Linux Regular Commands - Database Learners

Linux
Check number of CPU’s

$cat /proc/cpuinfo

Check memory information

cat /proc/meminfo

Check swap

$swapon -s

Kernel parameter

#/sbin/sysctl -a
cat /etc/sysctl.conf

System error log

/var/log/messages

Nic configuration

ifconfig -a

Directory Commands – mkdir

To create a new directory below current directory:

$mkdir new-dir-name

To create multiple versions of directory:

$mkdir -p dir1/dir2/dir3

Creates dir1 first. Creates dir2 under dir1. Creates dir3 under dir2
To create a directory with given permissions:

$mkdir -m 754 new-dir

new-dir will be created with 754 permissions

Directory Commands – rmdir

To remove a directory:

$rmdir dir-name

rmdir can remove only empty directories
To remove directory along parent directory:

$rmdir -p dir1/dir2/dir3

Deletes all the directories

Directory Commands – cd

Stands for change directory
Used to navigate among directories

$cd dir1

Takes to dir1 directory

$cd

Takes to the home directory
Can give full-path-name or relative-path-name as argument

UNIX commands – banner

Prints the given string in big letters

$banner <string>

UNIX commands – touch

Used to change the last access time of a file

$touch -a file1

Changes the access time of file1 with current time

$touch file1

Changes access time as well as modification time will be changed to current time

UNIX commands – file

Used to know the file type like shell script, executable

$file file-name

english text

UNIX file commands – wc

Used to know the number of characters, number of files and number of lines in a file

$wc file1

$wc -l file1

Displays the number of lines in file1
Can take the input from terminal directly

UNIX file commands – sort

Used to sort a file

$sort file1

Sorts file1 and displays the sorted file on terminal
Instead of displaying the sorted file on terminal, can be written in to a file by:

$sort –o file2 file1

Writes the sorted file1 in file2

UNIX file commands – cut

Cut is a filter.
Picks up given number of bytes from a given file by:

$cut -f 5,10 file1

Displays 5th field to 10th field in file1. Each field is assumed to be separated by TAB character

UNIX file commands – grep

This is the good example of unfriendly UNIX command
Acronym for ‘Globally search a Regular Expression and Print it’
Searches for a pattern in file(s)

$grep abc file1

Searches for the string ‘abc’ in file1

UNIX file commands – head

Displays only first few lines of a file as per the argument

$head -15 file1

Displays only first 15 lines of file1

UNIX file commands – tail

Displays only last few lines of a file as per the argument

$tail -15 file1

Displays the last 15 lines of file1

UNIX file commands – pg/more

Both commands work almost like the same with few differences

$pg +5 -20 -p -s file1

Starts displaying file1 20 lines per page, starting from 5th line, with prompt for each page of display

UNIX file commands – lp

Sends the user print job to the print queue

$lp file1 file2

request id is lp-23(2 files)
Many flags available for different ways of printing
Can take the input from terminal

UNIX file commands – lpstat

Gives the printer status like which is currently printing, how many jobs are still in queue etc

$lpstat -t

scheduler is running

system default destination : lp

device for lp: /dev/lp0

lp accepting requests since <date>

Queue information

UNIX file commands – cancel

Used to cancel the given print job

$cancel lp-15

request “lp-15,226 cancelled

UNIX file commands – compress

Two commands provided for file compression namely compress and pack
They behave the same way

$compress -v file1

file1: Compression: 90.1%

Compresses the file file1 and creates a file by name file1.z

UNIX file commands – uncompress

Used to unpack the compressed file by:

$uncompress file1

Pack and compress will untouch the file if there is no effective compression

UNIX file commands – man

UNIX provides on-line manuals by:

$man <command>

Displays all the information about <command>

I/O Redirection & Piping

I/O can be redirected using ‘<‘ & ‘>’
‘<‘ symbol implies – take the input from a file instead of keyboard
‘>’ symbol implies – write the output in the file instead of display unit
‘2>’ symbols implies – write the error in a file rather than display unit

e.g.:abc<in-file>out-file 2>err-file

A ‘>>’ indicates the appending of data to a file if it exists, instead of over-writing
1>&2 indicates – redirect Standard Output to Standard Error
<Command-1> | <Command-2> means – the standard output of Command-1 is going to be the standard input of Command-2
UNIX supports joining of commands through ‘|’ piping character

$ls | wc -l

The output of ls is the input for wc

$ls | wc -l > outfile

Output of wc will be written in outfile
Output of first process will be written to pipe and the pipe then hands it over to the next process.
tee can be used to capture the pipe file

$who | tee file1 | sort

Output of who will be written into file1 and the file will be sorted and the sorted file will be written to standard output

UNIX Command – ps

To see the processes running currently under UNIX can be had by:

$ps

PID TTY TIME COMMAND

2266 tty2 0:06 sh

2269 tty2 0.02 ps

Process Status command is shortly known as ps

UNIX Background Process

Time consuming tasks may be run in background
Place ‘&’ at the end of the command, the process will go to background
Displays the PID after submission
Success or failure of the background process will not be reported
Better to redirect the background process output to some file. Otherwise, it will disturb the foreground process messages on the screen
Too many background processes will degrade the performance
If logged out, all the background processes will be terminated

UNIX Command – Sleep

This command is used to delay a process for a period

of time

For Example :

To run a command at regular intervals, enter:

while true

date

sleep 60

done

This shell procedure displays the date and time once a minute.

