Commands

Difference between tail -f and tail -F

Basically these two command are very similar, except tail -F do one more thing: the retry.

This is useful when you are tailing a web server log, e.g. apache log. When the the log file got rotated, thetail -F will continue to work since it will retry for the rotated file, while tail -f can't.

Make Backup file
cp filename{,.bak}

Show disk usage of current dir or selected dir

 du -s <dir>

Write to stdout

 echo anything

Write to a file

 echo anything > <path> 

Append to a file

 echo anything >> <path> 

Update the modified time for a file

 touch <path> 

Quickly create an empty file

 > <path> 

Show differences between files

 diff -r leftDir rightDir 

Show files that differ without details

 diff -r -q leftDir rightDir 

Trace execution of a shell script

 sh -x <fileName> 

Monitor additions to a log file

 tail -f <fileName> 

Make a symbolic link

 ln -s <path> <name> 

List files in color

 ls --color=tty
 (Alias this to ls) 

List a single column of names only

 ls -1 

List directories only

 find -type d -maxdepth 1
 (Alias this to lsd) 

List files in order of modification time

 ls -lrt 

List files in order of size

 ls -lrS 

List all open files and sockets

 lsof 

Show the number of files in a directory

 ls -1 | wc -l 

Run a shell script so it changes the environment

 source .bash_profile (or whatever script you changed) 

Run a command relative to another root file system

 chroot newroot command 

Execute a shell script and echo the commands for debugging

 sh -x yourScript 

Low-level high speed copy

 The dd command can copy between files or devices.
 
  dd if=sourcePath of=destPath bs=1M

 Using the optional bs=n option can speed up the copy. You can use
 the suffix K, M, or G on numbers.

 By adding the count option, you can write an exact number of fixed-size records.
 For example, to destroy the partition table of a disk:

  dd if=/dev/zero of=/dev/sdb3 bs=1K count=1

 To display the status of a dd copy that's aready started, use this command
 in another shell window:
  kill -USR1 `pgrep '^dd$'`

 This will cause dd to show a status message in it's original window
 and continue with the copy operation. 

Write a line to the syslog

 logger This line is appended to /var/log/messages 
 logger -t MYTAG This line gets a MYTAG: prefix 

Serial Number of VMware
dmidecode  -t  1


To see the contents of a folder in human readable form
du -h --max-depth=1 

Currently mounted filesystems in nice layout
mount | column -t 

Commands to find out Hardware details:

a) lshw: lshw (Hardware Lister) is a small tool to provide detailed information on the hardware configuration of the machine. It can report exact memory configuration, firmware version, mainboard configuration, CPU version and speed, cache configuration, bus speed, etc. on DMI-capable x86 or EFI (IA-64) systems and on some PowerPC machines.

b) Dmidecode: reports information about your system's hardware as described in your system BIOS according to the SMBIOS/DMI standard (see a sample output). This information typically includes system manufacturer, model name, serial number, BIOS version, asset tag as well as a lot of other details of varying level of interest and reliability depending on the manufacturer. This will often include usage status for the CPU sockets, expansion slots (e.g. AGP, PCI, ISA) and memory module slots, and the list of I/O ports (e.g. serial, parallel, USB). http://www.nongnu.org/dmidecode/

c) lspci is a utility for displaying information about all PCI buses in the system and all devices connected to them. 

lspci -tv: Tree like Structure + Verbose

d) lsmod shows information about all loaded modules.The format is name, size, use count, list of referring modules. The information displayed is identical to that available from /proc/modules.

e) cat /etc/sysconfig/hwconf | less

f) dmesg is used to examine or control the kernel ring buffer and for hardware details too.

The program helps users to print out their bootup messages. Instead of copying the messages by hand, the user need only:

dmesg > boot.messages

and mail the boot.messages file to whoever can debug their problem.  

g) lsdev - display information about installed hardware.

h) cat/proc/options.....

