Processes

A process is a command that is running.
Opening a terminal is running a process.

Processes

Processes Start and Exit

They start.
They stop, which is also referred to as exit.
When processes exit, they return an "exit code" which can be useful. The exit code gives info about the "success" or "failure" of the process.

View a previous process exit code

# run a process, a small one here, "ps"
ubuntu@primary:~$ ps
    PID TTY          TIME CMD
  43759 pts/0    00:00:01 bash
  71881 pts/0    00:00:00 ps

# check out the exit code
ubuntu@primary:~$ echo $?
0

# try a process that doesn't exist, "horse"
ubuntu@primary:~$ horse

Command 'horse' not found, did you mean:

  command 'morse' from deb bsdgames (2.17-28build1)
  command 'morse' from deb morse (2.5-1build1)
  command 'morse' from deb morse-simulator (1.4-6build1)
  command 'hose' from deb netpipes (4.2-8build1)
  command 'horst' from deb horst (5.1-2)

Try: sudo apt install <deb name>

ubuntu@primary:~$ echo $?
127

Exit Codes Rundown

0: success
1: there was an err
2: a bash error: the process & program tried to use bash, itself, and a bash err happened
126: permission err
127: command not found
128: the exit command, itself, threw an err
130: program was ended with CTRL + C
137: program was ended with SIGKILL

See processes with ps

ps will output processes that are running.
ps stands for process snapshot.

# run "ps"
ubuntu@primary:~$ ps
    PID TTY          TIME CMD
  43759 pts/0    00:00:00 bash
  47519 pts/0    00:00:00 ps


# try ADDING a sleep command to the ps line
# see 'sleep' added to the list
ubuntu@primary:~$ sleep 5 & ps
[1] 58266
    PID TTY          TIME CMD
  43759 pts/0    00:00:00 bash
  58266 pts/0    00:00:00 sleep
  58267 pts/0    00:00:00 ps

note: sleep 5 creates a process that waits 5 sec. and then exits. sleep will be used throughout this doc as sleep is a low-impact command that consumes a process.

Monitor Processes with top

According to man top,

top – display sorted information about processes

Top stands for "table of processes", and shows an analysis of a bunch of data about running processes on a linux machine.

Using top in a mac

top will work on a mac shell, and I have node installed on my machine, so I'm going to use my mac, node, and the shell to illustrate top:

# in one terminal, start a node process
node

# in another terminal 
# get all the processes
Jakes-4:content Jake$ ps
  PID TTY           TIME CMD
  280 ttys000    0:00.09 -bash
  722 ttys000    0:00.10 node
89470 ttys001    0:00.08 /bin/bash -l
  726 ttys002    0:00.05 -bash

# NOTICE: node pid = 722
Jakes-4:content Jake$ top -pid 722


Processes: 404 total, 3 running, 401 sleeping, 2413 threads                                                              20:00:06
# 
# ...a bunch of other stuff...
# 
PID  COMMAND      %CPU TIME     #TH  #WQ  #POR MEM  PURG CMPR PGRP PPID STATE    BOOSTS    %CPU_ME %CPU_OTHRS UID  FAUL COW  MSGS
722  node         0.0  00:00.09 11   0    33   12M  0B   11M  722  280  sleeping *0[1]     0.00000 0.00000    501  2252 113  57

Kill a running process with Kill

# create a long-running 'sleep' process to kill
ubuntu@primary:~$ sleep 100 & ps
[1] 58271
    PID TTY          TIME CMD
  43759 pts/0    00:00:00 bash
  58271 pts/0    00:00:00 sleep
  58272 pts/0    00:00:00 ps
ubuntu@primary:~$ ps
    PID TTY          TIME CMD
  43759 pts/0    00:00:00 bash
  58271 pts/0    00:00:00 sleep
  58273 pts/0    00:00:00 ps

# use the process id to stop the process
ubuntu@primary:~$ kill -s SIGKILL 58271

# see that the process has been killed
ubuntu@primary:~$ ps
    PID TTY          TIME CMD
  43759 pts/0    00:00:00 bash
  58274 pts/0    00:00:00 ps
[1]+  Killed                  sleep 100

A brief overview of the command kill -s SIGKILL 58271:

kill is the command to send a signal
-s is the flag to send a signal name
- -n can be used instead, and pass the signal number
SIGKILL is the signal being sent
58271 is the signal number that is recieving the SIGKILL signal

Running in the Foreground or Background

Processes run either the foreground or background.

