Processes

Kevin Kuehler

Live Q&A: on Piazza

What’s a Process?

• Basically, an instance of a program being executed
• Hundreds of processes running on your system at any moment
• Every process has a unique PID

Basic Commands

• ps: print running processes
• Default: print processes in your current session
• -e: print all processes
• -f: print with full information
• -H: print a tree
• -u (--user): print a certain user’s processes
• kill: send a signal to a process (to end it)
• nice, ionice: process resource access
• pgrep: search for a process by its name
• htop: show live process stats

Process IO

• Every process on a Unix system initialized with three open file handles:
• Often referred to as streams
• stdin (fd 0): where a process looks for input
• stdout (fd 1): where a process can write output
• stderr (fd 2): where a process can write errors

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Review: Process IO Examples

• $ echo “stdout is the terminal”
• $ echo “stdout is the terminal” >&1
• $ echo “writing to stderr, also the terminal!” >&2
• $ ./program.sh 1>outfile 2>errfile
• stdin, new way to use the cat command?
• $ cat < somefile.txt

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Pipes Re-imagined: IPC

• Remember pipes from lecture 2?
• Now that we know about processes, we can think of pipes as a form of interprocess communication
• $ ps -ef | grep vim
• stdout of ps cmd becomes stdin for grep cmd

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The Tree of Life

• Every process has a parent process
• Initially, the parent of a process is the process which launched it
• Process 1 is at the very top of the tree

supernova Process Tree (feat. Shicheng)

Signals

• A means of sending messages to other processes
• SIGTERM (15): tell a process to exit now
• SIGKILL (9): kill a process without giving it the chance to clean up
• SIGINT (2): the signal sent when you press Ctrl+C
• SIGHUP (1): the signal sent when the user closes the terminal window
• SIGSTOP (varies; 19 on GNU/Linux): suspend a process
• SIGCONT (varies; 18 on GNU/Linux): resume a process
• The signals SIGKILL and SIGSTOP cannot be handled
• Can send using the kill, pkill, and killall commands�e.g. kill -STOP 3276, kill -9 7283

Orphans!

Orphan Processes

• Usually the result of a of the parent process crashing or being terminated by a signal like SIGKILL that doesn’t give the process time to clean up
• The orphan child process is “adopted” by the init process (PID 1)
• Called re-parenting

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Process Priority

• How much resources and time the CPU allocate for a process
• Determined by nice value on a scale from -20 to 19
• Set the niceness value for a new process:
• $ nice -n -20 ./sync_porn.sh & >/dev/null
• Set niceness value for existing process: renice -n -5 -p 30000
• Set priority for IO (disk usage)
• ionice, like nice, but for disk resources

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Background Processes (Primitive Daemons)

• Noninteractive processes
• Started on the shell by appending an ‘&’ to the command
• Give me an example of a long running command
• rsync
• $ rsync -ahz rsync://anime.girls.moe/waifus /home/keur/trash &
• We’ll try with something simpler...

yes, background me senpai

• Everyone open a terminal
• Run the command yes
• Review: How do we stop this command; what signal to we send it?
• Ctrl+C, which sends SIGINT
• Let’s background it: yes &
• Why is our terminal still getting spammed?!?! That doesn’t seem like a background process...
• Let’s investigate...

yes, background me senpai

• keur@supernova $ ps -fu

keur 30903 0.6 0.0 41468 5760 pts/4 Ss 01:58 0:00 -zsh

keur 30938 99.4 0.0 5840 704 pts/4 RN 01:59 1:28 \_ yes

keur 31013 0.0 0.0 29868 1552 pts/4 R+ 02:00 0:00 \_ ps -fu

• One quirk: stdout and stderr inherited from parent shell, only stdin detached
• keur@supernova $ yes > ~/yesfile.out &
• This is the behavior we’d expect of a background job, or is it…?

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Unix Job Control

• Ahh, time to leave my job running in the background and relax...
• keur@supernova $ logout

zsh: you have running jobs.

• WTF?!?!! (╯°□°)╯︵ ┻━┻
• keur@supernova $ jobs

[1] + running yes > ~/yesfile.out

• Run disown, removes this from shell’s job control
• Now we can logout and leave our job running

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Unix Job Control

• What would happen if we force logged out without disowning?
• Shell would send SIGHUP, killing its children
• This seems like a lot of work… is there a tool for this?
• Introducing nohup
• $ nohup sleep 1000 &
• Redirects stdout and stderr to file nohup.out
• Process handles SIGHUP (hence the name)

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More Job Control

• Let’s see the power of job control
• $ ping -c5 ocf.berkeley.edu
• Cntl+Z to suspend the process
• bg to have the process resume in the background
• fg to bring this to foreground
• You will get to play around with this more in lab!

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Some Theory: Process Creation on Unix

• Two important system calls
• fork(): create child process that is a copy of current task, differing only in its PID (some other minor differences that we won’t get into)
• exec(): load new executable in memory, replacing the current process image
• These two commands work in tandem to create new processes on Unix

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Some Theory: Process Creation on Unix

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Some Theory: Process Creation

keur 30916 0.0 0.0 4040 732 pts/17 S 17:06 0:00 \_ ./a.out

keur 30921 0.0 0.0 4288 720 pts/17 S 17:06 0:00 \_ [sh]

keur 30924 0.0 0.0 28196 1632 pts/17 R+ 17:06 0:00 \_ ps -fu

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What’s a Daemon?

• Noninteractive background process
• Etymology: daemon < Gk. δαίμων “spirit”
• Examples:
• crond: schedule commands to run at regular times
• sshd: run a command once at specific time
• Apache2: http server waiting for incoming http connections
• And more
• Daemon names usually end in d

Cron

• A daemon that lets you run tasks at scheduled times.
• Tasks are defined in crontabs.
• Example crontab entry:
• 26 4 */2 * * /opt/bin/find_aliens.sh

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• asterisk (*) = always

Minute (0 - 59)

Hour

(0 - 23)

Day of month

(1 - 31)

Month

(1 - 12)

Day of week

(0 - 6), Sunday = 0, 7

Editing Cron jobs

• Use the crontab command
• The command allows you to have per-user crontabs
• Use crontab -e to edit
• Use crontab -l to show
• The -u option lets you operate on another user’s crontab if you’re root
• System wide crontab in /etc/crontab

Cron is hard!

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That’s okay! Just visit https://crontab.guru

But crontab.guru is easy!

Init System

• A daemon which manages other daemons
• First process to be launched, has PID 1
• Started by the kernel using a hardcoded filename
• If initialization daemon fails, a kernel panic ensues :(
• Functions:
• Start daemons
• Stop daemons
• Coordinate system startup and shutdown
• Monitor status of daemons
• Restart daemons if they crash

Examples of Init Systems

• sysvinit (also called “init”)
• Developed for proprietary System V Unix systems by AT&T in the 1980s
• Largely replaced by systemd in 2015
• Upstart (discontinued)
• systemd
• Default init system on all major GNU/Linux distros except Gentoo
• Had controversial beginnings because people hate change… and Lennart Poettering
• Monolith that breaks the *nix philosophy of “small, sharp tools”
• OpenRC
• Popular alternative in anti-systemd circles
• runit
• s6
• GNU Shepherd

Process Monitoring

• Essential job of a sysadmin
• Many tools for this:
• ps, top and htop
• iotop: show processes using the most I/O
• iftop: show processes generating the most network traffic
• vmstat: show statistics about memory
• iostat: show statistics about I/O