2) ls -ltr: Lists according to last created Item.


3) ls -lta: Lists parent directory and present directory.


4) ntsysv : Lists services settings.


5) lokkit: Lists firewall configuration.


6) lsof -i : Lists the open files associated with current ports.


7) find / -type f -mmin 10   ( Here f=  normal files, -mmin 10=  finding files modified less then 10 min)

9) sysreport or sysreport -normp 

$ uname


$ uname -a
Linux box 2.4.10-4GB #1 Fri Sep 28 17:19:49 GMT 2001 i686 unknown


The output from uname indicates that the operating system is Linux, its
(kernel) release is 2.4.10, version #1 Fri Sep 28 17:19:49 GMT 2001, 34


Getting started and the name of the machine you are using is box. The hardware being
used (i.e. the type of physical machine, as opposed to the operating system, which is software), is an i686.


$ uname -r -v
2.4.10-4GB #1 Fri Sep 28 17:19:49 GMT 2001
$ uname -v -r
2.4.10-4GB #1 Fri Sep 28 17:19:49 GMT 2001
$ uname -rv
2.4.10-4GB #1 Fri Sep 28 17:19:49 GMT 2001
$ uname -vr
2.4.10-4GB #1 Fri Sep 28 17:19:49 GMT 2001




Options -m (machine), -n (nodename), -r (release), -s (system name)
or -v (version) can be used to print out part of the information that -a
(all) supplies. With no argument, -s is assumed by default. You can
combine options, for instance to print out the release and version of your
system, and can do so in one of four ways.

1. What does 'ps'mean?

ps is the shortage for Process Status. The command should be used to display the currently running processes on Unix/Linux systems. If you know the 'Task-Manager' which pops up under Windows NT/2000/XP when you press CTRL+ALT+DEL then you have a clue what ps does under Unix/Linux. Ps can show you the running processes on your system in different ways. I will describe the basic ones you should know.

2. Why is it good to know how the ps command works?

If you have a process which seems to hang (e.g. netscape navigator on some buggy websites) and you want to stop the process, then you can determine the process id of the process. Why do you need the process id? You can stop the process with the help of the 'kill' command. The kill command needs a process number otherwise it won't know to which process it should send the 'kill' signal. Other example. You have started a process and now your machines becomes slower and slower. You stop the process but your machine still gets slower and slower. Now it will be helpful if you can stop the process. In this case you also need its pid.
Next example: You want to know what processes your friend just uses (you know he has an remote session open), then you can also use the ps command to find out which processes belong to him.

Communicating with Users

Communicate with the system’s users and let them know what is going on! All Systems Administrators have the maintenance window where we can finally get control and handle some offline tasks. This is just one example of a need to communicate with the systems’ users, if any are still logged in. The most common way to get information to the system users is to use the /etc/motd file. This file is displayed each time the user logs in. If users stay logged in for days at a time they will not see any new messages of the day. This is one reason why real-time communication is needed. The commands shown in Table  allow communication to, or between, users who are currently logged in the system.

3. Some examples for the ps command

 

ps -f Full listing of the user’s currently running processes

ps -ef Full listing of all processes, except kernel processes

ps -A All processes including kernel processes

ps -Kf Full listing of kernel processes

ps auxw Wide listing sorted by percentage of CPU usage, %CPU

 

Executing the ps command

just enter 'ps' at the prompt:

$ ps
PID TTY          TIME CMD
3511 pts/1    00:00:00 bash
3514 pts/1    00:00:00 ps 

