Investigating and Managing Processes

31 May 2015 - Linux Linux Basics PS CRON CRONTAB

Overview

1. Process
2. Signals
- Sending Signals to Process
3. Scheduling Priority
- Altering Scheduling Priority
- Process Management Tools
  - CLI - top, htop
  - GUI - gnome-system-monitor
4. Job Control
5. Scheduling Process
6. Grouping Commands
- Compound
- Subshell
7. Exit Status
8. Conditional Execution Operators
9. test command
- File Tests
10. Scripting If Statements

1. Process

A process is a set of instructions loaded into memory.
Numeric Process ID (PID) used for identification.
UID, GID and SELinux context determines filesystem access.
The Linux Kernel tracks every aspect of a process by its PID under /proc/PID.

Listing Process

The ps command is used to view the process information.
By Default, shows processes from the current terminal

Options

a: Shows processes from all the terminals.
x: Shows all the processes owned by you, or shows all the processes when used together with the a option (such as: ps ax) Including processes that are not controlled by a terminal Such as Daemon processes, This shows up as ? in the tty column of the output
u: Shows process owner information.
f: Shows process parentage
o: Shows custom information Such as pid, tty, stat, nice, %cpu, %mem, time, comm, command, euser, ruser

Examples:

[mitesh@Matrix ~]$ ps
[mitesh@Matrix ~]$ ps a
[mitesh@Matrix ~]$ ps x
[mitesh@Matrix ~]$ ps u
[mitesh@Matrix ~]$ ps f
[mitesh@Matrix ~]$ ps xo pid,tty,stat,%cpu,%mem,time,command,euser,ruser
[mitesh@Matrix ~]$ ps axo pid,tty,stat,%cpu,%mem,time,command,euser,ruser

Process Status

Every process has a state property, which describes whether the process is actively using the cpu (Running), in memory but not doing anything (Sleeping), waiting for a resource to become available (Uninterruptable Sleep) or terminated but not flushed from the process list (Zombie).
Running and Sleeping are normal, but the presence of Uninterruptable Sleep or Zombie processes may indicate problems lurking on your system.

Uninterruptable Sleep

Process is sleeping and can not be woken up until an event occurs.
It can not be woken up by a signal.
Typically, the result of I/O operations, such as a failed network connections (For NFS Hard Mounts).

Zombie

Just before a process dies, it sends a signal to its parent and waits for an acknowledgment before terminating.
Even if the parent process does not immediately acknowledge this signal, all resources except for the process identity number (PID) are released.
Zombie processes are cleared from the system during the next system reboot And do not adversely affect system performance.

Finding Process

# Most Flexible
[mitesh@Matrix ~]$ ps axo pid,tty,comm | grep 'cups'
1516 ?        cupsd
3066 pts/1    eggcups

# By predefined patterns: pgrep
[mitesh@Matrix ~]$ pgrep -U root
[mitesh@Matrix ~]$ pgrep -G mitesh

[mitesh@Matrix ~]$ pgrep cups
1516
3066

# By exact program name:	pidof
[mitesh@Matrix ~]$ pidof cupsd
1516

2. Signals

Signals are simple messages that can be communicated to processes with commands like kill.
Sent directly to processes, no user interface required.
Programs associate actions with each signal.
Signals are specified by name or number when sent man 7 signals shows complete list
Signal 1 HUP (SIGHUP) Re-read Configuration Files
Signal 9 KILL (SIGKILL) Terminate Immediately
Signal 15 TERM (SIGTERM) Terminate Cleanly
Signal 18 CONT (SIGCONT) Continue If Stopped
Signal 19 STOP (SIGSTOP) Stop Process

Sending Signals to Process

By PID: kill [signal] pid ...
By Pattern: pkill [signal] pattern
By Name: killall [signal] command ...
kill can send many signals, but processes only respond to those signals whose they have been programmed to recognize.
For Example: Most services are programmed to reload their configuration when they receive a HUP(1) signals.
Some processes are terminated when they completed their tasks.
Interactive applications may need the user to issue a quit command.
In other cases, processes may need to be terminated with Ctrl+c, which sends an INT(2) signal to the process.
The process is shutdown cleanly means Terminate child process first & Complete any pending I/O operations.

NOTE!: The KILL(9) signal should be used only if a process will not respond to a Ctrl+c or a TERM(15) signals. Using KILL(9) signal on a routine basis may cause zombie processes and lost data.

