Linux is a family of open source Unix like operating system based on the Linux kernel, an operating system kernel first release on September 17,1991 by Linus Torvalds
Linux Kernel is the core interface between a computer's hardware and its processes
What does the kernel do?
The kernel has 4 jobs:
- Memory management: Keep track of how much memory is used to store what and where
- Process management: Determine which processes can use the central processing unit(CPU) when and for how long
- Device drivers: Act as mediator/interpreter between the hardware and processes
- System calls and security: Receive request for service from the process
Shell Types
In Unix, there are two major types of shells −
Bourne shell − If you are using a Bourne-type shell, the $ character is the default prompt.
C shell − If you are using a C-type shell, the % character is the default prompt.
The Bourne Shell has the following subcategories −
- Bourne shell (sh)
- Korn shell (ksh)
- Bourne Again shell (bash)
- POSIX shell (sh)
The different C-type shells follow −
- C shell (csh)
- TENEX/TOPS C shell (tcsh)
Shell scripts have certain advantages over compiler-based programs, such as C or C++ language. However, Shell scripting has certain limitations as well.
The following are the advantages:
- Script are easy to write
- scripts are quick to start and easy to debug
- They save the time of development
- Task of administration are automated
- No addition setup or tools are required for developing or testing shell script
- every line in shell scripts creates a new process in the OS. When we executes the compiled program such as C program it run as a single process for complete program
- since every command creates a new process , shell scripts are slow as compared to compiled programs
- Shell Scripts are not suitable if heavy math operation are involved
- There are problems with cross platform portability
- We cannot use shell scripts in the following situations when
- Extensive file operations are required
- we need data structure such as linked list or trees
- we need to generate or manipulate graphics or GUIs
- we need direct access to system hardware
- We need a port or socket I/O
- we need to use library or interface with legacy code
- Proprietary closed source application are used (Shell scripts put the source code right out in the open for the entire world to see)
pwd command stands for “print working directory,” and it outputs the absolute path of the directory you’re in. For example, if your username is “john” and you’re in your Documents directory, its absolute path would be: /home/john/DocumentsIt’s so easy to copy files and folders directly in the Linux terminal that sometimes it can replace conventional file managers.
To use the cp command, just type it along with the source and destination files:
cp file_to_copyt.txt new_file.txt
mv
You use the mv command to move (or rename) files and directories through your file system.
To use this command, you’d type its name with the source and destination files:
mv source_file destination_folder
mkdir
mkdir command is used to make new directory or folder
rmdir
rmdir command is used to delete directory
rm
rm command is used to remove objects such as computer files, directories and symbolic links from file systems and also special files such as device nodes, pipes and sockets, similar to the del command in MS-DOS, OS/2, and Microsoft Windows
head and tail
the head command will output the first part of the file, while the tail command will print the last part of the file. Both commands write the result to standard output.
cat
The cat command is a utility command in Linux. One of its most commonly known usages is to print the content of a file onto the standard output stream. Other than that, the cat command also allows us to write some texts into a file.
cat > sometxtfile
text to written
ctrl + C
more is the oldest , less is improvement
more : forward navigation and limited backward navigation
less: both forward and backward navigation and also has search option. You can go to the beginning and the end of file instantly. Plus you can switch to an editor (like open the file in vi or vm) It is noticeably quicker than editor for when the file is large
vi
The VI editor is the most popular and classic text editor in the Linux family.
write and edit text
:wq exit
nano command is used to open different types of file
sleep
grep
top
ps
free
find
I pipe
File permission and ownership
chmod
file permission and ownership
change file permission with chmod and ownership with chown
$chmod 755 filename.txt
changes file permission to rwx for owner and rx for group and others
read r= 4
write w = 2
execute x=1
chown chown owner: group filename
chown newuser: runner newFile
chown owner: group filename
chown ubntu:www-data newfile.ph
chgrp
this command changes the group ownership of each given file to group (which can be either a group or a numeric group id) or to match the same group as an existing reference file
chgrp oracleadin/usr/database
we can write these command in .sh file and run the script very conviently
# refers to comment
echo "hello world"
#echo is simply used to print string
ls
date
#we'll take input from user
#show output back to screen
read -p "What is your name?" name
echo "hello $name, let's learn shell scripting together"
passing input as a parameter while executing srits
$./name.sh "Jidesh"
read -p "What's your name?" "$1"
echo "Hello $1, shell scripting is cool"
operator
logical
&&
||
!
