Linux Guide to Linux Certification, Third Edition

1
Linux Guide to Linux Certification, Third Edition

• Chapter 9
• Managing Linux Processes

2
Objectives

• Categorize the different types of processes on a
  Linux system
• View processes using standard Linux utilities
• Explain the difference between common kill
  signals
• Describe how binary programs and shell scripts
  are executed

3
Objectives (continued)

• Create and manipulate background processes
• Use standard Linux utilities to modify the
  priority of a process
• Schedule commands to execute in the future using
  the at daemon
• Schedule commands to execute repetitively using
  the cron daemon

4
Linux Processes

• Program structured set of commands stored in an
  executable file
• Executed to create a process
• Process program running in memory and on CPU
• User process process begun by user on a terminal
• Daemon process system process
• Not associated with a terminal

5
Linux Processes (continued)

• Process ID (PID) unique identifier assigned to a
  process
• Child process process started by another process
  (parent process)
• Parent process process that has started other
  processes (child processes)
• Parent Process ID (PPID) PID of the parent
  process
• The init daemon has a PID of 1 and a PPID of 0

6
Linux Processes (continued)
Figure 9-1 Parent and child processes
7
Linux Processes (continued)
Figure 9-2 Process genealogy
8
Viewing Processes

• ps command view processes
• Most versatile and common process viewing utility
• No arguments lists all processes running in
  current shell
• PID, terminal, command that started process, CPU
  time
• f (full) option more complete information
• User identifier (UID), PPID, start time, CPU
  utilization
• -e option displays the entire list of processes
  across all terminals including daemons

9
Viewing Processes (continued)

• Process flag indicates particular features of
  the process
• Process state current processor state of process
• Most processes sleeping (S) or running (R)
• Zombie process process finished, but parent has
  not released child processs PID
• Defunct process
• Process state is Z

10
Viewing Processes (continued)

• Process priority (PRI) priority used by the
  kernel for the process
• Higher value means lower priority
• Nice value (NI) can be used to affect the
  process priority indirectly
• Higher value means a greater chance of low
  priority
• pstree command displays the lineage of a process
  by tracing its PPIDs until the init daemon

11
Viewing Processes (continued)
Table 9-1 Common options to the ps command
12
Viewing Processes (continued)

• top command displays interactive screen listing
  processes
• Organized by processor time
• Processes using most processor time listed first
• Rogue process faulty process
• Consumes excessive system resources
• top command can be used to change PRI or kill
  processes
• Rogue processes can be killed immediately when
  identified

13
Killing Processes

• kill command sends a kill signal to a process
  to terminate it
• 64 types of kill signals
• Affect processes in different ways
• -l option displays list of kill signal names and
  associated numbers
• To kill a process, give kill signal and PID
• If no kill signal given, the default kill signal,
  SIGTERM, is used

14
Killing Processes (continued)
Table 9-2 Common administrative kill signals
15
Killing Processes (continued)

• Trapping ignoring a kill signal
• The SIGKILL signal cannot be trapped by any
  process
• Use only as last resort
• When parent process receives a kill signal,
  parent process terminates all child processes
  before terminating itself
• To kill several related processes send signal to
  parent process
• Kill parent process in order to kill zombie
  processes

16
Killing Processes (continued)

• killall command kills multiple processes of the
  same name in one command
• Takes kill signal number as an option
• Uses process name instead of PID
• If no kill signal given, the default kill signal,
  SIGTERM, is used
• Can use top command to kill processes

17
Process Execution

• Three main types of executable commands
• Binary programs
• e.g., ls, find, grep
• Shell scripts
• Shell functions
• e.g., cd, exit

18
Process Execution (continued)

• Forking act of creating new BASH shell or
  subshell
• Carried out by fork function in BASH shell
• Subshell executes program or shell script using
  exec function
• Original shell waits for subshell to complete
• When done, subshell kills itself
• Control returns to original shell

19
Process Execution (continued)
Figure 9-3 Process forking
20
Running Processes in the Background

• Foreground processes BASH shell must wait for
  termination to display prompt and accept new
  commands
• Background processes BASH shell does not wait
  for termination
• Append metacharacter to command
• Upon execution, user receives BASH shell prompt
  immediately

21
Running Processes in the Background (continued)

• jobs command lists background job IDs for
  processes running in current shell
• To terminate background process
• Send kill signal to PID
• Send kill signal to background job ID
• Prefix job ID with character

22
Running Processes in the Background (continued)

• foreground (fg) command move a background
  process to the foreground
• Use Ctrlz to pause a foreground process
• Assigns the process a background job ID
• background (bg) command send an existing process
  to the background
• Provide background job ID as argument
• jobs command indicates the two most recent
  background processes
• By default, commands apply to most recent process

