Linux Basic/Common commands

Linux Basic/Common commands

Command      Description                 

adduser           Add a new user                    

arch                  Print machine architecture                

awk                 Find and Replace text within file(s)              

bc                    An arbitrary precision calculator language                 

cal                    Display a calendar                  

cat                   Concatenate files and print on the standard output              

chdir                Change working directory                 

chgrp               Change the group ownership of files             

chkconfig        Tool for maintaining the /etc/rc[0-6].d directory hierarchy               

chmod             Change the access permissions of files and directories                    

chown             Change the user and group ownership of files                      

chroot              Change root directory           

cksum              Print CRC checksum and byte counts                      

clear                 Clear terminal screen             

cmp                 Compare two files                 

comm              Compare two sorted files line by line           

cp                    Copy one or more files to another location               

cron                 Daemon to execute scheduled commands                

crontab            Schedule a command to run at a later time               

csplit                Split a file into context-determined pieces               

cut                   Divide a file into several parts                       

date                 Display or change the date & time               

dc                    Desk Calculator                    

dd                    Data Dump - Convert and copy a file                      

df                    Display free disk space                     

diff                  Display the differences between two files               

diff3                Show differences among three files             

dir                    Briefly list directory contents           

dircolors          Colour setup for `ls'              

dirname           Convert a full pathname to just a path                      

du                    Estimate file space usage                  

echo                 Display message on screen                

ed                    A line-oriented text editor (edlin)                 

egrep               Search file(s) for lines that match an extended expression               

eject                 Eject CD-ROM                     

env                  Display, set, or remove environment variables                     

expand            Convert tabs to spaces                      

expr                 Evaluate expressions             

factor               Print prime factors                

false                 Do nothing, unsuccessfully               

fdformat          Low-level format a floppy disk                    

fdisk                Partition table manipulator for Linux           

fgrep                Search file(s) for lines that match a fixed string                   

find                 Search for files that meet a desired criteria              

fmt                  Reformat paragraph text                   

fold                 Wrap text to fit a specified width                

format             Format disks or tapes            

free                  Display memory usage                      

fsck                 Filesystem consistency check and repair                  

gawk               Find and Replace text within file(s)             

grep                 Search file(s) for lines that match a given pattern                

groups             Print group names a user is in           

gzip                 Compress or decompress named file(s)                    

head                Output the first part of file(s)           

hostname         Print or set system name                   

id                     Print user and group id's                   

info                  Help info                   

install               Copy files and set attributes              

join                  Join lines on a common field            

kill                   Stop a process from running             

less                  Display output one screen at a time              

ln                     Make links between files                  

locate               Find files                    

logname           Print current login name                    

lpc                   Line printer control program             

lpr                    Off line print             

lprm                 Remove jobs from the print queue                

ls                      List information about file(s)             

man                 Help manual              

mkdir               Create new folder(s)             

mkfifo             Make FIFOs (named pipes)              

mknod             Make block or character special files            

more                Display output one screen at a time              

mount              Mount a file system              

mv                   Move or rename files or directories               

nice                  Set the priority of a command or job            

nl                     Number lines and write files             

nohup              Run a command immune to hangups            

passwd            Modify a user password                   

paste                Merge lines of files               

pathchk           Check file name portability               

pr                     Convert text files for printing           

printcap           Printer capability database                

printenv           Print environment variables              

printf               Format and print data           

ps                     Process status            

pwd                 Print Working Directory                    

quota               Display disk usage and limits           

quotacheck      Scan a file system for disk usage                  

quotactl           Set disk quotas                       

ram                  ram disk device                     

rcp                   Copy files between two machines                

rm                    Remove files             

rmdir               Remove folder(s)                  

rpm                  Remote Package Manager                

rsync                Remote file copy (Synchronize file trees)                  

screen              Terminal window manager               

sdiff                Merge two files interactively            

sed                   Stream Editor            

select               Accept keyboard input                     

seq                   Print numeric sequences                    

shutdown        Shutdown or restart linux                 

sleep                Delay for a specified time                 

sort                  Sort text files            

split                 Split a file into fixed-size pieces                   

su                     Substitute user identity                     

sum                  Print a checksum for a file                

symlink            Make a new name for a file              

sync                 Synchronize data on disk with memory                   

tac                   Concatenate and write files in reverse                      

tail                   Output the last part of files               

tar                    Tape Archiver           

tee                   Redirect output to multiple files                   

test                  Evaluate a conditional expression                 

time                 Measure Program Resource Use                   

touch               Change file timestamps                     

top                   List processes running on the system            

traceroute        Trace Route to Host              

tr                      Translate, squeeze, and/or delete characters             

true                  Do nothing, successfully                   

tsort                 Topological sort                    

tty                    Print filename of terminal on stdin               

umount            Unmount a device                 

uname              Print system information                   

unexpand        Convert spaces to tabs                      

uniq                 Uniquify files            

units                Convert units from one scale to another                   

unshar              Unpack shell archive scripts              

useradd           Create new user account                   

usermod          Modify user account             

users                List users currently logged in            

uuencode         Encode a binary file               

uudecode        Decode a file created by uuencode              

vdir                  Verbosely list directory contents (`ls -l -b')               