# The following are all identical and will send default TERM(15) signal to the process with PID number 3705
[mitesh@Matrix ~]$ kill 3705
[mitesh@Matrix ~]$ kill -15 3705
[mitesh@Matrix ~]$ kill -TERM 3705
[mitesh@Matrix ~]$ kill -SIGTERM 3705

3. Scheduling Priority

Every running process has a scheduling priority: A ranking among running processes determining which should get the attention of the processor.
Priority is affected by a process’ nice value.
The nice value range from -20 to 19 ( Default is 0 )
- -20: Highest CPU
- 19: Lowest CPU

Altering Scheduling Priority

Niceness value may be altered…
When starting a process

[mitesh@Matrix ~]$ nice -n 5 command

After Starting the process

[mitesh@Matrix ~]$ renice 5 -p PID

NOTE!: Only root may decrease nice value. Non-privileged users start a process at any positive nice value but cannot lower it once raised.

[mitesh@Matrix ~]$ nice -n 10 myprog
[mitesh@Matrix ~]$ renice 15 -p PID
[root@Matrix ~]# renice -19 -p PID

Process Management Tools

CLI - top, htop

Display list of processes running on your system, updated every 3 seconds.
You can use keystrokes to kill, renice and change the sorting order of processes.
Use ? key to view the complete list of hotkeys.
You can exit top by pressing the q key.

GUI - gnome-system-monitor

The gnome-system-monitor, which can be run from the console Or by selecting Applications -> System Tools -> System Monitor
Display real time process information
Allows killing, re-nicing, sorting

4. Job Control

Background Process

Append the ampersand to the command line: firefox &

Suspended Running Program

Use `Ctrl+z
Send STOP(19) signal

Manage Background Or Suspended Jobs

List Job Numbers and Names: jobs
Resume in the Background: bg [%jobnum]
Resume in the Foreground: fg [%jobnum]
Send a Signal: kill [SIGNAL] [%jobnum]

Examples:

[mitesh@Matrix ~]$ ping 127.0.0.1 &> /dev/null
^Z
[1]+  Stopped                 ping 127.0.0.1 &>/dev/null

[mitesh@Matrix ~]$ bg
[1]+ ping 127.0.0.1 &>/dev/null &

[mitesh@Matrix ~]$ firefox &
[2] 4162

NOTE!: The number next to [2] after backgrounding firefox is the PID

[mitesh@Matrix ~]$ jobs
[1]-  Running                 ping 127.0.0.1 &>/dev/null &
[2]+  Running                 firefox &

NOTE!: The + or -`` signs next to the job numbers tells which job is the default
+ sign is the default job

5. Scheduling Process

One time jobs use at, Recurring jobs use crontab

/-----------------------------------------------------------------------\
|									|
|	Create		|	at time		|	crontab -e	|
|	List		|	at -l		|	crontab -l	|
|	Details		|	at -c jobnum	|			|
|	Remove		|	at -d jobnum	|	crontab -r	|
|	Edit		|			|	crontab -e	|
|									|
\-----------------------------------------------------------------------/

Non-redirected output is mailed to the user
The root can modify jobs for other users

at command

Scheduling One Time Job with at command
One Command Per Line
Terminated With Ctrl+d

Options

/-----------------------------------------------------------------------\
|	at 8:00am December 7			at 7 am Thursday	|
|	at midnight + 23 minutes		at now + 5 minutes	|
\-----------------------------------------------------------------------/

/-------------------------------------------------------------------------------------------------------\
|	Command		Alias		        Meanning					        |
|--------------------------------------------------------------------------------------------------------
|													|
|	atq	|	at -l		|	Lists the jobs currently pending.			|
|	atrm	|	at -d jobnum	|	Deletes the job.					|
|		|	at -c jobnum	|	Cats the full environment for the specified job.	|
|													|
\-------------------------------------------------------------------------------------------------------/

Example:

[mitesh@Matrix ~]$ at 0200
at> date
at> cal
at> <EOT>
job 1 at 2011-08-26 02:00


[mitesh@Matrix ~]$ atq
[mitesh@Matrix ~]$ at -l
1	2011-08-26 02:00 a mitesh

crontab command

Scheduling Recurring Jobs with crontab command
The cron mechanism is controlled by a process named crond.
This process runs every minute and determines if an entry in user’s cron tables need to be executed.
The crontabs are stored in /var/spool/cron/
The root can modify the jobs for other users with crontab -u username and any of the other options, such as -e.