>
<
>=
<=
!=
==
arithmetic operator
+
-
/
*
%
** exponent
++ increase by 1
-- decrease by 1
conditional statement
if else
syntax
if [] [[]]
POSIX
test
-eq equal
-ne !=
-gt >
-lt<
ge ?=
le <=
eg
num1=123
num2=200
if [[ num1 -gt num2 ]];
echo "$num1 is greater than $num2"
else
echo "$num2 is greater than $num1"
fi
read -p "enter a number: " x
if [[ $x -ne 0 && $x%2 -eq 0 ]]; then
echo "$x s even number";
elif [[ $x%2 -ne 0 ]]; then
ehco "$x is odd number"
elif [[ $x -eq 0 ]];
echo "$x is neither even nor odd"
fi
switch case
read -p "Please give me a choice: " color
case $color in
red)
echo "The color is red";
;;
green)
echo "The color is green";
;;
yellow)
echo "the color is yellow";
;;
*)
echo :"couldn't guess"
esac
to run sub shell command $(data +%s)
epoch=$(date +%S)
echo $epoch
#!/bin/bash
#written in first line so that ./ will execute
array = (1 2 3 4 5 6 )
select user, address, gender from users;
select user || '-' || address || '-' || gender from users;
num = "1 ,2, 3, 4, 5"
echo "size of nums array: ${#nums[@]}"
IFS= ',' READ -r -a numArray <<<"$nums"
echo "New array: ${numArray[@]}"
len = ${#newArray[@]}
echo "Length of new array:$len"
#table multiple database servers user, password, hostname
select user||'|||password||'|'||'|'||hostname from db_server;
'oracle|admin|server1.cotiviti.com"
'oracle|admin|server2.cotiviti.com"
'oracle|admin|server3.cotiviti.com"
connection_string="string"
sqlplus user/password@connectionstring.com
IFS='|' read-r -a config <<<"$configurations"
user=${coonfig[0]}
passwd=${config[1]}
server=${config[2]}
mysql -u $user -p $password -h $server
loop
for loop syntax
for (( i=0; $i<10; i++)); do
echo "$i";
done
for i in {0..10}; do
echo "$i"
done
list="Apple cat orange cow";
for element in $list; do
echo "$element";
done
list="Apple| cat fish| orange| cow";
IFS='|' read -r -a animal <<<"$list"
len=${#animal[@]}
for (($i=0;$i<$len;$i++)); do
echo "Name of the anmal: ${animal[$i]}"
done
use $ sign only to access value
for name in $[animal[@]}; do
echo "Name of the animal $name"
done
for element in $list; do
echo "$element";
done
while loop
counter=1
while [[ $counter -le 10 ]]; do
echo $counter
((counter++))
done
using break in a loop
for ((i=1l<=10;i++)); do
echo $
if [$i -eq 3]; then
break
fi
done
using continue in a loop
for (( i=0;i<=20;i++)); do
if [$(($i%2)) -ne 1]; then
continue
fi
echo $i
done
#using continue n loop
for ((i=0;i<=20;i++); do
if[ $(($i %2)) -ne 10]; di
echo $counter
(counter++)
done
echo "Reached end of loop"
while loop
while [[ $counter -lt 10 ]]; do
echo $counter
((counter ++))
done
echo "reach end of loop"
until loop num=1
until [[ $num -gt 10 ]]; do
echo $(($num*3))
num=$(($num+1))
done
echo "reached end of loop:
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