• Shell :

• System and User :

• Networking :

• File Management :

A Bash script is a plain text file that contains a series of commands that are executed in sequence. Scripts have .sh extension (for example: script.sh). Shell scripts begin with the shebang (#!/bin/sh), which tells the system which shell interpreter to use to run the script.

Ex :

#!/bin/bash

mkdir Docs
cp doc1.txt Docs/
rm doc1.txt
clear

Executing Scripts :

Note : The script must have executable permission.

./[script]
sh [script]
bash [script]

# Output each command execution :
bash -x [script]

Ex :

./script.sh
bash script.sh

Declaring :

greet="Hello World"
num=17

Access :

echo $greet
echo "Number is ${num}"

Read-Only Variable :

Declare Integer :

Remove Variable :

Declaring :

Access :

Length of string :

String Replace :

greet="Hello World"
echo "Before: $greet"
greet=${greet/World/Universe}
echo "After: $greet"

Declaring Indexed Array :

Declaring Associative Array :

declare -A india["capital"]="New Delhi"

declare -A capital=(["India"]="New Delhi", ["Nepal"]="Kathmandu", ["Bhutan"]="Thimphu")

Indexing :

Access Element :

Print All Elements :

Print Key of Associative Array :

Length of Array :

Input :

Input (STDIN) :

Prompt Input :

Secure Prompt Input :

Input NChar :

Array Input :

Print (STDOUT) :

The echo prints output followed by a new line character.

echo "Hello World"

msg="Hello World"
echo $msg
echo "Hii, ${msg}"

Enable Backslash Escapes :

Special Char in Output :

Print (STDOUT) :

The printf prints output in the specified format; it does not append an implied newline.

• Arithmetic Operator :

Note : Arithmetic operations should be in double braces: count=$((count+1))

• Logical Operator :

• Relational Operator :

(Numerical Comparison)

Ex :

if [ 80 -gt 40 ]
then
    echo "Greater"
fi

• Relational Operator :

(String Comparison)

Ex :

if [ "Hello" = "Hi" ]
then
    echo "Matched"
else
    echo "Not Matched"
fi

• Lexicographic Comparison :

• File Testing Operator :

Ex :

if [ -f text.txt ]
then
    echo "text.txt exists and it is regular file."
fi

Note :

• Expressions can be grouped with ( [expression] )
• Conditional expressions must be in square braces []
• Space must be given between the operands and the operator.

Command substitution allows the output of a command to replace the command itself. Using a backquote is old-style; instead, the second form is recommended.

• Back-Quote :

Syntax :

Ex :

# Example 1:
DATE=`date`
echo "Date is $DATE"

# Example 2:
USERS=`who | wc -l`
echo "Logged in user are $USERS"

• Dollor Sign & Parenthesis :

Syntax :

Ex :

# Example 1:
echo "Today is $(date)"

# Example 2:
for i in $(cat file.txt)
do
    echo "$i"
done

• If Statement :

Syntax :

if [ condition ]
then
    #Statement
fi

# with OR
if [ condition ] || [ condition ]
then
    #Statement
fi

# with AND
if [ condition ] && [ condition ]
then
    #Statement
fi

Ex :

x="Y"
if [ "$x" = "y" ] || [ "$x" = "Y" ]
then
    echo "YES"
fi

• If-Else Statement :

Syntax :

if [ condition ]
then
    #Statement
else
    #Statement
fi

Ex :

name="admin"
if [ $name = "admin" ]
then
    echo "Hello BOSS"
else
    echo "Who Are You?"
fi

• Elif Statements :

Syntax :

If [ condition ]
then
    #Statement
elif [ condition ]
then
    #Statement
else
    #Statement
fi

Ex :

# Example 1:
x=10
y=20
if [ $x -lt $y ]
then
    echo "X is less than Y"
elif [ $x -gt $y ]
then
    echo "X is greater than Y"
elif [ $x -eq $y ]
then
    echo "X is equal to Y"
fi

# Example 2:
a=3
b=3
c=3
if [ $a -eq $b ] && [ $a -eq $c ]
then
    echo "Equilateral"
elif [ $a -eq $b ] || [ $b -eq $c ] || [ $a -eq $c ]
then
    echo "Isosceles"
else
    echo "Scalene"
fi

• Switch Statement :

Syntax :

case [var] in
case-1)
    #Statement
    ;;
case-2)
    #Statement
    ;;
case-3)
    #Statement
    ;;
*)
    #Default Statement 
    ;;
esac

Ex :

drink="tea"
case $drink in
tea|cofee|water)
    echo "Go to canteen"
    ;;
orange|apple)
    echo "Go to fruit market"
    ;;
none)
    echo "Okay"
    ;;
*)
    echo "ERROR: Invalid Selection"
    ;;
esac

• While Loop :

Syntax :

while [ condition ]
do
    #Statement
done

Ex :

