DEV Community: Madhvi Patel

Basic of Shell Scripting

Madhvi Patel — Sun, 03 Oct 2021 12:04:04 +0000

#! /bin/bash This statement marks the start of shell script.

Echo command:

Echo used for returning/printing the information on stdout(screen).
echo Hello World!

Variables:

Variables used for holding a value in memory. Should be Uppercase by convention. Any letters, numbers and underscores can be used. For accessing the variables, always use '$' or '${VAR-NAME}' in front of variable name.

NAME="Adam"
echo "My name is $NAME."
echo "My name is ${NAME}."

User Input:

read command is used for taking the input from stdin. -p is used for prompting the text to the stdout. NAME is used as variable for capturing the user entered data.

read -p "Enter your name:" NAME
echo "Your name is ${NAME}."

Incase you do not want to prompt any text at stdout then use following: Here read will read the user provided input and store it in variable NAME.

read NAME
echo "Your name is ${NAME}."

IF statement:

if condition then result and fi

if [ "$NAME" == "Thomas" ]
then
echo "Your name is Thomas"
fi

IF-ELSE statement:

if [ "$NAME" == "Thomas" ]
then
echo "Your name is Thomas"
else
echo "Your name is something else"
fi

IF-ELSE-IF-ELSE statement:

if [ "$NAME" == "Thomas" ]
then
echo "your name is Thomas"
elif [ "$NAME" == "Fred" ]
then
echo "your name is Fred"
else
echo "your name is not Thomas nor Fred"
fi

COMPARISON

NUM1=40
NUM2=35

if [ "$NUM1" -gt "$NUM2" ]
then
echo "$NUM1 is greater than $NUM2."
else
echo "$NUM2 is greater than $NUM1."
fi

Different comparison operators:

val1 -eq val2 => Returns true if the values are equal
val1 -ne val2 => Returns true if the values are not equal
val1 -gt val2 => Returns true if val1 is greater than val2
val1 -ge val2 => Returns true if val1 is greater than or equal to val2
val1 -lt val2 => Returns true if val1 is less than val2
val1 -le val2 => Returns true if val1 is less than or equal to val2

CASE STATEMENT:

read -p "Is your age above 18? Y/N" ANSWER
case "$ANSWER" in
[yY]|[yY][eE][sS])
echo "You are allowed to have drinks"
;;
[nN]|[nN][oO])
echo "You can enjoy juices"
;;
*)
echo "Enter your age"
;;
esac

FILE CONDITIONS:

FILE="file.txt"
if [ -e "$FILE" ]
then
echo "$FILE exists"
else
echo "$FILE does not exists"
fi

Different file operators:

-d file => True if file is an directory
-e file => True if file exists
-f file => True if provided string is file
-g file => True if the group id is set on file
-r file => True if file is readable
-s file => True if file has non zero size
-u file => True if the user id is set on a file
-w file => True if file is writtable
-x file => True is file is an executable

FOR LOOP:

NAMES="Brad Kevin Alice Mark"
for NAME in $NAMES
do
echo "Hello $NAME"
done

WHILE LOOP:

LINE=1
WHILE read -r CURRENT_LINE
do
echo "$LINE: $CURRENT_LINE"
((LINE++))
done < "./shell-script.sh"

FUNCTION:

function helloWorld(){
echo "Hello World"
}
helloWorld
helloWorld will call the function.

FUNCTION WITH PARAMS:

function intro(){
echo "Hello I am $1 and I am $2 years old"
}
intro "Brad" "36"
intro Brad will call the function with param Brad. You can pass in multiple params and access them using numbers $1, $2 and so on.