Crontab File Format

Comment lines begin with #.
One entry per line, no limit to line length.
Entry consist of five space-delimited fields followed by a command name.
Fields are Minute, Hour, Day Of Month, Month, Day Of week.
An asterisk (*) in a field represent all valid values.
Multiple values are separated by commas.
See man 5 crontab for more details

/-----------------------------------------------------------------------\
|									|
|	Minute		|	0-59					|
|	Hour		|	0-23					|
|	Day Of Month	|	1-31					|
|	Month		|	1-12 (Or Jan, Feb, Mar, Etc)		|
|	Day Of Week	|	0-7  (Or Sun, Mon, Tue, Etc)		|
|				(0 or 7 = Sunday, 1 = Monday)		|
|									|
\-----------------------------------------------------------------------/

 # * * * * *  command to execute
 # │ │ │ │ │
 # │ │ │ │ │
 # │ │ │ │ └───── day of week (0 - 6) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
 # │ │ │ └────────── month (1 - 12)
 # │ │ └─────────────── day of month (1 - 31)
 # │ └──────────────────── hour (0 - 23)
 # └───────────────────────── min (0 - 59)

Example:

[mitesh@Matrix ~]$ crontab -e
#Min	Hour	DOM	Month	DOW	Command
0	0	31	10	*	mail -s "boo" $LOGNAME < boo.txt
0	2	*	*	*	netstat -tulpn | diff - /media/cdrom/baseline
0	4	*	*	1,3,5	find ~ -name core | xargs rm -f {}

crontab in details

The cron mechanism is controlled by a process named crond.
This process runs every minute and determines if an entry in user’s cron tables need to be executed.
The crontabs are stored in /var/spool/cron/`
The root can modify the jobs for other users with crontab -u username and any of the other options, such as -e.

Crontab File Format

Comment lines begin with #.
One entry per line, no limit to line length.
Entry consist of five space-delimited fields followed by a command name.
Fields are Minute, Hour, Day Of Month, Month, Day Of week.
An asterisk (*) in a field represent all valid values.
Multiple values are separated by commas.
Special Time Specification Nicknames: @reboot, @yearly, @annually, @monthly, @weekly, @daily, @hourly
See man 5 crontab for more details

Examples:

[mitesh@Matrix ~]$ crontab -e
#Min    Hour    DOM     Month   DOW     Command
0       0       31      10      *       mail -s "boo" $LOGNAME < boo.txt
0       2       *       *       *       netstat -tulpn | diff - /media/cdrom/baseline
0       4       *       *       1,3,5   find ~ -name core | xargs rm -f {}

*/2	*	*	*	*	echo "Every 2 Minutes" &> /dev/tty1
*/5	*	*	*	*	echo "Every 5 Minutes" &> /dev/tty1
@reboot					echo "Runs Once After Reboot" &> /dev/tty1

[mitesh@Matrix ~]$ echo '*/15 8-17 * * 1-5 echo Breaktime' | crontab

The Cron Access Control

/-----------------------------------------------------------------------------------------------------------------------------------------------\
|	---------------		--------------		Only root can install the crontab files.						|
|																		|
|	/etc/cron.allow		--------------		The root & All The Listed users in cron.allow can install the crontab files.		|
|																		|
|	---------------		/etc/cron.deny		All The users except The users in cron.deny can install the crontab files.		|
|																		|
|	/etc/cron.allow		/etc/cron.deny		The cron.deny file is ignored.								|
|							The root & All The Listed users in cron.allow can install the crontab files.		|
\-----------------------------------------------------------------------------------------------------------------------------------------------/

NOTE!: Denying A User Through The Use Of Above Files Does Not Disable Their Installed crontab.

System Crontab Files

Different Format Than User Crontab Files
Default System Crontab File Is /etc/crontab
The /etc/cron.d/ Directory Contains The Additional System Crontab Files

Example:

[mitesh@Matrix ~]$ cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

#run-parts
01	*	*	*	*	root	run-parts	/etc/cron.hourly
02	4	*	*	*	root	run-parts	/etc/cron.daily
02	4	*	*	0	root	run-parts	/etc/cron.weekly
42	4	1	*	*	root	run-parts	/etc/cron.monthly

NOTE!: The System Crontab Files Are Different From The Users Crontab Files
In The System Crontab Files Sixth Field Is A Username, Which will Be Used To Execute The Commands.