# Example 1: print "hello" 5 times
count=0
while [ $count -lt 5 ]
do
    echo "Hello World"
    count=$((count+1))
done

# OR
count=0
while [ $count -lt 5 ]
do
    echo "Hello World"
    ((count++))
done

# Example 2: print file content by line
cat file.txt | while read line
do
    echo $line
done

# Example 3: read while loop
while read line
do
    echo $line
done < file.txt

# Example 4: infinite loop
while true
do
    echo "Infinite Loop..."
done

# Example 5: infinite loop
while :
do
    echo "Infinite Loop..."
done

# Example 6: factorial - pass number as command argument
counter=$1
factorial=1
while [ $counter -gt 0 ]
do
    factorial=$(( $factorial * $counter ))
    counter=$(( $counter - 1 ))
done
echo $factorial

• For Loop :

Syntax :

for [var] in [expression]
do
    #Statement
done

2nd Syntax :

for (( expr; expr; expr ))
do
    #Statement
done

Ex :

# Example 1: input array & print
read -a array
count=1
for i in ${array[@]}
do
    echo "Element $count : $i"
    ((count++))
done

# Example 2:
for i in 1 2 3 4 5
do
    echo $i
done

# Example 3:
for i in {1..25}
do
    echo $i
done

# Example 4: increment by 2 from 0 to 10
for i in {0..10..2}
do
    echo $i
done

# OR using seq - increment by 2 from 0 to 10
for i in $(seq 0 2 10)
do
    echo $i
done

# Example 5:
for (( i=1; i<=5; i++ ))
do
    echo $i
done

# Example 6: infinite loop
for (( ; ; ))
do
    echo "Infinite Loop..."
done

# Example 7: create 10 copies of test.txt
for i in $(seq 1 10)
do
    cp test.txt "test_${i}.txt"
done

# OR single line format
for i in $(seq 1 10); do cp test.txt "test_${i}.txt"; done

# Example 8: reading file
for line in $(cat test.txt); do
    echo $line
done

# Example 9: Iterating associative array
for key in ${!array[@]}; do
    echo "$key : ${array[$key]}"
done

• Until Loop :

Executes until the condition is false.

Syntax :

until [ condition ]
do
    #Statement
done

Ex :

count=5
until [ $count -le 0 ]
do
    echo "count = $count"
    count=$((count-1))
    sleep 1
done

• Select Loop :

Syntax :

select [var] in [expressions]
do
    #Statements
done

Ex :

select drink in tea cofee water orange apple none
do
    case $drink in
    tea|cofee|water)
        echo "Go to canteen"
        ;;
    orange|apple)
        echo "Go to fruit market"
        ;;
    none)
        break
        ;;
    *)
        echo "ERROR: Invalid Selection"
        ;;
    esac
done

• Loop Controls :

1. Break Statement -

a=1
while [ $a -lt 10 ]
do
    echo $a
    if [ $a -eq 5 ]
    then
        break
    fi
    a=$(expr $a + 1)
done

2. Continue Statement -

nums="1 2 3 4 5 6 7"
for num in $nums
do
    q=$(expr $num % 2)
    if [ $q -eq 0 ]
    then
        echo "Even"
        continue
    fi
    echo $num
done

Syntax :

function(){
    #Statements
}

Calling Function :

greet(){
    echo "Hello World"
}

greet

Function with Parameter :

greet(){
    echo "Hello $1"
}

greet World

Call within statement :

capital(){
    echo $1 | tr 'a-z' 'A-Z'
}

name="admin"
echo "Capital: $(capital $name)"

Local Variables :

Local variables are defined within a function and have local scope. They are only accessible within the function where they are defined, not outside of it. It is declared with the local keyword.

swap(){
    local t=$a
    a=$b
    b=$t
}

a=3
b=7
printf "\nA = $a\nb = $b"
swap
printf "\nA = $a\nb = $b"

Also called shell script parameters.

NOTE : 0,1,2 are global file descriptors.

• Custom File Descriptor :

Define own file descriptor and redirect to it.

1. Define/Create output file descriptor with x and redirect to file : exec x<> [file]
2. Redirect to it : [command] >&x
3. Undefine/Close output file descriptor x : exec x>&-

Ex :

$ exec 3<> log.txt
$ echo "Hello" >&3 #output is redirected to log.txt
$ cat log.txt
Hello

$ exec 3>&-

Pipelines or pipes (|) are used as a funnel to pass the output of one command to another command as an input or parameter.

Syntax :

Ex :

# Output 1 page at time:
ls -iR | less

# Display 10 rows:
ls -it | head -10

# Print line with word "Hello" in file.txt:
cat file.txt | grep Hello

# Sort output file having word "index":
grep -l index * | sort

Redefine aliases by assigning commands to words or short-names as substitutions.

Synatx :

Save aliases in the file .bashrc (in home directory) and execute the file with -

Ex :

alias cpp='g++ $1 && ./a.out && rm a.out'

• echo {1,2,3} : 1 2 3
• echo {1..5} : 1 2 3 4 5
• echo {5..1} : 5 4 3 2 1
• echo {1..-3} : 1 0 -1 -2 -3
• echo {1..10..2} : 1 3 5 7 9
• echo {A..E} : A B C D E
• echo {A..H..2} : A C E G
• echo a{2..5}b : a2b a3b a4b a5b
• echo a, b, c, d{1..3}, e : a, b, c, d1, d2, d3, e
• echo {code1,code2,code3}.c : code1.c code2.c code3.c
• echo code{.c,.cpp,.py,.sh} : code.c code.cpp code.py code.sh

A regular expression (shortened as regex) is a sequence of characters that defines a search pattern for use in pattern matching to find, match, or replace text.