To stop it, press the Interrupt key sequence.

Killing a Process

To terminate a process

$kill PID

An user can kill only his processes

UNIX Communication – write

Any user can ‘write’ a message on other user’s terminal.
Other user should give permissions to write message

$write <user-name>

hello

^d (Ctrl+d)

Hello will be written on user-name’s terminal
The <User-name> user should be logged-in to receive the message
The recipient should give permissions for message
Message writing can be denied by:

$mesg -n

Super user can write to any terminal without permissions

Shell Programming

A shell program is a series of UNIX commands
Instead of typing the commands one after the other, the commands may be written in a file and execute the file.
Shell programming offers much more versatility than mere UNIX commands
Almost a system can be developed by using shell script with UNIX commands
UNIX shell is the interface between the operating system & user
It is an interpreter, not an executable
A shell script can be executed by giving execute permissions ($chmod +x shell) or by shell command ($sh shell)
Shell programming is so powerful, even system shutdown like tasks can be handled
Three shells available under UNIX – Bourne, C & Korn shell
The following shell scripts were written in Bourne shell
Bourne shell script will work with Korn shell and the reverse is not true
Bourne is the default shell in most of the installations
Shell scripts are used to accomplish variety of tasks like:
Customizing the working environment
Automating the daily tasks like backups etc
Automating repetitive tasks
Executing important system procedures like shutdown
When the user logins into the system, the UNIX starts a shell (sh) on that terminal which accepts commands from ‘$’ prompt
When a shell-script is given for execution, the shell executes the commands from the shell-script

Interactive Shell Programming

Two basic words in shell are read & echo
read accepts input
echo writes the output

echo Enter your name ?

read name

echo Good morning $name

Write the above 3 lines in a file name sh01

$sh01

Enter your name ?

Vijay

Good morning Vijay

The ‘’ symbols makes the shell to understand ‘?’ as an ordinary character rather than special character
Double quotes (“) may be used to display a string

name is a shell variable receives data from keyboard
$name will display the contents of the variable

Interactive Shell Programming

echo Enter three values X Y Z

read x y z

echo $x $y $z

If 1 2 3 entered, x will have 1, 2 to y & 3 to z
If 1 2 3 4 are entered, then z will have 3 4
If only 1, 2 are entered, then z will be assigned null value
If more arguments entered, last variable will be assigned rest. If less, null will be assigned
echo This is new line nAnd this is second line
The above will be displayed as

This is new line

And this is second line

The escape sequence was brought from C language
b is blank, t is tab char etc.
To make the cursor at the end of the echoed line

echo “Enter your Name :c”

Enter your Name :_ (Cursor waits here)

echo “07” gives bell

Shell Variables

They are an integral part of shell programming
Provide ability to store and manipulate data
Fully under the control of the shell
Can create and destroy the variables as the user wants

Shell Variables – Guidelines

Any combination of alphabet, numbers and underscore( _ )
No editing characters like commas, blanks
The first character of the name should be alpha or underscore
May be of any reasonable length
Case sensitive ie abc & Abc are different

Shell – Assigning values

Values can be assigned to variable using ‘=‘ equal sign

$name=Vijay

$echo $name

displays Vijay

Shell Variables

There are 2 types of variables
1. UNIX defined or system defined variables
2. User defined variables

System Variables

UNIX defines certain variables for its own usage

e.g.: PS1 stands for prompt string1

$PS1=ABC

From this time onwards, UNIX displays ABC as prompt
PS2 is prompt string 2, default value is ‘>’
PATH defines the directories to searched by shell to execute any command or file
HOME defines the default working directory
LOGNAME stores the log name of the user

MAIL stores the file name of the mailbox along path
MAILCHECK duration of the mail checking
SHELL keeps the default shell name
TERM gives the name of the terminal the user is using
All the above variable values can be known by set command

$set

Displays all the values

Tips & Traps

All the variables are string variables
$a=20. 20 is stored as a string not as number. No arithmetic can be carried on this
Use double quotes if the value contains more than one word

$name=“Vijay Suri”

