Intro BASH

#bash #ysap
The main difference between scripting and programming languages is that we don't need to compile the code to execute the scripting language, as opposed to programming languages.

Like a programming language, a scripting language has almost the same structure, which can be divided into:

• Input & Output
• Arguments, Variables & Arrays
• Conditional execution
• Arithmetic
• Loops
• Comparison operators
• Functions

In general, a script does not create a process, but it is executed by the interpreter that executes the script, in this case, the Bash.

Execute a script in bash
bash hello.sh
or

sh script.sh
# or
./hello.sh

Shell

REPL = Read, Evaluate, Print, Loop this is what BASH does.

$ echo hello
hello

Where does BASH live? Bash process lives in the binary folder

$ which bash     
/usr/bin/bash

Navigation and Manipulation command

$ mkdir Bash
$ cd Bash

Create new files:

touch lesson-1.txt
touch lesson-2.txt
touch lesson-22.txt

We have a file with the typo? rename

mv lesson-22.txt lesson-3.txt

Remove all files:

rm lesson-*.txt

The * is a GLOB , called wildcard.

Find Terminal history

history 

Give all the ran commands in Terminal

Make a hidden file

touch .file.txt

Completely hidden

Show all the files:

ls -a

Searching and Paging

The most used command is grep.
It is a command to search for a pattern defined in the command.

Example: Find all the words in the system's dictionary

#!/bin/bash
cat usr/share/dict/words

Which will give a list of words from A-Z , like this:

Now , to find something with carl pattern:

#!/bin/bash
grep carl usr/share/dict/words
# or
grep 'carl' usr/share/dict/words

found this:

Or use special characters like : ^ to find words that starts with that sad pattern:

#!/bin/bash
grep '^sad' /usr/share/dict/words

$ at the end to find words that end with that er pattern

#!/bin/bash
grep 'er
which gives:
![Attachments/Pasted image 20260514175751.png](/img/user/HTB/CJCA/Bash/Attachments/Pasted%20image%2020260514175751.png)

---
###### Built-in commands vs external commands
Check the type of command:
```bash
type echo

type very useful command to check if command is built-in in shell or a binary:
┌──(kali[~]
└─$ type ls
ls is an alias for ls --color=auto

┌──(kali[~]
└─$ type cd
cd is a shell builtin

/usr/share/dict/words

which gives:
![Attachments/Pasted image 20260514175751.png](/img/user/HTB/CJCA/Bash/Attachments/Pasted%20image%2020260514175751.png)

---
###### Built-in commands vs external commands
Check the type of command:
{{CODE_BLOCK_14}}

`type` very useful command to check if command is built-in in shell or a binary:
┌──(kali[~]
└─$ type ls      
ls is an alias for ls --color=auto

┌──(kali[~]
└─$ type cd
cd is a shell builtin