The run-parts Is A Shell Script (/usr/bin/run-parts).
The run-parts Shell Scripts Take One Argument - A Directory Name And Invokes All Of The Program In That Directory.

Thus, At 4:02 Every Morning, All Of The Executables In The /etc/cron.daily/ Directory Will Be Run As The root User.

Default Daily Cron Jobs

The /etc/cron.daily Are Usually Used For:
- Clean Up Temporary Directories
- Update mlocate & whatis Database
- Perform Other Housekeeping Tasks
A) The tmpwatch:
- Deletes All Files In /tmp Directory Which Is Not Accessed For 240 Hours (10 Days)
- Deletes All Files In /var/tmp Directory Which Is Not Accessed For 720 Hours (30 Days)
B) The logrotate:
- Keeps Log Files From Getting Too Large
- Rotates Log Files On
- Predefined Intervals (Weekly)
- When Reach The Predefined Size
- Old Files Are Optionally Compressed
- Configuration Files:
  - /etc/logrotate.conf (Global Configuration)
  - /etc/logrotate.d/ (Override Global Configuration)

Example: The /var/log/messages Is Rotated Weekly To /var/log/messages-yyyymmdd

The Anacron System

The Anacron Runs The Missed Cron Jobs When The System Boots.
The Anacron Command Is Used To Run The Missed Daily,Weekly & Monthly Cron Jobs.

Example:

According To The /etc/crontab File
At 4:02 Every Morning, All Of The Executables In The /etc/cron.daily/ Directory Will Be Run As root User.
Now Suppose Your Laptop Is Almost Always Off At The 4:02 AM, Then The mlocate & whatis Database Is Never Be Updated.

Configuration File

/etc/anacrontab
Field1: If The Cron Jobs Not Been Run For The Specified No Of Days
Field2: Wait For The Specified No Of Minutes Before Runs
Field3: Job Identifier
Field4: The Cron Job To Run

Examples:

[mitesh@Matrix ~]$ cat /etc/anacrontab
# /etc/anacrontab: configuration file for anacron

# See anacron(8) and anacrontab(5) for details.

SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

# Period In Days	Delay In Minutes	Job-Identifier	Command
1		5			cron.daily	nice run-parts /etc/cron.daily
7		25			cron.weekly	nice run-parts /etc/cron.weekly
@monthly 	45			cron.monthly	nice run-parts /etc/cron.monthly

How Anacron Works

According To The /etc/crontab File
The 1st Command To Run Is 0anacron.
The 0anacron Command Sets The Last Run Timestamp In A /var/spool/anacron/cron.{daily,weekly,monthly} Files.
On The System Boot Up, The Anacron Commands Runs.
The /etc/anacrontab File Specify How Often The Commands In cron.daily/ cron.weekly/ and cron.monthly/ Should Be Runs.
If These Commands Are Not Runs In This Time Then
The Anacron Command Waits For The Specified No Of Minutes In The /etc/anacrontab File & Then Runs The Commands

6. Grouping Commands

Two ways to group commands

Compound

Example: date; who | wc -l
Commands run back to back

Subshell

Commands inside parentheses are run in their own instance of bash, called subshell.
Example: (date; who | wc -l)
All output is sent to a single STDOUT and STDERR
Suppose you want to maintain a count of the number of users logged on, Along with a time/date stamp, in a log file.

Examples:

[mitesh@Matrix ~]$ date >> logfile
[mitesh@Matrix ~]$ who | wc -l >> logfile

[mitesh@Matrix ~]$ date; who | wc -l
Tue Aug 30 14:04:31 IST 2011
3

[mitesh@Matrix ~]$ date; who | wc -l >> logfile
Tue Aug 30 14:05:08 IST 2011

[mitesh@Matrix ~]$ (date; who | wc -l) >> logfile

7. Exit Status

Processes report success or failure with an exit status.
0 for success
1-255 for failure
$? stores the exit status of the most recent command
exit [num] terminates and set status to num

Examples:

[mitesh@Matrix ~]$ ping -c1 -w1 localhost &> /dev/null
[mitesh@Matrix ~]$ echo $?
0

[mitesh@Matrix ~]$ ping -c1 -w1 station999 &> /dev/null
[mitesh@Matrix ~]$ echo $?
2

8. Conditional Execution Operators

* Commands can be run conditionally based on exit status.
&& Represents conditional AND THEN
|| Represents conditional OR ELSE

NOTE!: When executing two commands separated by &&,
The 2nd command runs if the 1st command exit successfully (Exit status 0).
When executing two commands separated by ||,
The 2nd command runs if the 1st command fails (Exit status in the range of 1 to 255).