Can assign more than one variable in a line

$name=“Vijay” age=20

Can display more than one variable in a line

$echo Name is $name and age is $age

All variables in a shell script are automatic variable. i.e. they will be created as soon as the shell script execution starts and dies as soon as the execution is over
A null variable can be created by

$a1=“” or a1=‘’ or a1=

Shell ignore if any shell variable is having null value

Unchanging Variable – readonly

User can make a variable unchanged during execution by:

$age=20

$readonly age

The the shell does not allow us to change the value

Wiping out Variable – unset

A variable can be removed from the shell by using unset command

$unset age

Unset can not be used for system variables
$unset PS1 is not allowed

Positional Parameters

Many occasions, a program expects the variables in a certain fashion. This is achieved through positional parameters from $0 through $9.
$0 is the program itself. Thus

$abc par1 par2 par3 par4 assigns

abc to $0, par1 to $1 … par4 to $4

Passing command line arguments

To write a shell by name sh02 which copies one file to another

echo Copying $1 to $2

cp $1 $2

By executing sh02

$sh02 file1 file2

Copying file1 to file2.

Positional parameters can not assigned values like $1=100

Shell Variables

To know the number of variables given for the shell through $#

$abc file1 file

echo $#

The above shell displays 3

Shell Variables – shift

Shell can handle only 9 variables at a time. To access more than 9, the shift command is used
$set You have the capacity to learn the shell programming in a very easy way
$echo $10 displays You0 as shell interprets $10 as $1 with 0.
$shift 5 : makes the 5th argument as $1 argument. Thus
$echo $1 displays learn
$* handles all positional parameters

Positional Parameters

Positional Parameters

If the number of parameters are greater than nine

The shift command can be used to shift the parameters

For Example :

$ set a b c d e f g h i j k l m n o p

$ echo $1 $2 $3 $4 $5 $6 $7 $8 $9

a b c d e f g h i

$ shift 7

$ echo $1 $2 $3 $4 $5 $6 $7 $8 $9

h I j k l m n o p

Arithmetic in Shell

Write the following in a file and execute:

a=10 b=4

echo ‘expr $a + $b’

echo ‘expr $a – $b’

echo ‘expr $a * $b’

echo ‘expr $a / $b’

echo ‘expr $a % $b’ #modulus

On execution, the output is:

Anything after ‘#’ sign will be treated as comment
expr is the key word for doing arithmetic
A multiplication symbol (*) should be preceded by ‘’
Terms in expr should be separated by space
Parenthesis may be used for clarity of expression
Arithmetic in shell scripts
- The expr can be used only for integers
- For Floating point the command bc is used

# example of floating point arithmetic program

a=15.5 b=7.8

c=‘echo $a + $b | bc‘ (returns 23.3)

d=‘echo $a – $b | bc‘ (returns 7.7)

e=‘echo $a * $b | bc‘ (returns 120.9)

f=‘echo $a / $b | bc‘ (returns 1)

UNIX decision loops

There are 4 decision making loops

if then fi

if then else fi

if then elif else fi

case – esac

UNIX – if then fi

Key word if and the delimiter is ‘fi’

if <command>

then

statements

e.g.: if cp $1 $2

then echo “Copied successfully”

UNIX – if then else fi

The structure of this construct is:

if <Condition>

then

statements

else

statements

e.g.:

if cp $1 $2

then

echo File copied successfully

else

echo File copy failed

Example “if – else – fi”

# Example of numeric test

echo Enter basic salary

read bs

if [ $bs -lt 2500 ]

then

hra=‘echo $bs * 10 / 100 | bc‘

da=‘echo $bs * 90 / 100 bc‘

else

hra=500

da=‘echo $bs * 98 / 100 | bc‘

gs=‘echo $bs + $hra + $da | bc‘

echo Gross salary = Rs. $gs

UNIX – if then elif else fi

Used for multilevel decision making.

if <condition> then

statements

elif <condition>

statements

else

statements

UNIX – test

if depends upon the exit status of the command given
test verbs translates the result into success or failure
There are three tests namely
Numerical test
String test
File test

UNIX – test – numerical

Used to compare numerical

-gt = greater than

-lt = less than

-ge = greater than or equal to

-le = less than or equal to

-ne = not equal

-eq = equal

if [ $1 -lt 5 ]

then

echo the value is < 5

elif [ $1 -le 7 ]

echo the value is <= or equal 7

else

echo the value is > 7

UNIX – test

Use square braces to avoid writing test
Provide a space after ‘[‘
Provide a space before ‘]’

UNIX – test – file

The following are the file related flags
-s returns True if the file exists and size > 0
-f returns True if the file exists and not directory
-d return True if the file exists and is a directory

e.g.:

if [ -f $1 ] then

echo File exists

UNIX – test – String

s1 = s2 returns true if both are same
-n returns true if string length > 0

if [ $1 = $2 ] then

echo Both strings are same

else

echo $?

