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Linux Basics

Wednesday, May 21, 2025 · 7 min read

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1. Saving Results to File

To save command output to a file:

Command > file
(Overwrites command output)
Command >> file
(Appends output)

Example:

echo $NAME > file

2. Basic Commands

mv - Move files/directories
cp - Copy files/directories
rm - Remove files/directories
mkdir - Create new directories
echo - Display text
ls - List files
- ls -f - Reverse file order
- ls -t - Sort by time
touch - Create empty files
pwd - Print working directory
cat - Concatenate file content
cd - Change directory
- cd .. - Move to parent directory
- cd - - Move to previous directory

3. File Permissions

Viewing Permissions

Use ls -l to view file permissions:

Permissions   Owner       Group       Size       Date       FileName
-rw-rw-r--    samarth     samarth     4096       Aug 10     SPOJ

Permission Types

Permissions are divided into:

User (u)
Group (g)
Other (o) (people outside the group)

Permissions:

r → Read
w → Write
x → Execute

Example

chmod u+w,g+w FileName

This gives the user and group write permissions.

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Removing Permissions

chmod u-rw,g-rwx FileName

Shorthand for Permissions

Permissions can also be set using numeric shorthand:

chmod 754 FileName

Where:

7 → User: read, write, execute
5 → Group: read, execute
4 → Other: read

Numeric Representation:

4 → Read
2 → Write
1 → Execute
0 → No Permissions

4. Checksum

Commands to generate checksums:

sha1sum FileName
sha256sum FileName
md5sum FileName

5. Compress & Extract `.tar` and `.gz` Files

Compress

gzip FileName
tar -cvf zipName.tar FileName

Extract

gunzip FileName.gz
tar -xvf zipName.tar

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6. Users

Viewing Users

Command:

useradd samarth

This creates a user named samarth.

Command:

sudo passwd samarth

Sets a password for samarth.

7. Groups

Managing Groups

Commands:

sudo groupadd <groupName>   # Add new group
sudo usermod -a -G <groupName> <user>   # Append user to group
sudo usermod -G <groupName> <user>      # Replace primary group

8. Viewing Files

Commands to view file contents:

cat FileName    # View entire file
more FileName   # View file content page-by-page
less FileName   # Scroll through file
head FileName   # View the first 10 lines
tail FileName   # View the last 10 lines

9. Wildcards

Wildcards are used for pattern matching in file names:

* → Matches zero or more characters
Example: a* → Matches abc, a123, etc.
? → Matches a single character
Example: a? → Matches ab, ac, etc.
[abc] → Matches any of the specified characters
Example: [a-c] → Matches a, b, c.

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Advanced Wildcards

[!ae] → Does not start with a or e
[a-g]* → Starts with a to g
[3-6]* → Starts with 3 to 6

10. `diff` & `grep`

Compare Files

diff file1 file2

Search Patterns in Files

grep <pattern> FileName

Options:

-i → Ignore case
-c → Count occurrences
-n → Include line numbers
-v → Inverse match (lines that don't match)

File Type Information

Commands:

file FileName    # Display file type
strings FileName # Display printable strings in file

11. Pipe Symbol

Use | to pass output of one command as input to another:

cat FileName | grep <pattern>
ls -au | grep <node>

12. Copying Files Over the Network

Commands:

Using scp:

scp <source> <destination>:/path

Using sftp:

sftp <host>   # Example: samarth@remote.com

Commands within sftp:

lcd          # Change to local directory
put FileName # Upload file
get FileName # Download file

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Processes

ps -e: Display all processes
ps -eH: Display process tree
top: Interactive process viewer
ps -p PID: Display process by PID

Background & Foreground

command &: Start command in the background
ctrl-c: Kill foreground process
ctrl-z: Suspend the foreground process
bg [%num]: Background a suspended process
fg [%num]: Foreground a background process
kill: Kill a process by PID or process number
kill -9 PID: Forcefully kill a process

Cron Service

cron: A time-based job scheduling service
crontab: Perform CRUD operations on job schedules
- Used to automate and schedule jobs

User Switching

whoami: Displays the current user
su [username]: Change user or become superuser
sudo: Superuser do (temporary elevated permissions)
sudo -u [user] command: Execute command as specified user
sudo su - [username]: Switch to another user

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Shell History

Shell history is stored in ~/.bash_history
!N: Repeat command line number N
!string: Repeat most recent command starting with "string"
history: View command history with numbers
ctrl-R: Reverse search
ctrl-C: Cancel search

Tab Autocompletion

Double tab: Get list of possible commands
Tab: Autocompletion

Package Management

DEB: Debian packages
APT: Advanced Packaging Tool

Common Commands

apt-cache search [string]: Search for a package
apt-get install [package]: Install a package
apt-get remove [package]: Remove a package (leaves configuration)
apt-get purge [package]: Delete package (including configuration)
apt-cache show [package]: Display details about a package

Examples

apt-cache search wireshark
apt-cache show wireshark
apt-get install wireshark
which wireshark: /usr/bin/wireshark
dpkg -i chrome.deb: Install chrome.deb package
dpkg -l: List all packages
dpkg -s /usr/bin/wireshark: Display package details

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Disk Management

fdisk: Manage partitions (allows data separation)

Common Commands

sudo fdisk -l: List all partitions
sudo fdisk /dev/sda: Manage partitions on device /dev/sda
- m: Help
- Partition management options

File Systems

ext: Linux file system

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User & Group Management

Account Details

Each account (user) has:

Username
UID
Default Group
Comments
Shell
Home Directory Location

User Information

All user details are stored in /etc/passwd file
- Format: username:password:UID:GID:comments:home_dir:shell
- Encrypted passwords are stored in /etc/shadow

User Management Commands

sudo useradd [username] -c "comment": Add a new user
passwd [username]: Set or change user password
There are other users apart from root, such as:
- sshd, mongo, syslog: Used for system or application accounts

User Deletion

sudo userdel [username]: Delete a user
sudo userdel -r [username]: Delete user and associated files

User Update

sudo usermod [username] [options]: Update user properties

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Managing Groups in Linux

All group details are stored in /etc/group.

The format of a group entry is:

groupname:password:GID:account1,account2,...,accountN

password: Encrypted (shadow)
GID: Group ID
account1, account2, ...: List of accounts in the group.

Commands for Group Management

List all groups:
```
groups username
```
Add a new group:
```
groupadd group-name
```
Delete a group:
```
groupdel group-name
```
Modify a group:
```
groupmod group-name
```

Note: Replace group-name and username with the appropriate group name and username values.

References & Related Topics

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1. Saving Results to File

2. Basic Commands

3. File Permissions

Viewing Permissions

Permission Types

Example

Removing Permissions

Shorthand for Permissions

4. Checksum

5. Compress & Extract .tar and .gz Files

Compress

Extract

6. Users

Viewing Users

7. Groups

Managing Groups

8. Viewing Files

9. Wildcards

Advanced Wildcards

10. diff & grep

Compare Files

Search Patterns in Files

File Type Information

11. Pipe Symbol

12. Copying Files Over the Network

Processes

Background & Foreground

Cron Service

User Switching

Shell History

Tab Autocompletion

Package Management

Common Commands

Examples

Disk Management

Common Commands

File Systems

User & Group Management

Account Details

User Information

User Management Commands

User Deletion

User Update

Managing Groups in Linux

Commands for Group Management

References & Related Topics

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5. Compress & Extract `.tar` and `.gz` Files

10. `diff` & `grep`