GNU Presentation¶

Managing Processes in Linux¶

Foreground and Background Jobs¶

Linux allows the management of multiple processes concurrently, which is one of its key advantages. You can run different programs and jobs simultaneously, often by sending them to the background.

Running Jobs in the Background:
Normally, when you run a program in the terminal, it blocks the terminal until the process completes. To run a command in the background, you can append & at the end of the command. For example: bash xeyes & sleep 60 & This allows you to continue using the terminal while these programs run in the background.
Suspending and Resuming Jobs:
If a job is running in the foreground, you can:
Pause it with Ctrl + Z
Bring it to the foreground using fg
Send it to the background using bg

You can list all running jobs with the jobs command, and view the Process ID (PID) with jobs -l.

Useful Commands for Process Management¶

nohup:
The nohup command allows a process to continue running even after the terminal or session is closed. By default, output is written to nohup.out. bash nohup command &
kill:
The kill command sends signals to processes. While Ctrl + C and Ctrl + Z also send signals, kill allows more control:
SIGTERM (15): Graceful termination.
SIGKILL (9): Forceful termination.

Syntax:
bash kill -9 <PID> # Force kill a process

killall:
The killall command sends signals to all processes by name.
Example: bash killall -9 firefox
pkill:
Similar to killall, but with a more flexible pattern matching approach. bash pkill -9 <pattern>
ps:
The ps command lists running processes.
Example: bash ps -ef # List all processes
pgrep:
Find processes by name and return their PID: bash pgrep <process-name>
top:
top is the go-to tool for live system monitoring. It provides real-time information about system resource usage.

Key Top Commands: - h for help - q to quit - M to sort by memory usage - k to kill a process

free:
Displays system memory usage, with options for different units: bash free -h # Human-readable format
uptime:
Shows how long the system has been running, load averages, and logged-in users.
watch:
Repeatedly runs a command at specified intervals (default: 2 seconds). Example: bash watch free -h

Terminal Multiplexers¶

screen¶

The screen command allows you to multiplex multiple terminal sessions inside a single terminal window. You can detach and reattach to sessions. Key commands: - Ctrl + A + D: Detach from the current session. - screen -r: Reattach to a session.

Other Useful Keybindings: - |: Split the screen vertically. - C: Create a new window. - N: Move to the next window. - K: Kill the current window.

tmux¶

tmux is a more advanced terminal multiplexer, often preferred for its features. After installing, you can use: - Ctrl + B + %: Split the window vertically. - Ctrl + B + ": Split the window horizontally. - Ctrl + B + D: Detach from the current session.

To list and reattach to a session:

tmux ls  # List sessions
tmux attach-session -t <session_name>  # Reattach to a session

Change Process Execution Priorities¶

Nice and Renice¶

Nice Values:
The nice value determines the priority of a process. The range is from -20 (highest priority) to 19 (lowest priority).
Setting Nice Values:
To run a program with a custom priority, use: bash nice -n <nice_value> <command>
Renicing Running Processes:
To change the priority of a running process: bash sudo renice -n <nice_value> <PID>

Use the top command to monitor process priorities. In the top output, the NI column shows the "nice" value.

Search Text Files Using Regular Expressions¶

grep, egrep, fgrep, sed, and Regex¶

Regular Expressions (Regex) Basics¶

Regular expressions are patterns used to match text. For example: - a matches the character 'a'. - .* matches any sequence of characters.

Regex Metacharacters¶

*: Matches 0 or more of the preceding character.
+: Matches 1 or more of the preceding character.
?: Matches 0 or 1 of the preceding character.
{n,m}: Matches between n and m occurrences.
|: Alternation (OR).
.: Any single character.

Character Classes¶

\d: Any digit (0-9).
\w: Any word character (alphanumeric + underscore).
[a-z]: Any lowercase letter.

grep¶

grep is used to search for patterns in files. Key options: - -i: Case-insensitive search. - -v: Invert match. - -n: Show line numbers. - -r: Recursive search.

Extended grep¶

Use egrep or grep -E for extended regex features:

egrep 'a|b' filename  # Matches lines containing 'a' or 'b'

Fixed grep¶

fgrep or grep -F is used for fixed string matching (no regex interpretation).

sed¶

sed is a stream editor for manipulating text.
For example:

sed -r "s/(Z|R|J)/starts with ZRJ/" myfile.txt

Basic File Editing in vi/vim¶

vi Modes¶

vi operates in two modes: - Command Mode: For navigation and issuing commands. - Insert Mode: For editing text.

Navigating in vi¶

In command mode, use: - h: Move left. - j: Move down. - k: Move up. - l: Move right.

Editing Text¶

i: Enter insert mode at the cursor position.
a: Enter insert mode after the cursor.
x: Delete a character.
dd: Delete a line.
yy: Copy a line.
p: Paste text.

Search and Replace¶

/pattern: Search forward for "pattern".
?pattern: Search backward.
:s/old/new/g: Replace "old" with "new" in the current line.

Exiting vi¶

:wq: Save and exit.
:q!: Exit without saving.

Alternative Editors¶

nano: A simple terminal-based text editor.
emacs: A powerful and extensible text editor.
neovim: An improved version of vim.

To set your default editor:

export EDITOR='vim'  # Set vim as the default editor