$? Contains the value of the last command
0 = true, 1 = false

UNIX logical conditions

-a stands for AND condition
-o stands for OR condition
-! Is negation
if <Condition-1> -a <Condition-2>
Returns true if both the conditions are true

if [ $1 -gt 60 ] -a [ $2 -lt 50 ] then

statements…

UNIX case

To handle multiple choices
case value in
choice 1) statements;;
choice 2) statements;;
*) statements;;
esac
esac is the delimiter of case
Used for menus
*) is the default choice
All choice statements should be terminated by double semicolon(;;)

UNIX case example

case $option in

1) echo Financial accounting;;

2) echo Materiel accounting;;

*) echo Invalid Opt – Try;;

esac

UNIX case

The choices many be in any order
case statement may be a shell variable or shell argument or output of a command
Need not be numbers – may be strings too

e.g.:

banana) statements;;

orange) statements;;

Multiple options can be grouped

e.g.:

case $1 in

banana | orange) echo Fruit;;

dog | pig) echo Animal;;

lion) echo Wild animal;;

esac

Can use shell’s pattern matching

e.g.:

case $1 in

[a-z]) echo Small alpha;;

[A-Z]) echo Capital;;

esac

UNIX Loop Controls

Provided 3 loop constructs namely:
while loop
for loop
until loop

UNIX – while loop

while <condition>

statements

done

done is the delimiter of do

UNIX – while example

count = 1

while [ $count -le 3 ]

echo Loop value $count

count = `expr $count + 1`

done

Examples of “while” loop

# calculation of simple interest

count=1

while [ $count -le 3 ]

echo “nEnter p,n,rc”

read p n r

si=‘echo $p * $n * $r /100 | bc‘

echo Simple interest = Rs.$si

count=‘expr $count + 1‘

done

# printing numbers using while

i=1

while [ $i -le 10 ]

echo $i

i=‘expr $i + 1‘

done

UNIX – while

The loop will continue so long as the condition is TRUE
When the condition is false, the next command after done will be executed
The condition can be any valid UNIX command
The while condition can be simple condition or complex condition
The condition should have an exit status. Otherwise, it may go into infinite loop

UNIX – until loop

until <condition>

statements

done

until continues its loop so long as the condition is false
except this, while & until are identical

Example – until loop

# printing numbers using until

i=1

until [ $i -gt 10 ]

echo $1

i=‘expr $i + 1‘

done

UNIX – for loop

Most frequently used loop

for control-var in value1 value2…

statements

done

for takes a list of variables

UNIX – for example

for word in $*

echo $word

done

Example – for loop

Example

# To print names of all sub-directories

for entry in *

if [ -d $entry ]

then

echo $entry

done

UNIX – break statement

Used to break the current loop and comes out of the loop
Usually associated with if

if [ $1 -eq 5 ]

then

I = 2

break

UNIX – continue statement

To take the control to the beginning of the loop bypassing the statements

I=1

while [ $I -le 5 ] then

I = `expr $I + 1`

continue

done

UNIX – Metacharacters

Called as regular expressions.
Classified as follows:

File name : ? * […] [!…]

I/O redirection : < > >> << m> m>&n

Process execution: ; ( ) & && ||

Positional paras : $1..$9

Spl characters : $0 $* [email protected] $# $! $$ $-

UNIX – File name

ls ?? – lists all files with 2 chars long
ls a* – lists all file names begin with a
ls [a-c]* -file names begin with a,b &c
ls [!a-c] – file names not starting with a,b,c

UNIX – I/O Redirection

< – take input from
> write output to
>> append output
<< abc – takes the input till ‘abc’ encountered
m> filename – Makes filename as output of m
m<&n – Merges stdin, stdout and stderr

UNIX – Process Execution

; – multiple commands separated by ‘;’
& – puts to background process
&& means AND – Comd1 && Comd2 : Executes Comd2 if and only if Comd1 is successful
|| means OR – Comd1 || Comd2 – Executes Comd2 if Comd1 fails

UNIX – Special Characters

$$ – Gives the Process id of the command
$? – Exit status of the last executed command
$! – PID of last background process
$# – Total number of positional parameters
$0 – Command name
$* – list all shell arguments

Tips & Traps

Try to use the absolute paths in the shell scripts. This saves the search time of the system for the command
Remove write permissions to files that are important. This even prevents the accidental update of the files
Compress the print/report files. These files occupies lot of disk space. Better pack them and keep
The compressed print files can be printed directly without uncompress them by using the pcat command
Untouched files in the directory may be found using find command.
$find . -mtime 10 -print
displays the files that are untouched for the last 10 days
Clean up of the directory should be done periodically. Otherwise, any amount disk space will be inadequate.

$pcat textfile.txt.z > /dev/lp0

The above command will send the print file directly to printer

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