Linux Guide to Linux Certification, Second Edition

1
Linux Guide to Linux Certification, Second
Edition

• Chapter 10
• Managing Linux Processes

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Objectives

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

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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

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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

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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) Parent processs PID

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Linux Processes (continued)
Figure 10-1 Parent and child processes
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Linux Processes (continued)
Figure 10-2 Process genealogy
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Viewing Processes

• ps command View processes
• Most 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

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Viewing Processes (continued)

• Process state Current processor state of process
• Most processes sleeping (S) or running (R)
• Zombie process Process finished, but parent has
  not released PID
• Defunct process
• Process state is Z

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Viewing Processes (continued)

• Process priority (PRI) Determines how many
  processor time slices process will receive
• Higher value means lower priority
• Nice value (NI) Indirectly represents priority
• Higher value means lower priority

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Viewing Processes (continued)
Table 10-1 Common options to the ps command
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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

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Killing Processes

• kill signal Sent to a process by kill command
• 64 types of kill signals
• Affect processes in different ways
• kill command Kills all instances of a process by
  command name
• To kill a process, give kill signal and PID
• If no kill signal given, SIGTERM assumed

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Killing Processes (continued)
Table 10-2 Common administrative kill signals
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Killing Processes (continued)

• Trapping Ignore a kill signal
• Kill signals sent to processes having children
• Parent process terminates all child processes
  before terminating itself
• Often necessary to kill parent process in order
  to kill zombie processes

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Killing Processes (continued)

• killall command Kills all instances of a process
  by command name
• Uses process name instead of PID
• Can use top command to kill processes

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Process Execution

• Three main types of executable commands
• Binary programs
• Shell scripts
• Shell functions

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Process Execution (continued)

• Forking Act of creating new BASH shell child
  process from a parent
• 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

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Process Execution (continued)
Figure 10-3 Process forking
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Running Processes in the Background

• Foreground processes BASH shell must wait for
  termination
• Background processes BASH shell does not wait
  for termination
• metacharacter
• Upon execution, user receives BASH shell prompt
  immediately

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Running Processes in the Background (continued)

• jobs command Lists background processes running
  in current shell
• foreground (fg) command Run a background process
  in foreground
• Foreground process may be paused with ctrl Z
• background (bg) command Run a foreground process
  in background

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Running Processes in the Background (continued)

• Jobs command marks two most recent background
  processes
• Commands operate on most recent process, by
  default

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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

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Process Priorities (continued)

• PRI dictates number of time slices a process gets
• Cannot change PRI value directly
• Set NI to indirectly affect priority
• Processes start with NI of 0
• nice command Change a processs priority as it
  starts

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Process Priorities (continued)
Figure 10-4 The nice value scale
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Process Priorities (continued)

• renice command Alter NI of a process after it is
  running
• Only root user may change NI to a negative value
• May also change NI of running processes using top
  utility

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Scheduling Commands

• at daemon (atd) System daemon that executes
  tasks at a future time
• cron daemon (crond) System daemon that executes
  tasks repetitively in the future

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Scheduling Commands with atd

• at command Schedule commands and tasks to run at
  a preset time
• l option View a list of scheduled jobs
• Regular users see only their own jobs
• c option View system environment at scheduling
  time
• d option Delete a job
• f option Run scheduled jobs from shell script

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Scheduling Commands with atd (continued)

• atq command Alternative method to view scheduled
  jobs
• 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 file, mailed to user

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Scheduling Commands with atd (continued)
Table 10-3 Common at commands
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Scheduling Commands with atd (continued)

• /etc/at.allow File listing all users able to use
  at command
• /etc/at.deny File listing all the users not able
  to use at command
• On Fedora Linux systems, no /etc/at.allow file
  exists
• Empty by default
• All users allowed to use at daemon

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Scheduling Commands with crond

• Cron tables Files specifying tasks for cron
  daemon to run
• Specify when commands should be executed
• User and system cron tables
• Six fields separated by space or tab characters

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Scheduling Commands with crond (continued)
Figure 10-5 User cron table format
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Scheduling Commands with crond (continued)
Figure 10-6 Sample user cron table entry
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Scheduling Commands with crond (continued)

• /var/spool/cron Stores user cron tables
• /etc/crontab Contains default system cron table
• /etc/cron.d Contains additional system cron
  tables

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User Cron Tables

• /etc/cron.allow Lists users able to use cron
  command
• /etc/cron.deny Lists users not able to access
  cron command

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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 all scheduled jobs

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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/crontable)

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The System Cron Table (continued)

• Initial section of cron table specifies execution
  environment
• Remainder similar to user cron table entries
• Sixth field specifies who to run command as
• Remaining fields represent command to run
• run-parts command Execute all files in a
  directory

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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

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Summary (continued)

• Process information is stored in the /proc
  filesystem the ps 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
• You can send kill signals to a process using the
  kill, killall, and top commands

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Summary (continued)

• The BASH shell 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 NI
• Commands can be scheduled to run at a later time
  using the at and cron daemons