watch              Execute/display a program periodically                    

wc                   Print byte, word, and line counts                  

whereis            Report all known instances of a command               

which              Locate a program file in the user's path                    

who                 Print all usernames currently logged in                     

whoami           Print the current user id and name (`id -un')             

xargs                Execute utility, passing constructed argument list(s)           

yes                   Print a string until interrupted     

Basic and Common commands

Before you turn over to the next chapter where you are going to meet a plethora of commands, remember a few things that apply to all UNIX commands.

•      All UNIX commands must always be entered in small case letters

•      Between the command name and the options that may be available with the command there must always be a space or a tab, for example, ls –l. Here is the command whereas –l is the option and the two have been separated by space. The option is usually preceded by a minus (-) sign. The option available with a command are often known as switches

•       Two or more options available with command can usually be combined, for example, the command ls –l –a is same as ls –la

•        If you make a typing mistake, press backspace to erase characters Don’t try back using arrow keys and then attempt deleting using the del key

•      To cancel the entire command before you press Enter, press ctrl+c Or del key

Basic Commands:

~]$ date             #To see the date of the system.

~]$ date +%d-%Y-%H-%M  #To see particular date format you always use date options

~]$ cal                #To see the current month calendar.

~]$ cal 11 2014            #To display the calendar, Month November (11) and year 2014


cal
      May 2016
Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31

~]$ clear            #To clear the screen

~]$ ls –l            #List files, directories with their properties

~]$ ls                   #To list files and directories

~]$ ls –a            #To list all hidden files and directories

~]$ ls –d            #To list only directories

 ~]$ pwd             #Print working directory

~]$ who am I    #To see from which user you have logged in

~]$ who             #To see all who is logged in yet this point of time from which IP

~]$ w                  #More details about user related info

 ~]$ uptime        #To see the server up time, boot time, users and load

 ~]$ uname –a     #Verify Operating system version, kernel version and architecture

~]$ touch <File Name>         #Create an empty file / Multiple empty files yet a time

Options:

~]$ touch –am                                   #it will change a file time to current time

~]$ touch –r file1 –B 30 file2  #it will create two files with 30 seconds time difference

~]$ cat /dev/null > file                       #To empty the data file

~]$ cat > <File Name>                      #Create an single file with text

       In above example two files 'kumar' and 'ravi' where created because we have provided the space in between 'kumar' and 'ravi' file names.

~]$ rm –rf <File / Directory Name>    #Delete files and directories forcefully

~]$rmdir <directory>    #Delete directories only

 ~]$ mkdir <Directory Name> #Create an empty directory / directories

~]$ mkdir –p <directory/directory/directory> #to create parent directories

 ~]$ cd <Path of the directory>         #Change directory

 ~]$ cat <File Name>              #View content of file

 ~]$ time             #Calculate response time of the activity / command

 ~]$ hwclock                  #to see detailed date and time with time zone

~]$ cp <Source path> <Destination path>  #Copy the files from one path to another path

~]$cp –Rv <source> <destination> #copy directories from source to destination

Options:

-R, -r,              #copy directories recursively

-v                     #verbose to see progress of copy job

-p                     #preserve

-f                      #forcefully

-I                     #interactive: Ask before overwriting file

 Copying directory must use –R to copy directories

 ~]$ mv <source> <destination>       #Move files/directories

~]$ mv <old name> <new name>    #Rename the file and directory

 ~]$ last                          #Check who logged in and when logged in duration

~]$ arch             #to know architecture

 

 ~]$ reboot / init 6                     #Restart server

~]$poweroff / init 0                  #To shut down the server

~]$ dmesg                                 #Check boot process logs

 ~]$ nsloookup <Server Address>    #check dns resolution

~]$ dig  <server address>      #check dns resolution to debug

~]$ tree <directory>               #it will show the tree of parent directory

~]$ stat <file name>               #detailed information about file

 ~]$ wc                            #word count, character count and line count

Options:

-l          #Check line count

-c         #Character Count

-w        #Word Count

 Help relate commands:

~]$ whatis <Command Name>  #It will display single line description about command

