Bash command line for Linux

Linux Bash commands with definitions,

adduser	Add a user to the system
addgroup	Add a group to the system
alias	Create an alias
apropos	Search Help manual pages (man -k)
apt-get	Search for and install software packages (Debian/Ubuntu)
aptitude	Search for and install software packages (Debian/Ubuntu)
aspell	Spell Checker
awk	Find and Replace text, database sort/validate/index
basename	Strip directory and suffix from filenames
bash	GNU Bourne-Again SHell
bc	Arbitrary precision calculator language
bg	Send to background
break	Exit from a loop
builtin	Run a shell builtin
bzip2	Compress or decompress named file(s)
cal	Display a calendar
case	Conditionally perform a command
cat	Concatenate and print (display) the content of files
cd	Change Directory
cfdisk	Partition table manipulator for Linux
chgrp	Change group ownership
chmod	Change access permissions
chown	Change file owner and group
chroot	Run a command with a different root directory
chkconfig	System services (runlevel)
cksum	Print CRC checksum and byte counts
clear	Clear terminal screen
cmp	Compare two files
comm	Compare two sorted files line by line
command	Run a command - ignoring shell functions
continue	Resume the next iteration of a loop
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	Convert and copy a file, write disk headers, boot records
ddrescue	Data recovery tool
declare	Declare variables and give them attributes
df	Display free disk space
diff	Display the differences between two files
diff3	Show differences among three files
dig	DNS lookup
dir	Briefly list directory contents
dircolors	Colour setup for `ls'
dirname	Convert a full pathname to just a path
dirs	Display list of remembered directories
dmesg	Print kernel & driver messages
du	Estimate file space usage
echo	Display message on screen
egrep	Search file(s) for lines that match an extended expression
eject	Eject removable media
enable	Enable and disable builtin shell commands
env	Environment variables
ethtool	Ethernet card settings
eval	Evaluate several commands/arguments
exec	Execute a command
exit	Exit the shell
expect	Automate arbitrary applications accessed over a terminal
expand	Convert tabs to spaces
export	Set an environment variable
expr	Evaluate expressions
FALSE	Do nothing, unsuccessfully
fdformat	Low-level format a floppy disk
fdisk	Partition table manipulator for Linux
fg	Send job to foreground
fgrep	Search file(s) for lines that match a fixed string
file	Determine file type
find	Search for files that meet a desired criteria
fmt	Reformat paragraph text
fold	Wrap text to fit a specified width.
for	Expand words, and execute commands
format	Format disks or tapes
free	Display memory usage
fsck	File system consistency check and repair
ftp	File Transfer Protocol
function	Define Function Macros
fuser	Identify/kill the process that is accessing a file
gawk	Find and Replace text within file(s)
getopts	Parse positional parameters
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)
hash	Remember the full pathname of a name argument
head	Output the first part of file(s)
help	Display help for a built-in command
history	Command History
hostname	Print or set system name
iconv	Convert the character set of a file
id	Print user and group id's
if	Conditionally perform a command
ifconfig	Configure a network interface
ifdown	Stop a network interface
ifup	Start a network interface up
import	Capture an X server screen and save the image to file
install	Copy files and set attributes
jobs	List active jobs
join	Join lines on a common field
kill	Stop a process from running
killall	Kill processes by name
less	Display output one screen at a time
let	Perform arithmetic on shell variables
ln	Make links between files
local	Create variables
locate	Find files
logname	Print current login name
logout	Exit a login shell
look	Display lines beginning with a given string
lpc	Line printer control program
lpr	Off line print
lprint	Print a file
lprintd	Abort a print job
lprintq	List the print queue
lprm	Remove jobs from the print queue
ls	List information about file(s)
lsof	List open files
make	Recompile a group of programs
