Bash Scripting
Bash scripting must have/know tips.
Intro script
Section titled “Intro script”#!/bin/bash
# Hello World Bash Script
echo "Hello World!"- Line 1: #! is commonly known as the shebang and is ignored by the Bash interpreter. The second part, /bin/bash, is the absolute path to the interpreter, which is used to run the script. This is what makes this a “Bash script” as opposed to another type of shell script, like a “C Shell script”, for example.
- Line 2: # is used to add a comment, so all text that follows it is ignored.
- Line 3: echo “Hello World!” uses the echo Linux command utility to print a given string to the terminal, which in this case is “Hello World!”.
Results in:
./hello-world.sh
Hello World!Special Bash variables
Section titled “Special Bash variables”| Variable Name | Description |
|---|---|
| $0 | The name of the Bash script |
| $1 - $9 | The first 9 arguments to the Bash script |
| $# | Number of arguments passed to the Bash script |
| $@ | All arguments passed to the Bash script |
| $? | The exit status of the most recently run process |
| $$ | The process ID of the current script |
| $USER | The username of the user running the script |
| $HOSTNAME | The hostname of the machine |
| $RANDOM | A random number |
| $LINENO | The current line number in the scrip |
Common test command operators
Section titled “Common test command operators”| Operator | Description: Expression True if… |
|---|---|
| !EXPRESSION | The EXPRESSION is false. |
| -n STRING | STRING length is greater than zero |
| -z STRING | The length of STRING is zero (empty) |
| STRING1 != STRING2 | STRING1 is not equal to STRING2 |
| STRING1 = STRING2 | STRING1 is equal to STRING2 |
| INTEGER1 -eq INTEGER2 | INTEGER1 is equal to INTEGER2 |
| INTEGER1 -ne INTEGER2 | INTEGER1 is not equal to INTEGER2 |
| INTEGER1 -gt INTEGER2 | INTEGER1 is greater than INTEGER2 |
| INTEGER1 -lt INTEGER2 | INTEGER1 is less than INTEGER2 |
| INTEGER1 -ge INTEGER2 | INTEGER1 is greater than or equal to INTEGER 2 |
| INTEGER1 -le INTEGER2 | INTEGER1 is less than or equal to INTEGER 2 |
| -d FILE | FILE exists and is a directory |
| -e FILE | FILE exists |
| -r FILE | FILE exists and has read permission |
| -s FILE | FILE exists and it is not empty |
| -w FILE | FILE exists and has write permission |
| -x FILE | FILE exists and has execute permission |
| && | AND |
| || | OR |
Reading user input
Section titled “Reading user input”Prompting user for input and silently reading it using read.
-p- prompts the user to supply input, while the input is typed it is shown in the terminal-sp- prompts the user to supply input, while the input is typed it is NOT shown in the terminal
#!/bin/bash
# Prompt the user for credentials
read -p 'Username: ' username
read -sp 'Password: ' password
echo "Thanks, your credentials are as follows: " $username " and " $password
$ ./input2.sh
Username: crypt0rr
Password:
Thanks, your credentials are as follows: crypt0rr and HaveYouSeenMyPassword?For Loop
Section titled “For Loop”$ for ip in $(seq 1 5); do echo 10.11.1.$ip; done
10.11.1.1
10.11.1.2
10.11.1.3
10.11.1.4
10.11.1.5$ for i in {1..5}; do echo 10.11.1.$i;done
10.11.1.1
10.11.1.2
10.11.1.3
10.11.1.4
10.11.1.5While Loop
Section titled “While Loop”#!/bin/bash
# while loop example
counter=1
while [ $counter -lt 6 ]
do
echo "10.11.1.$counter"
((counter++))
done./while.sh
10.11.1.1
10.11.1.2
10.11.1.3
10.11.1.4
10.11.1.5#!/bin/bash
# while loop example 2
counter=1
while [ $counter -le 5 ]
do
echo "10.11.1.$counter"
((counter++))
done
./while2.sh 10.11.1.1
10.11.1.2
10.11.1.3
10.11.1.4
10.11.1.5Functions
Section titled “Functions”Two methods of declaring functions.
function function_name {
commands...
}function_name () {
commands...
}