101 3.5 create, monitor and kill processes v2

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Linux Essenciais and System Administration

Key Knowledge Areas
Run jobs in the foreground and background.
Signal a program to continue running after logout.
Monitor active processes.
Select and sort processes for display.
Send signals to processes.
Unix Commands
Create, monitor and kill processes
Terms and Utilities
& bg
fg jobs
kill nohup
ps top
free uptime
killall
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Process wraps up everything needed to know about a running piece of software.
Meta information includes the machine code for the software, and things like what
user/group pair is running the process, when it was started, what command line was, etc.
Pertinent parts of a process:
PID; PPID; UID/GID; Command; Start Time; CPU Time; CWD; State; TTY; Environment; Priority; Nice; Level
Processes structure
3
PID
Process ID
•Linux uses this number to uniquely identify every process on the computer
•Number from 1-32768 ( default - can change the maximum )
•Assigns new PIDs incrementally by 1, 2 or 4
•Loops back to 1 after hitting the maximum
PPID
Parent Process ID
•PID of the process that started this one

UID/GID
The User and Group running the process
•Very important! Defines access and permissions to file system and operating system.
•Inherited from Parent process unless: SetUID/SetGID bits on executable
•Completes the Circle of Security
Processes structure
4
command
The command (and arguments) for the process
•Identifies the executable running, as well as the arguments passed at invocation
start & cpu time
Start Time tracks when the process was started
•CPU Time tracks time the process actually spends running on the CPU
CWD
Current Working Directory
•Inherited from parent process

state
State of the process:
•Runnable; Stopped; Blocked – Interruptible; Blocked - Non-interruptible; Zombie
Processes structure
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TTY
•Connected terminal
•Mostly informational
environment
•Every process has it’s own Environment
priority
•Priority is a read-only value showing current priority assigned by the scheduler
•Ranges from 0-99, with higher values representing higher priorities.
•The scheduler constantly adjusts priorities to balance efficiency, performance and responsiveness

nice
nice level represents the influence on the calculations the kernel uses when assigning priorities.
•Originally designed and named to allow users to be “nice” to other users of the system by assigning a
higher nice value to an intensive process, which in turn lowers it’s priority.
•Ranges from -20 to 19. Default nice level is 0.
•Only root can assign negative nice values.
See slides: Modify process execution priorities v2.ppt
Processes structure
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Job control - Ability to selectively suspend execution of processes and continue their execution later.
A job is a process or a pipeline of processes that were started by the shell.
Job control
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job which receives keyboard input is called the foreground job.
•When a foreground process is running, it receives keyboard input and signals.
•Processes started are run in foreground by default and continue until they exit.
To run a process in background – input command followed by special character &
•Processes running in the background may still send output to the terminal.
•They do not receive keyboard input unless they are brought to the foreground.
When bash starts a background job, it prints a line with:
- job number and - process ID of last process in pipeline

command jobs displays current list of jobs either running or stopped.
jobs
8
foo:~ $ dd if=/dev/zero of=/dev/null bs=1 &
[1] 1181
foo:~ $ cat /dev/urandom | grep hello &
[2] 1183
foo:~ $ jobs
[1]- Running dd if=/dev/zero of=/dev/null bs=1 &
[2]+ Running cat /dev/urandom | grep hello &
Ex:

Control jobs
9
key sequences entered to foreground processes:
Key Signal Meaning Usage
Ctrl+C SIGINT Interrupt Interrupt the program running in the foreground
Ctrl+Z SIGSTOP Suspend Suspend the program running in the foreground
cmds entered to control background processes.
Command Meaning Usage
fg foreground Run the background job in the foreground. If it has suspended, restart it.
bg background Restart a suspended job.
fg makes most recently executed job a foreground job.
You can specify a specific job number. (Ex. fg 2 will make job 2 run in the foreground)
bg makes most recently executed job continue to run in background.
You can make a specific job run in the background by specifying a job number (Ex. bg 2)

Control jobs
10
kill command
kill job based on job number (instead of PID) using percentage sign to specify the job number
foo:~ $ jobs
[1]- Running dd if=/dev/zero of=/dev/null bs=1 &
foo:~ $ kill %1
foo:~ $
[1]- Terminated dd if=/dev/zero of=/dev/null bs=1
foo:~ $ jobs
foo:~ $ kill %2
foo:~ $
[2]+ Terminated cat /dev/urandom | grep hello
Ex:

Monitoring processes
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ps – process status
options supported by ps are somewhat complex.
GNU version supports: Unix98 options (letters); BSD options (dash); GNU options (two dashes).
To ... Unix98 BSD
- Show all processes ps -ax ps -A ps -e
- Show full info ps -u (user format) ps -f (full listing)
- Show full info for all processes ps -aux ps -ef ps -Af
foo:~ $ ps -f
UID PID PPID C STIME TTY TIME CMD
georgem 987 612 0 20:32 pts/2 00:00:00 /bin/bash
georgem 3398 987 0 21:11 pts/2 00:00:00 ps -f
foo:~ $ ps u
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
georgem 987 0.0 1.7 4676 2040 pts/2 S 20:32 0:00 /bin/bash
georgem 3399 0.0 0.5 2524 696 pts/2 R 21:11 0:00 ps u
Ex:
ps -w – display in wide format
ps -f – display in forest of processes, similar to output of pstree - Generate hierarchical view of
processes.

Processes states
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5 basic process states:
1. Runnable Process is running, or is set to run.
•Linux is a multi-tasking it’s hard to see exactly when processes are running (switched so quickly),
•state is runnable, indicating that the scheduler will provide CPU time when it’s available.
2. Stopped Process will not get CPU time
•Nothing happens to the process - it’s still in memory, poised, ready to go.
•But when it’s put in the stopped state, scheduler will not put it on the CPU.
•Files/network connections remain open, but network connections may drop after a time (timeout).
3. Blocked/Sleeping – interrutible Process is waiting for some event
•Event can be an alarm from a sleep system call, or a signal or other external event.
•Interruptible means that other processes/events can break the sleep.
4. Blocked/Sleeping - non-interrutible Sleep state generally caused by IO operations
accessing a drive, communicating with network, etc.
•Non-interruptible means that other processes/events can not break this sleep.
•This process is unable to respond to signals.
5. Zombie/Defunct Exited process whose parent did not wait() on child
•Does not consume resources beyond a PID and meta information storage ( < 1k generally )
•Generally caused by2 situations:
- Bug in software
- Overly taxed machine

Monitoring processes
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top – displays processes that use up the most CPU or memory.
Ex:

Disconnected processes
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commands nohup and setsid
- used to run processes disconnected from terminal that started them.
nohup sets the signal mask for the process it starts to ignore the SIGHUP signal.
SIGHUP signal is sent to each process when the shell exits. - happens when you exit a session on console, or via network connection.
•nohup writes the output to a file - nohup.out
•nohup is generally used - When you know that the process you are going to run should continue to
run after your session ends.

Signals
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Processes in Linux do not communicate with each other directly, but send each other signals
via kernel.
most common signal sent is SIGTERM,
means Terminate, unless you know what to do
signal(7) man page.
Ex:
http://linux.about.com/od/commands/l/blcmdl7_signal.htm
First form of Interprocess Communication ( IPC )
A signal is a message sent to a process to indicate
events or other conditions.
The signal itself is the message – there around
three dozen defined signals...

Fim de sessão
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101 3.5 create, monitor and kill processes v2

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101 3.5 create, monitor and kill processes v2