Linux is an open-source kernel that powers many operating systems, called distributions (or distros). This documentation mainly focuses on Debian, one of the Linux distros that is free and widely used.
- No previous knowledge is required!
Note
This is not a complete guide to Linux. It's just a collection of specific concepts and commands.
Basic Linux commands and concepts.
Definitions:
.,./: Current directory.<variable>: A variable.<variable>...: Multiple variables.
which <command>...: Show the path of a command.whatis <command>...: Briefly describe a command in one line.wc -<option>... <file>...: Count lines, words, and characters.find <directory>... -name <pattern> -<option>: Search for files and directories.find ./dir1 -name file.txt # A specific file in dir1 and its subdirectories find ./dir1 -name "*.txt" # All .txt files find ./dir1 -name file.txt -delete # Find and delete
grep -<option>... <pattern> <file>...: Search for patterns in a file and return matching lines.-i: Case-insensitive search.-r: Recursive search in directories.
grep "hello" file.txt # Search for "hello"
less: Open output in a controllable pager.q: Quit.space: Scroll down (next page).b: Scroll up (previous page).g: Go to the beginning.G: Go to the end./<pattern>: Search forward (down).?<pattern>: Search backward (up).n: Repeat search in the same direction (next result).N: Repeat search in the opposite direction (previous result).h: Help.
less file.txt # View a file in a pager find ./dir1 -name "*.txt" | less # View the output of find in a pager
cat -<option>... <file>...: Print file contents to the standard output (terminal).-n: Number all lines.-b: Number non-empty lines.-s: Squeeze blank lines into one.
getent <database> <key>: Get entries from system databases.getent group # All groups and their members getent group docker # Select a specific group (docker in this case) getent services # All services and their ports getent hosts # All hosts and their IP addresses
OUTPUT > <file>...: Redirect output to a file.>: Overwrite.>>: Append.
> file.txt # Create/empty a file and start writing in it pwd > file.txt # Save the command (pwd) output to file cat file1.txt > file2.txt # Copy contents of file1.txt to file2.txt cat file1.txt file2.txt >> file3.txt # Append contents of file1.txt and file2.txt to file3.txt
mkdir -<option>... <directory>...: Create a directory.-p: Create parent directories as needed.
mkdir -p ./sub1/sub2 # Create a nested directory structure mkdir -p ./new_dir/{dir1,dir2,dir3} # Create a directory with multiple subdirectories in it
rm -<option>... <file/directory>...: Delete a file/directory.-i: Prompt before each removal.-r: Recursively remove a directory and its contents.-f: Force delete without prompt.-v: Verbose mode (show what is being done).
cp -<option>... <source>... <destination>: Copy files and directories (mvto move).Overwrites if already exists.
-i: Prompt before overwrite.-r: Recursively copy directories and their contents.
cp file.txt ./dir1 # Copy a file to dir cp file.txt ./dir1/file2.txt # Copy and rename in destination
touch <file>...: Create an empty file.
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du -<option>... <file/directory>...: Show disk usage.-a: Show all files and directories.-h: Human-readable format (e.g., KB, MB).-s: Summary total for each argument.
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pidof <program_name>...: Show the PID(s) of a running process.pidof bash # PID of bash pidof docker # PID of Docker
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ps -<option>...: View running processes.-u <user>: Show processes for a specific user.-ef: More detailed list.
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lsof -<option>... <pattern>...: List open files and their processes.-p <PID>: Show files opened by a specific process.-i: Show network connections.
lsof -i tcp # TCP connections lsof -i tcp:3000 # TCP on port 3000
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kill -<signal_num> <PID>: Send a signal to a process.Signal Number Description SIGHUP1Reload config. Tells a program to restart or reload its settings. Often used after editing config files. SIGINT2Graceful stop. Sent when you press Ctrl + C to stop a program. SIGQUIT3Like SIGINT(2), but also creates a debug log (core dump).SIGABRT6Tells a program to abort/cancel (used when something goes wrong). SIGKILL9Force-stops a program immediately. SIGTERM15Asks a program to stop nicely. (Default signal for kill)kill -9 <PID> # Force kill a process with given PID kill -9 $(pidof nginx) # Force kill all processes with the name nginx
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source <file>...: Execute commands from a file in the current shell.Apply changes without needing to start a new terminal session.