man	Help manual
mkdir	Create new folder(s)
mkfifo	Make FIFOs (named pipes)
mkisofs	Create an hybrid ISO9660/JOLIET/HFS filesystem
mknod	Make block or character special files
more	Display output one screen at a time
mount	Mount a file system
mtools	Manipulate MS-DOS files
mtr	Network diagnostics (traceroute/ping)
mv	Move or rename files or directories
mmv	Mass Move and rename (files)
netstat	Networking information
nice	Set the priority of a command or job
nl	Number lines and write files
nohup	Run a command immune to hangups
notify-send	Send desktop notifications
nslookup	Query Internet name servers interactively
open	Open a file in its default application
op	Operator access
passwd	Modify a user password
paste	Merge lines of files
pathchk	Check file name portability
ping	Test a network connection
pkill	Stop processes from running
popd	Restore the previous value of the current directory
pr	Prepare files for printing
printcap	Printer capability database
printenv	Print environment variables
printf	Format and print data
ps	Process status
pushd	Save and then change the current directory
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
read	Read a line from standard input
readarray	Read from stdin into an array variable
readonly	Mark variables/functions as readonly
reboot	Reboot the system
rename	Rename files
renice	Alter priority of running processes
remsync	Synchronize remote files via email
return	Exit a shell function
rev	Reverse lines of a file
rm	Remove files
rmdir	Remove folder(s)
rsync	Remote file copy (Synchronize file trees)
screen	Multiplex terminal, run remote shells via ssh
scp	Secure copy (remote file copy)
sdiff	Merge two files interactively
sed	Stream Editor
select	Accept keyboard input
seq	Print numeric sequences
set	Manipulate shell variables and functions
sftp	Secure File Transfer Program
shift	Shift positional parameters
shopt	Shell Options
shutdown	Shutdown or restart linux
sleep	Delay for a specified time
slocate	Find files
sort	Sort text files
source	Run commands from a file `.'
split	Split a file into fixed-size pieces
ssh	Secure Shell client (remote login program)
strace	Trace system calls and signals
su	Substitute user identity
sudo	Execute a command as another user
sum	Print a checksum for a file
suspend	Suspend execution of this shell
symlink	Make a new name for a file
sync	Synchronize data on disk with memory
tail	Output the last part of files
tar	Tape ARchiver
tee	Redirect output to multiple files
test	Evaluate a conditional expression
time	Measure Program running time
times	User and system times
touch	Change file timestamps
top	List processes running on the system
traceroute	Trace Route to Host
trap	Run a command when a signal is set(bourne)
tr	Translate, squeeze, and/or delete characters
TRUE	Do nothing, successfully
tsort	Topological sort
tty	Print filename of terminal on stdin
type	Describe a command
ulimit	Limit user resources
umask	Users file creation mask
umount	Unmount a device
unalias	Remove an alias
uname	Print system information
unexpand	Convert spaces to tabs
uniq	Uniquify files
units	Convert units from one scale to another
unset	Remove variable or function names
unshar	Unpack shell archive scripts
until	Execute commands (until error)
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
v	Verbosely list directory contents (`ls -l -b')
vdir	Verbosely list directory contents (`ls -l -b')
vi	Text Editor
vmstat	Report virtual memory statistics
watch	Execute/display a program periodically
wc	Print byte, word, and line counts
whereis	Search the user's $path, man pages and source files for a program
which	Search the user's $path for a program file
while	Execute commands
who	Print all usernames currently logged in
whoami	Print the current user id and name (`id -un')
Wget	Retrieve web pages or files via HTTP, HTTPS or FTP
write	Send a message to another user
xargs	Execute utility, passing constructed argument list(s)
xdg-open	Open a file or URL in the user's preferred application.
yes	Print a string until interrupted
.	Run a command script in the current shell
!!	Run the last command again
###	Comment / Remark