The Foreground

In the foreground, processes run and the output appears in the shell.
In the foreground, the shell is consumed by the running process && no other processes can be run whil the foreground is consumed.
The foreground is where processes run by default: if you see it, its probably in the foreground. That's why running ps shows 2 processes: the shell and the ps that was just run:

ubuntu@primary:~$ ps
    PID TTY          TIME CMD
  43759 pts/0    00:00:00 bash
  58518 pts/0    00:00:00 ps

There are a lot of other processes running, but those are in the background.

The Background

In the bg, processes can be run that "free up" the shell. The terminal/cmd prompt can be used while processes run in the bg.
Working with background processes feels to me a little mysteryious. To "see" them running is not as clear, obviously, as what is in the terminal, making process-management... a whole new world.

# start a short-lived process in the bg
ubuntu@primary:~$ sleep 2 &
[1] 58543

# wait a few seconds till after that is done
# just press enter
ubuntu@primary:~$ 
[1]+  Done                    sleep 2

It looks like linux will "tell" the shell that the bg process is done on any command run after the bg process finished. Interesting Detail

Working With Background Jobs

Some processes might be "long-running": things like installing dependencies, updating dependencies, etc. Using the background can be helpful when working with a long-running job and using the background to run other processes at the same time.

jobs is a command that shows background processes
CTRL + Z can be used to stop a process from completing
bg can be used to "move" a process from the running foreground to the background
fg can be used to "move" a process from the background to the running foreground
- NOTE: bg && fg get a number added after the command - here the number will be 1 && is incremental by the number of jobs in.... the job line...?

# make a foreground "sleep"
ubuntu@primary:~$ sleep 20

# can't do anything with the current terminal, waiting 20....

# quickly press CTRL+Z to stop the running process
^Z
[1]+  Stopped                 sleep 20

# inspect the process, see that it is stopped
ubuntu@primary:~$ jobs
[1]+  Stopped                 sleep 20
# NOTE: use the -l flag to get the pid, why not!
ubuntu@primary:~$ jobs -l
[1]+ 71862 Running                 sleep 20 &

# "move" the stopped job to the gb
ubuntu@primary:~$ bg 1
[1]+ sleep 20 &

# see the status of the bg process, "job"
ubuntu@primary:~$ jobs
[1]+  Running                 sleep 20 &

# bring the running job back to the foreground
ubuntu@primary:~$ fg 1
sleep 20

# cant do anything till the 20s passes

Run Processes Synchronously

If Previous Process Succeeds

&& can be used to run a program when a previous program succeeds...

ubuntu@primary:~$ touch this-file.txt && ls >> this-file.txt && ps >> this-file.txt

That command uses the && operator a few times so that if any program fails the following program(s) will not run.

If Previous Process Fails

|| can be used to catch when the previous program fails.

# typo, forcing a failure that the || will catch
ubuntu@primary:~$ touc bad-command.txt || echo "dang"

Command 'touc' not found, did you mean:

  command 'touch' from deb coreutils (8.30-3ubuntu2)

Try: sudo apt install <deb name>

dang

# notice the "dang" above!

Regardless of Previous Command output

; can be used to run commands regardless of previous command's output status.

ubuntu@primary:~$ touc bad-command.txt; echo "echo string"; echo "runs no matter what"

Command 'touc' not found, did you mean:

  command 'touch' from deb coreutils (8.30-3ubuntu2)

Try: sudo apt install <deb name>

echo string
runs no matter what

Subcommands

These.
Run commands in commands.
Inception.
Use this: $(sub-command here):

ubuntu@primary:~$ echo Hi, my name is $(whoami)
Hi, my name is ubuntu

Tags:

linux