CREATE FOLDER AND WRITE TO A FILE

mkdir hello
touch hello/world.txt
echo "Hello World" >> hello/world.txt
echo "created hello/world.txt"

Linux Basic Commands Cheetsheet

Madhvi Patel — Sun, 03 Oct 2021 11:48:17 +0000

Basic commands for working with text:

Ctrl + A => move cursor in the front of text
Ctrl + E => move cursor in the end of text
Ctrl + U => remove(crop) the word from cursor till start of the word
Ctrl + K => remove(crop) the word from cursor till end of word
Ctrl + Y => Paste cropped text
Ctrl + Shift + C => Copy text to clipboard
Ctrl + Shift + P => Paste text to clipboard
Ctrl + C or Ctrl + Z => Stop running the command

Commands for files and folder:

~ : Represents users home directory. cd ~ or cd => return to users home directory.
pwd => Represents your absolute path of current working directory.
. => Represents current directory.
cd => Change directory
cd .. => Moves one directory backwards
cd ../../ => Moves two directory backwards
mkdir dir-name => Create new directory if does not exist.
rmdir dir-name => Removes directory if exists
cp file1 file2 => Copies content of file1 to file2
mv ./file1 ../ => Move file1 from current directory to backward directory
rm file1.txt => Remove file1.txt
rm -r dummy => Removes recursively all files and directory from dummy directory
ls => List all files and directories within your current working directory.
touch file-name => Will create a file with provided file-name.

Commands for changing file/directory permissions:

when you run ls -la on terminal you might see below:

drwxrwxrwx : d => directory, r => read, w => write, x => excute, -
=> no permission.

if d is blank then, its a file (-rwxr-xr-x)
first group(drwx) - represents owner( who created the file)
second(rwx) - represents user group (user groups other than owner, who has access to file)
third(rwx) - represents others

Numerical value for permissions r, w ,x: r - 4 w - 2 x - 1

chmod XXX : this is used to change the permission associated with file/directory. It's called change mode.
X denotes permission number for an group(owner, user-group, other).
chmod 644 - will assign read write to owner and read to both user group and other group. The default permission is 644. permission 777 - means all users(owner, user, others) have got full permission(rwx) on file/directory.
chown user fileName: change ownership of a file or directory
chown user:group filename

Commands to create hard and soft links for file:

Link => A link in UNIX are pointers pointing to a file or a directory. Links allow more than one file name to refer to the same file, elsewhere.

Two ways to create symlinks:

ln -s file1 file 2 => This will create a soft link file2 for referring file1. Soft links break if the source file(file1) is moved or moved.
ln file 1 file 2 => This will create a hard link file2 for referring file1. Hard links work even if then source(file1) is moved.

Commands for searching/displaying text:

grep -n “the” file1.txt => searches "the" in file1.txt with numbered line.
cat file1.txt => Displays the content of file.
| => Pipe is used to send the result of one command to next command after pipe to execute on it.
cat file.txt | wc : here result of cat command is send to wc command for further processing.
head filename => Display first 10 lines of a file.

head file1.txt => Displays first 10 lines of file1.txt

head -n5 file1.txt => Displays first 5 lines of file1.txt

tail filename => Display last 10 lines of a file.

tail file1.txt => Displays last 10 lines of file1.txt

tail -n5 file1.txt => Displays last 5 lines of file1.txt

cat file1.txt | tail -n5 | cat -n => This displays the last 5 lines of the file1.txt numbered as 1, 2, 3, 4, 5.

less filename => Less will open the entire file but displays the part of file in scrollable mode, user can navigate in the file. Displays the content of file page by page.

Redirection of Inputs/Outputs/Stderr:

### File Descriptor and their representations:

stdin => 0 => Input from the console
stdout => 1 => Output to console
stderr => 2 => Error to console

### Redirection:

' > ' => output of command1 is redirected to file.txt/stdout (Output Redirection).
' >> ' => output of command1 is appended to file.txt/stdout (Output Redirection).
' < ' => command 1 reads its input from file.txt/stdin (Input Redirection).

RAM CPU and Disk Info:

free -h => Displays available and used RAM(memory).
cat /proc/cpuinfo => Displays information about cpu like cores etc.
df -h => Displays information about available and used Disks. -h here is used to display the information in human readable format

Other useful Command:

man option command => Displays manual page for a command.
help command => Display help information related to a command.
uname -a => Displays information about linux system on your machine.
hostname => Display the hostname and ip for your machine in the network. hostname -I => Will Display the ip of your system in network.
ping => To test the reachability of the system in network. ping google.com => Will ping to google.com and if that available in network it will return the data packets