23
Process Priorities

• Time slice amount of time a process is given on
  a CPU
• More time slices mean more execution time on CPU
• Executes faster
• Usually measured in milliseconds

24
Process Priorities (continued)

• PRI dictates number of time slices a process gets
• Low PRI is likely to get more time slices than
  high PRI
• Cannot change PRI value directly
• Set NI to indirectly affect priority
• Negative NI value, more time slices positive NI
  value, less time slices
• Processes start with NI of 0
• nice command change a processs priority as it
  starts

25
Process Priorities (continued)
Figure 9-4 The nice value scale
26
Process Priorities (continued)

• renice command alter NI of a process after it
  has been started
• Only root user may change NI to a negative value
• -u option change the NI for all processes owned
  by the specified user or group
• May also change NI of running processes using top
  utility

27
Scheduling Commands

• To schedule commands to execute in the future
• at daemon (atd) system daemon that executes
  tasks at a future time
• cron daemon (crond) system daemon that executes
  tasks repetitively in the future

28
Scheduling Commands with atd

• at command schedule commands and tasks to run at
  a preset time
• Specify the time as an argument
• l option view a list of at job IDs (regular
  users see only their own jobs)
• atq command alias to at -l
• c option view content of a specified at job
• d option delete the specified at job
• f option list commands to be scheduled by at
  from a shell script

29
Scheduling Commands with atd (continued)
Table 9-3 Common at commands
30
Scheduling Commands with atd (continued)

• at daemon uses current shells environment for
  execution
• Shell environment and scheduled commands stored
  in /var/spool/at
• If stdout of scheduled command has not been
  redirected to a file, it is mailed to user

31
Scheduling Commands with atd (continued)

• /etc/at.allow file listing all users allowed to
  use the at daemon
• /etc/at.deny file listing all the users not
  allowed to use the at daemon
• If both files exist, only /etc/at.allow file is
  processed
• On Fedora Linux systems, only /etc/at.deny file
  exists by default
• Initially left blank, all users allowed to use at
  daemon

32
Scheduling Commands with crond

• Suitable for scheduling repetitive tasks
• Cron tables configuration files specifying when
  commands should be executed
• Six fields separated by space or tab characters
• First five specify times to run the command
• Sixth absolute pathname to command to be executed

33
Scheduling Commands with crond (continued)

• User cron tables represent tasks scheduled by
  individual users
• System cron tables contains system tasks
• /var/spool/cron stores user cron tables
• /etc/crontab file contains system cron tables
• /etc/cron.d directory contains system cron
  tables

34
Scheduling Commands with crond (continued)
Figure 9-5 User cron table format
35
Scheduling Commands with crond (continued)
Figure 9-6 Sample user cron table entry
36
User Cron Tables

• /etc/cron.allow lists users allowed to use the
  cron daemon
• /etc/cron.deny lists users not allowed to use
  the cron daemon
• If both files exist, only /etc/cron.allow file is
  processed
• On Fedora Linux systems, only /etc/cron.deny file
  exists by default
• Initially left blank, all users allowed to use
  cron daemon

37
User Cron Tables (continued)

• crontab command view and edit user cron tables
• e option edit cron tables in vi editor
• l option list a user cron table
• r option remove cron table and all scheduled
  jobs
• -u option used by root user to edit, list, or
  remove a specified users cron table

38
The System Cron Table

• System maintenance, backups, and CPU-intensive
  tasks often scheduled for non-business hours
• Often scheduled by cron daemon
• Entries in system cron table (/etc/crontab)

39
The System Cron Table (continued)

• Initial section of cron table specifies execution
  environment
• Remainder similar to user cron table entries
• Cron tables located in the /etc/cron.d directory
  are run by the system as a specified user
• Can schedule administrative task by placing a
  shell script in the appropriate one of the
  following directories /etc/cron.hourly/,
  /etc/cron.daily/, /etc/cron.weekly, and
  etc/cron.monthly/

40
Summary

• Processes are programs that are executing on the
  system
• User processes are run in the same terminal as
  the user who executed them, whereas daemon
  processes are system processes that do not run on
  a terminal
• Every process has a parent process associated
  with it and, optionally, several child processes

41
Summary (continued)

• Process information is stored in the /proc
  filesystem the ps, pstree and top commands can
  be used to view this information
• Zombie and rogue processes that exist for long
  periods of time use up system resources and
  should be killed to improve system performance
• You can send kill signals to a process using the
  kill, killall, and top commands

42
Summary (continued)

• The BASH shell creates, or forks, a subshell to
  execute most commands
• Processes can be run in the background by
  appending an to the command name
• The priority of a process can be affected
  indirectly by altering its nice value
• Commands can be scheduled to run at a later time
  using the at and cron daemons