3.2. Displaying all processes owned by a specific user

$ ps ux
USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
heyne      691  0.0  2.4 19272 9576 ?        S    13:35   0:00 kdeinit: kded    
heyne      700  0.1  1.0  5880 3944 ?        S    13:35   0:01 artsd -F 10 -S 40
heyne      710  0.0  2.8 21876 11072 ?       S    13:35   0:00 kdeinit: knotify 
heyne      711  0.0  0.0  1344  352 ?        S    13:35   0:00 kwrapper ksmserve
heyne      713  0.0  2.4 18900 9304 ?        S    13:35   0:00 kdeinit: ksmserve
heyne      714  0.0  2.9 21548 11528 ?       S    13:35   0:00 kdeinit: kwin -se
heyne      715  0.3  4.8 31096 18820 ?       S    13:35   0:03 /usr/lib/mozilla/
heyne      719  0.1  3.5 22548 13680 ?       S    13:35   0:01 kdeinit: kdesktop
heyne      721  0.5  3.6 23904 14028 ?       S    13:35   0:05 kdeinit: kicker  
heyne      722  0.0  2.0 18504 7824 ?        S    13:35   0:00 kdeinit: kio_file
heyne      723  0.0  4.8 31096 18820 ?       S    13:36   0:00 /usr/lib/mozilla/
heyne      724  0.0  4.8 31096 18820 ?       S    13:36   0:00 /usr/lib/mozilla/
heyne      725  0.0  4.8 31096 18820 ?       S    13:36   0:00 /usr/lib/mozilla/
heyne      729  0.0  2.4 19408 9404 ?        S    13:36   0:00 kdeinit: klaptopd
heyne      730  0.0  2.3 17044 9152 ?        S    13:36   0:00 kteatime -session
heyne      731  0.0  3.0 21236 11848 ?       S    13:36   0:00 kdeinit: kmix -se
heyne      735  0.0  4.8 31096 18820 ?       S    13:36   0:00 /usr/lib/mozilla/
heyne      736  0.0  2.7 20492 10600 ?       S    13:36   0:00 korgac --miniicon
heyne      745  0.0  2.4 19232 9528 ?        S    13:36   0:00 kalarmd --login
heyne      753  0.0  0.3  2108 1160 pts/0    S    13:36   0:00 bash
heyne      787  0.7  1.7  9520 6784 ?        S    13:50   0:00 emacs
heyne      789  0.2  0.3  2112 1164 pts/1    S    13:51   0:00 bash
heyne      794  0.0  0.4  3560 1576 pts/1    R    13:51   0:00 ps ux                     

You can also use the syntax "ps U username". In my case I use ps U heyne. If you use this syntax you get a result like the following:

PID TTY      STAT   TIME COMMAND
721 ?        S      0:08 kdeinit: kicker          
722 ?        S      0:00 kdeinit: kio_file file /tmp/ksocket-heyne/klauncherx8
723 ?        S      0:00 /usr/lib/mozilla/mozilla-bin
724 ?        S      0:00 /usr/lib/mozilla/mozilla-bin
725 ?        S      0:00 /usr/lib/mozilla/mozilla-bin
729 ?        S      0:00 kdeinit: klaptopdaemon -session 11c0a8021400010381450
730 ?        S      0:00 kteatime -session 11c0a802140001038168018000001282500
731 ?        S      0:00 kdeinit: kmix -session 11c0a8021400010368648780000000
735 ?        S      0:00 /usr/lib/mozilla/mozilla-bin
736 ?        S      0:00 korgac --miniicon korganizer
745 ?        S      0:00 kalarmd --login
753 pts/0    S      0:00 bash
787 ?        S      0:01 emacs
789 pts/1    S      0:00 bash
796 ?        S      0:07 kdeinit: konqueror --silent
800 ?        S      0:00 kdeinit: kio_uiserver    
801 ?        S      0:02 xmms /files/mp3/00's/Badly Drawn Boy-About a Boy-03-S
802 ?        S      0:00 xmms /files/mp3/00's/Badly Drawn Boy-About a Boy-03-S
803 ?        S      0:00 xmms /files/mp3/00's/Badly Drawn Boy-About a Boy-03-S
804 ?        S      0:00 xmms /files/mp3/00's/Badly Drawn Boy-About a Boy-03-S
837 ?        S      0:00 xmms /files/mp3/00's/Badly Drawn Boy-About a Boy-03-S
838 ?        S      0:00 xmms /files/mp3/00's/Badly Drawn Boy-About a Boy-03-S
860 pts/1    R      0:00 ps U heyne

As you can see, the ps command can give you a lot of interesting information. If you for example want to know what your friend actually does, just replace your login name with her/his name and you see all processe belonging to her/him.

