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yuna song
yuna song

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Linux Basics: Permissions and Process Management

#linux

1. chmod — Change File Permissions

Changes the permissions of files and directories.

Syntax 

chmod [options] <mode> <file>
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Numeric (Octal) Mode

Permissions are represented by the sum of:

  • 4 = Read (r)
  • 2 = Write (w)
  • 1 = Execute (x)
Mode Permission Description
755 rwxr-xr-x Owner: full access; Group/Others: read & execute
644 rw-r--r-- Owner: read & write; Group/Others: read only
777 rwxrwxrwx Full access for everyone (not recommended)

Symbolic Mode 

chmod u+x file.sh      # Add execute permission for owner
chmod g-w file.txt     # Remove write permission from group
chmod o+r file.txt     # Add read permission for others
chmod a+x script.sh    # Add execute permission for everyone
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Notes

  • u = owner
  • g = group
  • o = others
  • a = all users

2. chown — Change File Ownership

Changes the owner and/or group of files and directories.

Syntax 

chown [options] <owner>[:group] <file>
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Common Usage 

chown user1 file.txt
chown user1:group1 file.txt
chown :group1 file.txt
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Useful Options 

chown -R user1:group1 /var/www/html
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  • -R : Change ownership recursively.

Notes

  • Changing ownership usually requires sudo. 
sudo chown webuser:webgroup config.php
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3. ps — Display Running Processes

Displays information about currently running processes.

Common Usage 

ps -ef
ps aux
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Practical Examples 

ps aux | grep nginx          # Find nginx processes
ps aux --sort=-%mem          # Sort by memory usage
ps aux --sort=-%cpu          # Sort by CPU usage
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Common Output Fields

Field Description
PID Process ID
TTY Associated terminal
STAT Process state
TIME Total CPU time used
COMMAND Executed command

4. top — Monitor System Processes

Displays CPU, memory, and process information in real time.

Syntax 

top
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The display refreshes every 3 seconds by default.

Useful Shortcuts

Key Action
P Sort by CPU usage
M Sort by memory usage
T Sort by CPU time
k Kill a process (enter PID)
q Quit top

Useful Options 

top -d 1              # Refresh every second
top -u username       # Show processes for a specific user
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Notes

The top section shows system-wide CPU, memory, swap, and task information, while the bottom section lists running processes.

5. kill — Send Signals to Processes

Sends signals to running processes, most commonly to terminate them.

Syntax 

kill [signal] <PID>
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Common Usage 

kill 1234
kill -15 1234
kill -9 1234
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Common Signals

Signal Description
15 (SIGTERM) Gracefully terminate a process (default)
9 (SIGKILL) Forcefully terminate a process

Kill by Process Name 

pkill nginx
killall chrome
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Notes

  • Use SIGTERM (kill or kill -15) whenever possible.
  • Use SIGKILL (kill -9) only if the process does not respond.
  • Find a process ID with: 
ps aux | grep <process_name>
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  • Administrative privileges may be required: 
sudo kill -9 1234
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Note: AI was used to assist in the drafting and formatting of this post.

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