Linux commands for Compressing and Archiving File

1. gzip and gunzip

gzip command is used to compress the file, and gunzip is used to de-compress it.

Usage:

gzip <file name>

It provides the extension .gz and removes the original file

Example:

$ wc sample_copy.txt

      65 2776 17333 sample_copy.txt

$ gzip sample_copy.txt

$ wc sample_copy.txt.gz

      26 155 7095 sample_copy.txt.gz

The compression ratio depends on the type, size and nature of the file

Usage:

gunzip <file name with.gz>

Example:

$ gunzip sample_copy.txt.gz

$ /*do ls and you can see the original file*/

If you want to compress the directory contents recursively, use -r option with gzip command and unzip it use the same option with gunzip command.

2. tar : The archival program

tar command is used to create archive that contains a group or file or entire directory structure.

It is generally used for back ups.

Usage:

tar [options]

<file1 or dir> . . .

Options:

• -c Create an archive
• -x Extract files from archive
• -t Display files in archive
• -f arch Name the archive arch

Examples:

$ tar -cvf compression.tar compression

compression/ //v for verbose

compression/temp/

compression/temp/sample2.txt

compression/sample1.txt

• We can use tar and gzip command in succession to compress the tar file.

$ tar -cvf compression.tar compression
$ gzip compression.tar
$ //will create compression.tar.gz file

• For un-compression the file first use gunzip command, which will create a tar file and then use tar command to untar the contents

$ gunzip compression.tar.gz
$ tar -xvf compression.tar

• To just view the contents of the tar file use -t option 

$ tar -tvf compression.tar

• Instead of doing tar first and then gzip next, we can combine both of them using the option -z

$ tar -cvzf compression.tar.gz compression
compression/
compression/temp/
compression/temp/sample2.txt
compression/sample1.txt

•  We can de-compress .tar.gz agin in a single command using the option -z with -x

$ tar -xvzf compression.tar.gz

 3. zip and unzip: compressing and archiving

zip command can be used for archiving as well as compressing the contents of the directory or the file.

Usage:

zip [options] output.zip <files to be zipped or directory>

Example:

$ zip sample1.zip sample1.txt

//will create sample1.zip file

• Use -r option to recursively zip the contents of the directory

$ zip -r compression.zip compression
// will create compression.zip file

• To un-compress the file use unzip command 

$ unzip compression.zip
// will uncompress the compression.zip file

Linux File Handling Commands

1. cat : Concatenate Files

cat command is used to display the contents of a small file on terminal
Usage:
cat <file name>
Example:

$ cat sample3.txt
Unix (officially trademarked as UNIX, sometimes...


cat when supplied with more than one file will concatenate the files without any header information
Usage: 
$ cat sample3.txt sample4.txt


2. tac : concatenate files in reverse

tac command is used to display the contents of a small file in reverse order on terminal
Usage:
tac <file name>
Example:

$ tac sample3.txt
/*displays sample3.txt in reverse order*/


tac when supplied with more than one file will concatenate the reverse contents of files without any header information
Usage:

$ tac sample3.txt sample4.txt

3. more, less : paging output

more and less commands are used to view large files one page at a time
Usage:
more <file name>
less <file name>
Example:

$ more sample1.txt
/*sample1.txt will be displayed one page at a time */
$ less sample1.txt
/*sample1.txt will be displayed one page at a time */








less is the standard pager for linux and in general less is more powerful than more


4. wc : statistic of file

wc command is used to count lines, words and characters, depending on the option used.
Usage:
wc [options] [file name]
Example:

$ wc sample1.txt
65 2776 17333 sample1.txt

• Which means sample1.txt file has 65 lines, 2776 words, and 17333 characters
• you can just print number of lines, number of words or number of charcters by using following options:
• -l : Number of lines
• -w : Number of words
• -c : Number of characters

5. cmp: comparing two files

cmp command is used to compare two files whether they are identical or not

Usage:

cmp <file1> <file2>

Example:

$ cmp sample1.txt sample2.txt

sample1.txt sample2.txt differ: byte 1, line 1

$ cmp sample1.txt sample1_copy.txt

$ /*No output prompt returns back*/

• The two files are compared byte by byte and the location of the first mismatch is printed on the screen.
• If two files are identical, then it doesnot print anything on the screen.

6. comm : what is common?

comm command displays what is common between both the files

Usage:

comm <file1> <file2>

Example:

$ comm sample5.txt sample6.txt

• The input files to comm command should be sorted alphabetically.