3.3. Own output format

If you are bored by the regular output, you could simply change the format. To do so use the formatting characters which are supported by the ps command.
If you execute the ps command with the 'o' parameter you can tell the ps command what you want to see:
e.g.
Odd display with AIX field descriptors:

$ ps -o "%u : %U : %p : %a"
RUSER    : USER     :   PID : COMMAND
heyne    : heyne    :  3363 : bash
heyne    : heyne    :  3367 : ps -o %u : %U : %p : %a

• / - root directory
• ./ - current directory
• ./command_name - run a command in the current directory when the current directory is not on the path
• ../ - parent directory
• ~ - home directory
• $ - typical prompt when logged in as ordinary user
• # - typical prompt when logged in as root or superuser
• ! - repeat specified command
• !! - repeat previous command
• ^^ - repeat previous command with substitution
• & - run a program in background mode
• [Tab][Tab] - prints a list of all available commands. This is just an example of autocomplete with no restriction on the first letter.
• x[Tab][Tab] - prints a list of all available completions for a command, where the beginning is ``x''
• [Alt][Ctrl][F1] - switch to the first virtual text console
• [Alt][Ctrl][Fn] - switch to the n^th virtual text console. Typically, there are six on a Linux PC system.
• [Alt][Ctrl][F7] - switch to the first GUI console, if there is one running. If the graphical console freezes, one can switch to a nongraphical console, kill the process that is giving problems, and switch back to the graphical console using this shortcut.
• [ArrowUp] - scroll through the command history (in bash)
• [Shift][PageUp] - scroll terminal output up. This also works at the login prompt, so you can scroll through your boot messages.
• [Shift][PageDown] - scroll terminal output down
• [Ctrl][Alt][+] - switch to next X server resolution (if the server is set up for more than one resolution)
• [Ctrl][Alt][-] - change to previous X server resolution
• [Ctrl][Alt][BkSpc] - kill the current X server. Used when normal exit is not possible.
• [Ctrl][Alt][Del] - shut down the system and reboot
• [Ctrl]c - kill the current process
• [Ctrl]d - logout from the current terminal
• [Ctrl]s - stop transfer to current terminal
• [Ctrl]q - resume transfer to current terminal. This should be tried if the terminal stops responding.
• [Ctrl]z - send current process to the background
• reset - restore a terminal to its default settings
• [Leftmousebutton] - Hold down left mouse button and drag to highlight text. Releasing the button copies the region to the text buffer under X and (if gpm is installed) in console mode.
• [Middlemousebutton] - Copies text from the text buffer and inserts it at the cursor location. With a two-button mouse, click on both buttons simultaneously. It is necessary for three-button emulation to be enabled, either under gpm or in XF86Config.

Turning off PING reply:

Disable ping reply Temporarily

You can temporarily disable the ping reply using the following method.

# echo "1" >  /proc/sys/net/ipv4/icmp_echo_ignore_all

Please note that this setting will be erased after the reboot. To disable ping reply permanently (even after the reboot), follow the step mentioned below.
Also, to enable the ping reply back, set the value to “0″ as shown below.

# echo "0" >  /proc/sys/net/ipv4/icmp_echo_ignore_all

Disable ping reply Permanently

You can permanently disable the ping reply using the following method.
Step 1: Edit the sysctl.conf file and add the following line.

net.ipv4.icmp_echo_ignore_all = 1

Step 2: Execute sysctl -p to enforce this setting immediately.