• grep ^The file.txt : print line start with "The"
• grep The^ file.txt : print line end with "The"
• grep [Hh]ello file.txt : print line with word "Hello" or "hello"
• grep hello|HELLO file.txt : print line with word "hello" or "HELLO"
• ls file[1-9] : print files with name file1 to file9
• grep '[0-9][0-9][0-9][0-9]' file.txt : match for 4 digit number from file.txt with range 0-9
• ls *[0-9]* : print files containing numbers 0 to 9 (file1.txt test2.sh note5.pdf test_20)
• ls test.* : print files start with "test" (test.txt test.pdf)
• ls ab* : print files start with "ab" (ab.txt abcd abort.sh)
• ls {*.pdf,*.doc,*.exe} : print files end with .pdf/.doc/.exe (file.txt project.doc run.exe)
• ls a[a-d]b : print files with name bet a[a to d]b (abd acd)
• ls test_[!3] : print files with number except 3 (test_1 test_2 test_4)

Global regular expression print (grep) prints lines that match patterns from text or files.

Syntax :

• grep hello file.txt : match for word "hello" in file.txt and print matched line
• grep -i hello file.txt : case insensitive match for "hello" in file.txt
• grep -r hello file.txt : recursively match for "hello" in file.txt
• grep -v hello * : match for all files & folders which doesn`t have word "hello"
• grep -c hello file.txt : count occurance of "hello" in file.txt
• grep -w hello file.txt : match for whole word "hello" in file.txt
• grep -n hello file.txt : match for "hello" in file.txt and print line number with matched lines
• grep -l hello * : match for "hello" in all files and print name of each matched files instead of matched line
• grep -B 2 -A 3 hello file.txt : print before 2 lines and after 3 lines of matched case

Translate, squeeze, and/or delete characters from standard input and write them to standard output.

Syntax :

• tr "ab" "xy" <<< "abcd" : replace occurrence of "a" with "x" and "c" with "z"
• tr [a-z] [A-Z] <<< "abcd" : convert lower case characters to upper case
• tr [:lower:] [:upper:] <<< "abcd" : convert lower case characters to upper case
• tr [:space:] "\t" <<< "Hello World" : translate white-space characters to tabs
• tr -s [a-z] <<< "Hellooo Worrld" : squeeze sequence of repetitive characters using -s option and replaces with single occurrence of character
• tr -d "l" <<< "Hello World" : delete specified characters using -d option
• tr -d [:digit:] <<< "Hello 123 World6789" : remove all the digits from the string
• tr -c "a" "x" <<< "ababc" : invert the selection by using -c option (complement)
• tr -cd [:digit:] <<< "abc123def456" : delete everything except digits

Tokens :

It searches for files and directories in the directory hierarchy.

Syntax :

• find . -name file.txt : find "file.txt" and is case sensitive
• find . -iname file.txt : find "file.txt" and is case insensitive
• find . -name "*.html" : find files end with ".html" and is case sensitive
• find . -maxdepth 2 "*.cpp" : find files end with ".html" in 2 level depth only
• find . -not "*.cpp" : find all files except those end with ".cpp"
• find / -mtime 10 : find files that modified in last 10 days
• find / -atime 10 : find files whose access time is within last 10 days
• find / -cmin 10 : find files that created within last 10 days

Eval evaluates the specified parameters as a command.

Syntax :

Ex :

list="ls | more"
    eval $list

Expr evaluates a given expression and prints it to standard output.

Syntax :

Ex :

# Example 1:
expr 1 + 2
expr 5 - 3
expr 2 \* 3
expr 8 / 2

# Example 2:
read x
read y
sum=$(expr $x + $y)
echo "Sum = $sum"

# Example 3: comparing numbers with '=' (0=false & 1=true)
res=$(expr $x = $y)
echo $res

# Example 4: 1 when arg1 < arg2
res=$(expr $x \< $y)
echo $res

# Example 5: length of string
x="Hello World"
echo $(expr length $x)

# Example 6: Extract substring from string
x="Hello World"
sub=$(expr substr $x 2 3)
echo $sub

# Example 7: index of character
x="Hello World"
c=$(expr index $x)
echo $c

Modifies file permissions.

Syntax :

Change Permission Recursively :

Format of Permission :

• 1st bit is file type
• 2nd 3 bits are user permission
• 3rd 3 bits are group permission
• 4th 3 bits are others permission

Ex :

chmod u+wr-r text.txt
    chmod o+wrx script.sh

• Permission using Integers :

• wrx wrx --- = 4+2+1 4+2+1 0+0+0 = 770
• r-x r-- r-- = 4+0+1 4+0+0 4+0+0 = 544

Ex :

chmod 777 file.sh