Process Status in Operating Systems

Study Notes

A process can have three statuses: foreground, background, or stopped (suspended).
Only one job can be in the foreground at a time, receiving keyboard input and sending output to the screen.
Background jobs run quietly without terminal input and require user attention only if they need interaction.
Jobs can be suspended using Ctrl-Z; suspended jobs can continue running in the foreground with fg or in the background with bg.
Interrupting a job with Ctrl-C kills it permanently, unlike suspending which allows resuming.
To run a command in the background, append an '&' to the command line.
Shell returns the job number and PID of the background process for management.

ps lists processes and their details, including PIDs.
kill can terminate processes using either PID or job ID.
Use kill -9 to forcefully kill a process with a higher success probability.
Commands for job control:
- jobs: List all current jobs.
- bg: Resume a stopped job in the background.
- fg: Resume a stopped job in the foreground.
- stop: Stop execution of a job.

Shell variables are typically set when a new shell is initiated.
Environmental variables (global) are accessible in all shells; can be viewed using env or printenv.
Local variables are accessible only in the current shell; view using set.
Use echo $VARIABLE_NAME to retrieve the value of a shell variable.
Variables can be defined using the syntax VAR_NAME=value.

PS1 controls the appearance of the shell prompt.
Special characters in PS1:
- \u: Username
- \h: Hostname
- \w: Current working directory
To change an environment variable, use export VARIABLE=VALUE.
PATH: Contains directories for executable file searches, separated by colons.
Modifying PS1 can customize the command line prompt to include additional information like date and time.

A process is defined as a program in execution, created when a command is invoked from a shell.
Each process is identified by a unique Process Identifier (PID).
The init process is a vital process that is always running in Linux.
Types of processes include:
- Interactive processes
- Automatic processes
- Daemons
The ps command displays process information, with options to show various process attributes.
Example usage of the ps command displays current shell processes with their PIDs.

Job control allows management of multiple processes in the shell.
Processes can be in one of three statuses:
- Foreground: interacts with the user via keyboard and screen.
- Background: runs without user input.
- Stopped: temporarily suspended.
Use Ctrl-Z to suspend a foreground process and resume it with fg (foreground) or bg (background).
Ctrl-C sends an interrupt to terminate a foreground job, unlike suspending.

kill command terminates a job using its PID or job number.
The jobs command lists jobs associated with the current shell.
& runs commands in the background.
Other job control commands:
- bg: Resume a stopped job in the background.
- fg: Resume a stopped job in the foreground.
- stop: Stop the execution of a job.

Shell variables typically use uppercase characters and can be defined using single or double quotes for strings.
Double quotes allow variable resolution, whereas single quotes do not.
Use the export command to make a local variable available as an environment variable for child processes.

PATH variable specifies directories for the shell to search for executables, separated by colons.
The PS1 variable defines the format of the shell prompt, comprising components like username (\u), hostname (\h), and current working directory (\W).
Changing the Prompt: Modify PS1 to adjust prompt details; special characters can customize prompt appearances.

Set environment variables with export VARIABLE=VALUE.
To see a variable's value, use echo $VARIABLE.
To add directories to the PATH, you can append them using:
- export PATH="$PATH:/new/directory".
Remove an environment variable to eliminate its current value and associated components.