 ~]$ whereis <Command Name> #It will provide you path of the command

~]$ man <command>                 #manual page of the command

~]$ info <command>              #information about the command

~]$ <command> --help           #it will gives a command options and there usage

~]$ apropos <keyword>         #to know about the command use

Linux Booting procedure

The stages involved in Linux Booting Process are:
1. BIOS

2. Boot Loader
    - MBR
    - GRUB

3. Kernel
4. Init
5. Runlevel scripts

6. User Interface

STEP 1. BIOS

• This is the first thing which loads once you power on your machine.
• When you press the power button of the machine, CPU looks out into ROM for further instruction.
• The ROM contains JUMP function in the form of instruction which tells the CPU to bring up the BIOS
• BIOS determine all the list of bootable devices available in the system.
• Prompts to select bootable device which can be Hard Disk, CD/DVD-ROM, Floppy Drive, USB Flash Memory Stick Etc..
• Operating System tries to boot from Hard Disk where the MBR contains primary boot loader.

STEP 2. Boot Loader 

To be very brief this phase includes loading of the boot loader (MBR and GRUB/LILO) into memory to bring up the kernel.

MBR (Master Boot Record)

• It is the first sector of the Hard Disk with a size of 512 bytes.
• The first 434 - 446 bytes are the primary boot loader, 64 bytes for partition table and 6 bytes for MBR validation timestamp.

NOTE: Now MBR directly cannot load the kernel as it is unaware of the file system concept and requires a boot loader with file system driver for each supported file systems, so that they can be understood and accessed by the boot loader itself.

To overcome this situation GRUB is used with the details of the file system in /boot/grub.conf and file system drivers

GRUB (Grand Unified Boot loader)

this loads the kernel in 3 stages

GRUB stage 1: 

• The primary boot loader takes up less than 512 bytes of disk space in the MBR - too small a space to contain the instructions necessary to load a complex operating system. 
• Instead the primary boot loader performs the function of loading either the stage 1.5 or stage 2 boot loader.

GRUB Stage 1.5: 

• Stage 1 can load the stage 2 directly, but it is normally set up to load the stage 1.5. 
• This can happen when the /boot partition is situated beyond the 1024 cylinder head of the hard drive. 
• GRUB Stage 1.5 is located in the first 30 KB of Hard Disk immediately after MBR and before the first partition.
• This space is utilized to store file system drivers and modules.
• This enabled stage 1.5 to load stage 2 to load from any known location on the file system i.e. /boot/grub

GRUB Stage 2:

• This is responsible for loading kernel from /boot/grub/grub.conf and any other modules needed
• Loads a GUI interface i.e. splash image located at /grub/splash.xpm.gz with list of available kernels where you can manually select the kernel or else after the default timeout value the selected kernel will boot

The original file is /etc/grub.conf of which you can observe a symlink file at /boot/grub/grub.conf

STEP 3. Kernel

This can be considered the heart of operating system responsible for handling all system processes.

Kernel is loaded in the following stages:

1. Kernel as soon as it is loaded configures hardware and memory allocated to the system.
2. Next it uncompresses the initrd image (compressed using zlib into  zImage or bzImage formats) and mounts it and loads all the necessary drivers.
3. Loading and unloading of kernel modules is done with the help of programs like insmod, and rmmod present in the initrd image.
4. Looks out for hard disk types be it a LVM or RAID.
5. Unmounts initrd image and frees up all the memory occupied by the disk image.
6. Then kernel mounts the root partition as specified in grub.conf as read-only.
7. Next it runs the init process

STEP 4. Init Process

• Executes the system to boot into the run level as specified in /etc/inittab

You can check current runlevel details of your system using below command on the terminal

# who -r

         run-level 3  Jan 28 23:29                   last=S

• Next as per the fstab entry file system's integrity is checked and root partition is re-mounted as read-write (earlier it was mounted as read-only).

STEP 5. Runlevel scripts

A no. of runlevel scripts are defined inside /etc/rc.d/rcx.d

• Based on the selected runlevel, the init process then executes startup scripts located in subdirectories of the /etc/rc.d directory.
• Scripts used for runlevels 0 to 6 are located in subdirectories /etc/rc.d/rc0.d through /etc/rc.d/rc6.d, respectively.
• For more details on scripts inside /etc/rc.d follow the below link
  What are the s and k scripts in the etc rcx.d directories
• Lastly, init runs whatever it finds in /etc/rc.d/rc.local (regardless of run level). rc.local is rather special in that it is executed every time that you change run levels.

NOTE: rc.local is not used in all the distros as for example Debian.

Next if everything goes fine you should be able to see the Login Screen on your system.