# sysctl -p

The above command loads the sysctl settings from the sysctl.conf file.
After the ping reply is disabled using one of the above method, when somebody tries to ping your machine they will end up waiting without getting a ping reply packet even when the machine is up and running.

###
exportfs -a : Exports all shared directories from /etc/exports
exportfs -r  : Revises the list of shared directories after you have changed /etc/exports
This sends the appropriate signals to the rpc.nfsd and rpc.mountd daemons to reread the /etc/exports file and update their internal tables

exportfs -u : Unexports all directories
exportfs -v : Display currently shared directories.

If you have an error in your /etc/exports file, it is reported when NFS starts up in syslog

Commands Operations.

Ctrl-G                           Display current line number.

gg                                 Move to first line in file. {vim}

numG                           Move to line number num.

G                                   Move to last line in file.

:num                            Move to line number num.

H                                   Top line of screen (Home position).

M                                   Middle line of screen.

L                                    Last line of screen.

CTRL + C                   Terminate current program

CTRL + Z                     Suspend current program

CTRL + D                    Exit the current shell

Tab                               Command/file auto-completion

Home                           Go to the beginning of the command line

End                              Go to the end of the command line

CTRL + L                     Clear the screen

Backspace                 Deletes text behind the cursor

ATL + Backspace     Delete entire word behind the cursor

Delete                          Delete text in front of the cursor

CTRL + R                    Search command history

Up Arrow                    Cycle backward through command history

Down Arrow               Cycle forward through command history

Left Arrow                   Move the cursor left one character

Right Arrow                Move the cursor right one character

ALT + B                        Move the cursor back one word

ALT + F                         Move the cursor forward one word

CTRL + U                    Cut all text before the cursor

CTRL + K                    Cut all text after the cursor

ALT + D                       Cut the currently selected word

CTRL + Y                    Paste previously cut text

CTRL + _ (Underscore)    Undo changes typed on the command line

CTRL + T                     Transpose the previous two characters on the command line

ALT + T                        Transpose the previous two words on the command line

ATL + L                        Convert word to lowercase

ATL + U                       Convert word to uppercase

C-f	Move forward a character
C-b	Move backward a character
C-n	Move to next line
C-p	Move to previous line
C-a	Move to beginning of line
C-e	Move to end of line

Linux Shortcuts for Shell Commands :

• Ctrl + D : exit a terminal, same as typing `exit`
• Ctrl + L : clears the screen, same as `clear`
• Ctrl + C : breaks/cancels an ongoing operation
• Ctrl + Z : pauses (stops) an ongoing operation. Type `fg` (foreground) to resume it or `bg` (background) to continue the process in the background
• Ctrl + Alt + Del : shuts down computer

Other bash Shell shortcuts:

Bash Shell shortcuts for Moving backward Forward :

• Ctrl + A : Go to the beginning of the line you are currently typing on
• Ctrl + E : Go to the end of the line you are currently typing on
• Ctrl + U : Clears the line before the cursor position. If you are at the end of the line, clears the entire line.
• Ctrl + W : Delete the word before the cursor.
• Ctrl + K : Clear the line after the cursor.
• Ctrl + T : Swap the last two characters before the cursor.
• Esc + T : Swap the last two words before the cursor.
• Alt + F : Move cursor forward one word on the current line.
• Alt + B : Move cursor backward one word on the current line.

History related bash shell shortcuts:

• Ctrl + R : Let’s you search through previously used commands.
• Alt + < : Move to the first line in the history.
• Alt + > : Move to the last line in the history.

Other bash shell shortcuts:

• Ctrl + H : Same as backspace.
• Tab : Auto:complete files and folder names.
• Ctrl + Y : to paste it (as in delete and copy) all text in front of the cursor.
• Alt + ? : Show current completion list.
• Alt + * : Insert all possible completions.
• Alt + / : Attempt to complete filename.
• Alt + c : Capitalize the word.
• Alt + l : Make word lowercase.
• Alt + r : Recall command.