Shell Scripting

Bash Scripting

0. Execution

./filename.sh
sh filename.sh
source filename.sh

1. Shebang

#!/bin/bash

2. Display

echo "Hello, World"

3. Comments

# This is a comment

4. Here Documnet (here-doc)

To input multiple lines of text to a command

command << delimeter
    text...
    statements...
delimeter

5. `if` Statement

if [ condition ]
then
    statements
elif [ condition ]
then
    statements
else
    statements    
fi

If we are using signs (>, <, =, …) instead of letters (-gt, -lt, -eq, …) in the condition, use (()) instead.

if (( condition ))

We can use logical operators like

if [ condition1 ] && [ condition2 ]
                or
if [[ condition1 && condition2 ]]

These can also be implemented using the corresponding letters for the logical operators:

if [ condition1 -a condition2 ]

6. `case` Statement

case expression in
    pattern1)
        commands
        ;;
    pattern2)
        commands
        ;;
    *)
        default commands
        ;;
esac

7. `while` loop

The while loop is executed as long as the condition evaluates to true.

while [ condition_is_true ]
do 
    statements
done

8. `until` loop

The until loop is executed as many times the condition or command evaluates to false. The loop terminates when the condition or command becomes true.

until [ condition_becomes_true ]
do
    statements
done

9. `for` loop

for variable in {start...stop...step}
do 
    statements
done

Or we can do

for (( i=0; i<5; i++ ))

10. `break` and `continue`

for (( i=0; i<=10; i++ ))
do
    if [ $i -eq 5 ]
    then 
        continue
    fi
    echo $i
done

11. Script Input: STDIN

Command-line Arguments

./filename.sh input1 input2 input3
    ↑           ↑       ↑     ↑
    $0          $1      $2    $3

We can use any number of command-line arguments given, by using an array

args=("$@")
echo [args[0]]

Reading a file

Reading a file using the filename

while read line
do 
    echo $line
done < "{$1:-/dev/stdin}"

To run

./filename.sh   file_to_read

If we give no filename as the argument, the program will consider the terminal as the file, and the input given via the command line will be taken as $line.

12. Script Output: STDOUT, STDERR

We can redirect the Standard Output and the Standard Error after running a command to two different files.

command 1>file1.txt 2>file2.txt
        ↑           ↑
      STDOUT      STDERR
        ↓
command >file.txt

To redirect the output and error to the same file, do

command 1>file1.txt 2>$1
        or
command >& file1.txt

13. Send the output of one script to another

In script1,

x = 1234
export x
./script2.sh

In script2,

echo $x

14. String Processing

We can compare 2 strings using ==
We can compare their lengths using \> and />
Use ^ for lowercase and ^^ for uppercase

15. Numbers and Arithmetic

n1 = 4
n2 = 5
echo $(( n1 + n2 ))
echo $(( n1 - n2 ))
echo $(( n1 * n2 ))
echo $(( n1 / n2 ))
echo $(( n1 % n2 ))

    or

echo $(expr $n1 + $n2 )
echo $(expr $n1 \* $n2 ) 
...

To convert a hexadecimal number to decimal,

hexa=f23a2b
echo "obase=10; ibase=16; $hexa" | bc  

16. Declare command

declare command is used to update variables within the shell.

We can see all variables in the system by running declare -p To create a variable,

declare myvariable

To give a value to it,

declare myvariable=12

To create a read only variable, do

declare -r myvariable="value"

17. Arrays

To create an array and print out the entire array,

name = ('element1' 'element2' 'element3')
echo "${name[@]}"

To print the element at a particular index, do

echo "${name[1]}"

To print the indexes in the array, do

echo "${!name[@]}"

To get the length of an array,

echo "${#name[@]}"

To remove the value at an index, run

unset name[2]

To insert a value at a particular index, do

name[index]=value

We can use seq to get a sequence of values

seq [last]
seq [first] [last]
seq [first] [step] [last]

For example, do

for i in $(seq 5) 
do 
    echo $i
done

18. Functions

To declare a function,

function funcName()
{
    statements...
}

funcName() {
    statements...
}

To call a function, run

funcName

We can use positional arguments in a function

funcName() {
    echo $1 $2
}

funcName "Hello" "World"

19. Files and Directories

To create a directory, run

mkdir -p dir_name

To check if a directory exists, do

if [ -d $dir_name ]
...

To create an empty file, run touch filename.

To chek if a file exists, do

if [ -f $file_name ]
...

To overwrite a file or append to an existing file, respectively do

echo "$filename" > "Something to overwrite"
echo "$filename" >> "Something to append"

To read line by line from a file,

while IFS= read -r line
do
    echo "$line"
done < $filename

To delete a file, do

rm filename

20. Sending Email

Install ssmtp
```
 sudo apt install ssmtp
```

Edit the file /etc/ssmtp/ssmtp.conf

 root=email@123.com
 mailhub=ssmtp.gmail.com:587
 AuthUser=email@123.com
 AuthPass=
 UseSTARTTLS=yes

Run
```
 ssmtp <email>
```

Write the mail.

 To:
 From:
 Cc:
 Subject:
 this is the body of the mail.

21. `curl`

To download a file, run

curl <url> -O

To rename the file

curl <url> -o new_name
        or
curl <url> > new_name

To download the header, run

curl -I <url>

22. `select` loop

select variable_name in option1 option2 option3 option4
do 
    echo $variable_name
done

To avoid invalid inputs, we can use switch to do

select ...
do 
    case $name in 
    option1)
        echo option1
    ...
done

To wait for a keypress,

while [ true ]
do 
    read -t 3 -n 1
if [ $? = 0 ]
then 
    exit;
else
    echo "Waiting for keypress"
fi

23. inotify

Inode Notify is used to monitor the files and directories.

inotifywait -m /path/to/directory

This constantly notifies us of any operation inside our directory being watched.

24. grep

Global Regular ExPression is used for searching text for lines that match a regular expression.

grep <regex> <file>

Searching case insensitively,

grep -i $grepVar $filename

For the count or line number, do respectively

grep -c ...
grep -n ...

25. awk

awk is a programming language used to manipulate data. awk allows us to search for a particular pattern, and also to specify the action to be taken when the pattern is found. We can run awk inside bash. To print using awk, do

awk '{print}' /path/to/filename

To search for a pattern,

awk '/pattern/ {print}' /path/to/filename

To print out a specific field

# Prints out the second field: "is" from "This is linux"
awk '/linux/ {print $2}' $filename

# Fields 3 and 4
awk '/linux/ {print $3, $4}' $filename

26. sed

Stream EDitor is used to manipulate text files. To substitute all occurences of a ‘i’ with ‘I’ and display it, do

cat filename.txt | sed 's/i/I/'
            or
sed 's/i/I/g' <filename>

To modify the file, we need the i switch

sed -i 's/i/I/g' <filename>

27. comm

The comm command is used to compare two files line by line.

comm [OPTION]... FILE1 FILE2

For example, to compare two sorted files named file1.txt and file2.txt and display only the lines unique to file1.txt, you can use:

comm -23 file1.txt file2.txt

Similarly, to display only the lines common to both files, you can use:

comm -12 file1.txt file2.txt

28. Debugging

Run
```
 bash -x ./filename.sh
```
Which enables us to see step-by-step which lines are being executed.
We can also do this by adding -x to the shebang
```
#!/bin/bash -x
```

We can select the lines to be executed by

 set -x
     ...
     statements to be executed
     ...
 set +x