source ~/.bashrc # Apply changes made to .bashrc
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watch -<option>... <command -option>...: Re-run a command periodically.watch -n 5 free -h # Show memory usage with a 5 seconds of refresh rate
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cmd1 ; cmd2 ; cmd3: Run commands in order, regardless of success. -
cmd1 && cmd2 && cmd3: Run next only if the previous succeeds. -
cmd1 || cmd2 || cmd3: Run next only if the previous fails. -
cmd1 | cmd2 | cmd3: Pipe output of one command to the next.
find . *.* | less # View the command result in a pagerUnderstanding the file and directory permission system in Linux.
Permissions determine who can access, modify, or execute files and directories, and ensure that users and programs can only interact with the system in permitted ways.
Permission types determine what kind of access a user (or program) has to a file or directory.
- Read (
r):View file contents or list a directory's contents.
- Write (
w):Modify file contents or add/remove files in a directory.
- Execute (
x):Run a file as a program or enter a directory.
Ownership determines who has control over a file or directory.
- User (
u):The owner of the file or directory (usually the creator).
- Group (
g):Members of a group associated with the file or directory.
- Others (
o):Everyone else on the system.
The permission structure is represented in a 10-character string, such as -rwxr-xr--, which can be broken down as follows:
[identifier] [owner] [group] [others]
- rwx r-x r--
- Identifier: The 1st char.
-: File.d: Directory.l: Symbolic link.
- User/Group/Others: Combination of 3 chars each.
r: Read.w: Write.x: Execute.-: No permission.
The permission structure can also be represented in octal format, where each permission type is assigned a number:
r: 4w: 2x: 1-: 0
A complete table:
| Octal | Binary | File Mode |
|---|---|---|
| 0 | 000 | --- |
| 1 | 001 | --x |
| 2 | 010 | -w- |
| 3 | 011 | -wx |
| 4 | 100 | r-- |
| 5 | 101 | r-x |
| 6 | 110 | rw- |
| 7 | 111 | rwx |
Example:
-rwxr-xr--translated as754.
7for the owner (rwx).5for the group (r-x).4for others (r--).
chmod -<option>... <mode> <file/directory>...: Change file/directory permissions.<mode>:- Symbolic:
<targets><actions><permissions>.- Targets:
u: User.g: Group.o: Others.a: Everyone (user, group, others).
- Actions:
+: Add permission.-: Remove permission.=: Set permission.
- Permissions:
r,w,x.
- Targets:
- Octal.
- Symbolic:
chmod g+x file.txt # Add execute permission for group chmod u-rw file.txt # Remove read and write permissions for user chmod a=rwx file.txt # Set all permissions for everyone chmod ug-x file.txt # Remove execute permission for user and group chmod +x file.txt # Add execute permission for everyone (when the target is not specified it defaults to all) chmod 755 file.txt # Set permissions to rwxr-xr-x chmod -R 755 ./dir1 # Set permissions for all files and directories in dir1 recursively
chown -<option>... <user>:<group> <file/directory>...: Change file/directory ownership.$USERrefers to the current user.-R: Change ownership of all files and directories recursively.
chown $USER:$USER file.txt # To the current user and group chown john ./dir1 # To user john and group staff
chgrp -<option>... <group> <file/directory>...: Change file/directory group ownership.-R: Change group ownership of all files and directories recursively.
groups <user>: List groups a user belongs to.usermod -aG <group> <user>: Add a user to a group.gpasswd -d <user> <group>: Remove a user from a group.passwd -<option>... <user>: Change a user's password.-d: Delete a user's password.-e: Expire a user's password (force change on next login).