beginners
33. chown command examples
chown command is used to change the owner and group of a file. \
To change owner to oracle and group to db on a file. i.e Change both owner and group at the same time.
$ chown oracle:dba dbora.sh
Use -R to change the ownership recursively.
$ chown -R oracle:dba /home/oracle
34. passwd command examplesChange your password from command line using passwd. This will prompt for the old password followed by the new password.
$ passwd
Super user can use passwd command to reset others password. This will not prompt for current password of the user.
# passwd USERNAME
Remove password for a specific user. Root user can disable password for a specific user. Once the password is disabled, the user can login without entering the password.
# passwd -d USERNAME
35. mkdir command examples
Following example creates a directory called temp under your home directory.
$ mkdir ~/temp
Create nested directories using one mkdir command. If any of these directories exist already, it will not display any error. If any of these directories doesn’t exist, it will create them.
$ mkdir -p dir1/dir2/dir3/dir4/
36. ifconfig command examplesUse ifconfig command to view or configure a network interface on the Linux system.
View all the interfaces along with status.
$ ifconfig -a
Start or stop a specific interface using up and down command as shown below.
$ ifconfig eth0 up
$ ifconfig eth0 down
More ifconfig examples: Ifconfig: 7 Examples To Configure Network Interface
37. uname command examples
Uname command displays important information about the system such as — Kernel name, Host name, Kernel release number,Processor type, etc.,
Sample uname output from a Ubuntu laptop is shown below.
$ uname -a
Linux john-laptop 2.6.32-24-generic #41-Ubuntu SMP Thu Aug 19 01:12:52 UTC 2010 i686 GNU/Linux
38. whereis command examples
When you want to find out where a specific Unix command exists (for example, where does ls command exists?), you can execute the following command.
$ whereis ls
ls: /bin/ls /usr/share/man/man1/ls.1.gz /usr/share/man/man1p/ls.1p.gz
When you want to search an executable from a path other than the whereis default path, you can use -B option and give path as argument to it. This searches for the executable lsmk in the /tmp directory, and displays it, if it is available.
$ whereis -u -B /tmp -f lsmk
lsmk: /tmp/lsmk
39. whatis command examples
Whatis command displays a single line description about a command.
$ whatis ls
ls (1) - list directory contents
$ whatis ifconfig
ifconfig (8) - configure a network interface
40. locate command examples
Using locate command you can quickly search for the location of a specific file (or group of files). Locate command uses the database created by updatedb.
The example below shows all files in the system that contains the word crontab in it.
$ locate crontab
/etc/anacrontab
/etc/crontab
/usr/bin/crontab
/usr/share/doc/cron/examples/crontab2english.pl.gz
/usr/share/man/man1/crontab.1.gz
/usr/share/man/man5/anacrontab.5.gz
/usr/share/man/man5/crontab.5.gz
/usr/share/vim/vim72/syntax/crontab.vim
41. man command examples
Display the man page of a specific command.
$ man crontab
When a man page for a command is located under more than one section, you can view the man page for that command from a specific section as shown below.
$ man SECTION-NUMBER commandname
Following 8 sections are available in the man page.
General commands
System calls
C library functions
Special files (usually devices, those found in /dev) and drivers
File formats and conventions
Games and screensavers
Miscellaneous
System administration commands and daemons
For example, when you do whatis crontab, you’ll notice that crontab has two man pages (section 1 and section 5). To view section 5 of crontab man page, do the following.
$ whatis crontab
crontab (1) - maintain crontab files for individual users (V3)
crontab (5) - tables for driving cron
$ man 5 crontab
42. tail command examples
Print the last 10 lines of a file by default.
$ tail filename.txt
Print N number of lines from the file named filename.txt
$ tail -n N filename.txt
View the content of the file in real time using tail -f. This is useful to view the log files, that keeps growing. The command can be terminated using CTRL-C.
$ tail -f log-file
More tail examples: 3 Methods To View tail -f output of Multiple Log Files in One Terminal
43. less command examples
less is very efficient while viewing huge log files, as it doesn’t need to load the full file while opening.
$ less huge-log-file.log
One you open a file using less command, following two keys are very helpful.
CTRL+F – forward one window
CTRL+B – backward one window
More less examples: Unix Less Command: 10 Tips for Effective Navigation
44. su command examplesSwitch to a different user account using su command. Super user can switch to any other user without entering their password.
$ su - USERNAME
Execute a single command from a different account name. In the following example, john can execute the ls command as raj username. Once the command is executed, it will come back to john’s account.
[john@dev-server]$ su - raj -c 'ls'
[john@dev-server]$
Login to a specified user account, and execute the specified shell instead of the default shell.
$ su -s 'SHELLNAME' USERNAME
45. mysql command examples
mysql is probably the most widely used open source database on Linux. Even if you don’t run a mysql database on your server, you might end-up using the mysql command ( client ) to connect to a mysql database running on the remote server.
To connect to a remote mysql database. This will prompt for a password.
$ mysql -u root -p -h 192.168.1.2
To connect to a local mysql database.
$ mysql -u root -p
If you want to specify the mysql root password in the command line itself, enter it immediately after -p (without any space).
46. yum command examples
To install apache using yum.
$ yum install httpd
To upgrade apache using yum.
$ yum update httpd
To uninstall/remove apache using yum.
$ yum remove httpd
47. rpm command examples
To install apache using rpm.
# rpm -ivh httpd-2.2.3-22.0.1.el5.i386.rpm
To upgrade apache using rpm.
# rpm -uvh httpd-2.2.3-22.0.1.el5.i386.rpm
To uninstall/remove apache using rpm.
# rpm -ev httpd
More rpm examples: RPM Command: 15 Examples to Install, Uninstall, Upgrade, Query RPM Packages
48. ping command examples
Ping a remote host by sending only 5 packets.
$ ping -c 5 gmail.com
More ping examples: Ping Tutorial: 15 Effective Ping Command Examples
49. date command examples
Set the system date:
# date -s "01/31/2010 23:59:53"
Once you’ve changed the system date, you should syncronize the hardware clock with the system date as shown below.
# hwclock –systohc
# hwclock --systohc –utc
50. wget command examples
The quick and effective method to download software, music, video from internet is using wget command.
$ wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-3.2.1.tar.gz
Download and store it with a different name.
$ wget -O taglist.zip http://www.vim.org/scripts/download_script.php?src_id=7701
　

Wednesday, June 8, 2011

Scanning LUNS on Linux without reboot

9:32 AM

SADBAD

#for host in /sys/class/scsi_host/host*; do echo - - - > $host/scan; done

or

echo "- - -" > /sys/class/scsi_host/hostX/scan

X is hba port 0/1/2/3

Note: OS is RHEL