Linux file system

1. Standard directory structure

• / - the topmost
• /dev - all the devices are accessible as files
• /var - "variable" data such as mails, log files, databases
• /usr - almost all the packages installed
• /etc - con?guration files
• /home - home directories for all the users
• /root - home directory of the privileged user root
• /mnt - used to mount other directories/partitions

2. File Attributes

• To see the file attributes type ls -l on your terminal

ls -l

total 156-rw-r--r--  1 root root   1053 Sep 22  2010 anaconda-ks.cfg-rw-r--r--  1 root root 127004 Sep 22  2010 CHECKSUMSdrwxr-xr-x  2 root root   4096 Aug  3 00:02 Desktop-rw-r--r--  1 root root    100 Jun  6 22:14 svn-checkout-command

•  It has 10 characters, first character is d if its directory and - if its file.
• Next 9 characters are divided into three groups with a set of 3 characters each
• First 3 characters - Owner of the file or directory
• Next 3 characters - Group
• Last 3 characters - Others
• r - Read i.e. File or directory is readable
• w - Write i.e. File or directory is writable
• x - Execute i.e. File or directory is executable
• -rw-r–r– means it has read, write but not execute permissions for the owner of the file, only read permissions for the group and only read permissions for others.
• The third column of the command ls -l tells about the owner of the file, next column tells to which group it belongs

3. Changing the File attributes

chmod Changing the permissions of the file

Usage: 

$ chmod o+x Testing.java

$ chmod 655 Testing.java

4. Changing ownership

chown command is used for changing the ownership and also group of the file

Usage:

$ chown guest Testing.java

$ chown guest:guest Testing.java

5. File system commands

• Deleting Files - rm
• Copying and moving files - cp, mv
• Creating directories - mkdir
• Deleting Empty Directory - rmdir

$ rm Testing.java

//deletes the file Testing.java

$ cp Testing.java Copy.java

//creates the copy of Testing.java

$ mv Testing.java Test.java

//renames the file Testing.java to Test.java

$ mkdir newDir

//Creates directory newDir

$ rmdir newDir

//deletes directory newDir newDir should be empty.

General Purpose utilities commands in Linux

1. Calender
cal: Command to see calender for any specific month or a complete year
Usage: 
$ cal [ [month] year]
Example:
$ cal april 2009
Output:
April 2009
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

2. Date
date: displays the current date
Usage:
$ date
Output:
Tue Apr 21 21:33:49 IST 2009
user$ date +"%D %H:%M:%S"
04/21/09 21:35:02
Options:
d - The da of the month (1-31)
y - The last two digits of the year
H,M,S - Hour Minute and second respectively
D - the date in mm/dd/yy

3. echo
echo: Print message on the terminal
Usage:
echo ""
Example:
$ echo "Welcome to the workshop"
Output:
Welcome to the workshop

4. printf
printf: Print the formatted message on the terminal
Usage:
printf ""
Example:
$ printf "the amount is %d\n" 100
Output:
the amount is 100

5. Calculator
bc: A text based calculator
Usage:
$ bc
2*10+20-9+4/2 [Input]
33 [Output]
[ctrl+d] [Quit]

xcalc is graphical based calculator

6. script
script command records your session and stores it in a file
Usage:
$ script
Script started, file is typescript
$ echo "this is a sample script"
this is a sample script
$ [ctrl+d]
Script done, file is typescript

By default if you dont specify any file name the contents will be stored in file name typescipt
Usage:
$ cat typescript
Script started on Tuesday 21 April 2009 10:07:00
$ echo "this is a sample script"
this is a sample script
$
Script done on Tuesday 21 April 2009 10:07:34 PM

7. passwd
passwd command allows you to change your password
Usage:
$ passwd
Changing password for user123.
(current) UNIX password:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

8. WHO: Who are the users?
who command tells you the users currently logged on to the system
Usage:
$ who
root     :0           Sep  4 22:47
root     pts/1        Sep  4 22:48 (:0.0)

9. man - The reference Manual
man displays the documentation for a command
Usage:
man
Example:
$ man ls
ls - list directory contents
SYNOPSIS
ls [OPTION]... [FILE]...
DESCRIPTION
List information about the FILEs (the
none of -cftuvSUX nor --sort.