Editing Content of file

sed is a powerful command-line tool in Unix/Linux that allows you to perform basic text transformations on an input stream (a file or input from a pipeline). It’s commonly used for:

• Searching: Finding lines that match a pattern.

• Editing: Modifying text by substituting, inserting, deleting, or appending text.

• Displaying: Printing specific lines based on patterns.

The head command in Unix/Linux is used to display the beginning of a file or the first few lines of its content.

Case 1: To collect data from a file that contains continuous lines of text.

# Example:
# Suppose we have data like this
──(kali㉿kali)-[/tmp/temp_workspace]
└─$ cat note.txt
Dave's password list:

Window
rickc137
dave
superdave
megadave
umbrella

Note to myself:
New password policy starting in January 2022. Passwords need 3 numbers, a capital letter and a special character

We want to fetch the data from note.txt. We want all the value from Window to umbrella .

# Testing command:
sed -n '/Window/,/umbrella/p' note.txt

# sed -n '/start_pattern/,/end_pattern/p': Ideal for ranges if lines are grouped together.

# If got the result we want then we will save the result in new file.
sed -n '/Window/,/umbrella/p' note.txt > username.list

sed -i "s/192\.168\.45\.[0-9]\+/$IP_KALI/" shell.php

sed 's/\bbuf\b/shell/g'

The command sed 's/\bbuf\b/shell/g' replaces every whole word buf with shell in the input text, ensuring it doesn't affect words like buffer.

Case 2: If we want to delete last few lines from file.

Suppose we want to delete last four lines from /etc/john/john.conf file. So, We can use head command with -n and -4 (It is minues 4).

head -n -4 /etc/john/john.conf
# This command will display all the content of mentioned file expect last four lines.

# Way to save result:
head -n -4 /etc/john/john.conf > temp && sudo mv temp /etc/john/john.conf

Case 3: Deleting particular line number and displaying its content.

We will use sed command for this.

• 1d: Tells sed to delete the first line.

• -i: Edits the file in place.

# Example:
sed -i '1d' passwd_list.txt

sudo sed -i '$d' /etc/hosts

This deletes the last line from Hosts the file. Run Multiple times to delete multiple last lines.

Deprecated function

Case 4: Special case of changing the value of x in ~/.zshrc file

I store the value in ~/.zshrc file to ease the process of fatching ip.

I will be changing value in one go.

# ip Setting.

# export VM1=172.16.244.10
export IP_KALI=$(ifconfig tun0 | grep 'inet ' | awk '{print $2}')
x=237
export web02=192.168.$x.247
export external=192.168.$x.248
export legacy=192.168.$x.249
export mail=192.168.$x.189
export login=192.168.$x.191
export web01=192.168.$x.245
export demo=192.168.$x.246

Changing value

sed -i 's/^x=[0-9]\+/x=NEW_VALUE/' ~/.zshrc ; source ~/.zshrc

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