Examples:

[mitesh@Matrix ~]$ grep -q 'no_such_user' /etc/passwd || echo "No such user"
No such user

[mitesh@Matrix ~]$ ping -c1 -w2 localhost &> /dev/null \
> && echo "Localhost is up" \
> || (echo "Localhost is unreachable"; exit 1)
Localhost is up
[mitesh@Matrix ~]$ echo $?
0


[mitesh@Matrix ~]$ ping -c1 -w2 station999 &> /dev/null \
> && echo "Station999 is up" \
> || (echo "station999 is unreachable"; exit 1)
station999 is unreachable
[mitesh@Matrix ~]$ echo $?
1

#!/bin/bash
for x in $(seq 1 10)
do
  echo adding test$x
  (
    echo -ne "test$x\t"
    useradd test$x 2>&1 > /dev/null && mkpasswd test$x
  ) >> /tmp/userlog
done
echo 'cat /tmp/userlog to see new passwords'

9. test command

The test command evaluates true or false scenarios to simplify conditional execution.
Returns 0 for true
Returns 1 for false

NOTE!: Strings should be compared using a Mathematical Operator, While Numbers are compared using an Abbreviation (-eq).

Examples:

# Long Form
$ test "$A"  =  "$B" && echo "Strings are equal"
$ test "$A" -eq "$B" && echo "Integers are equal"

# Shorthand
$ [ "$A"  =  "$B" ] && echo "Strings are equal"
$ [ "$A" -eq "$B" ] && echo "Integers are equal"

File Tests

Use the following command for complete list man test

/-----------------------------------------------------------------------------------------------\
|												|
|	-d FILE		|	FILE exists and is a directory					|
|	-e FILE		|	FILE exists							|
|	-f FILE		|	FILE exists and is a regular file				|
|	-h FILE		|	FILE exists and is a symbolic link (same as -L)			|
|	-L FILE		|	FILE exists and is a symbolic link (same as -h)			|
|	-r FILE		|	FILE exists and read permission is granted			|
|	-s FILE		|	FILE exists and has a size greater than zero			|
|	-w FILE		|	FILE exists and write permission is granted			|
|	-x FILE		|	FILE exists and execute (or search) permission is granted	|
|	-O FILE		|	FILE exists and is owned by the effective user ID		|
|	-G FILE		|	FILE exists and is owned by the effective group ID		|
|												|
\-----------------------------------------------------------------------------------------------/

Example:

$ [ -f ~/lib/functions ] && source ~/lib/functions

10. Scripting If Statements

Every process reports an exit status.
0 for success
1-255 for failure
Execute instructions based on a exit status of the command.

#!/bin/bash
if ping -c1 -w2 station1 &> /dev/null
then
  echo "Station1 is up"
elif grep "station1" ~/maintenance.txt &> /dev/null
then
  echo "Station1 is undergoing maintenance"
else
  echo "Station1 is unexpectedly DOWN!"
  exit 1
fi

The exit status can be checked within the body of the if as shown in the example, Or you can assign the exit status to a variable using a subshell, as in:
- STATUS=$(test -x /bin/ping6)
The if structure can be combined with conditional operator

#!/bin/bash
if test -x /bin/ping6; then
  ping6 -c1 ::1 &> /dev/null && echo "IPv6 stack is up"
elif test -x /bin/ping; then
  ping -c1 127.0.0.1 &> /dev/null && echo "No IPv6, but IPv4 stack is up"
else
  echo "Oops! This should not happen."
  exit 255
fi

NOTE!: This script checks for the IPv6 version of the ping command (ping6) exists.
If it does, it uses ping6 to send a test packet to the system’s IPv6 Loopback Interface.
Else The script checks for the IPv4 version of the ping command (ping) exists.
If it does, it uses ping to send a test packet to the system’s IPv4 Loopback Interface.
If neither IPv6 nor IPv4 exists,
Something is probably wrong and a non-zero return code is issued along with a warning message.

FOR GOOD EXAMPLE OF REAL WORLD SCRIPTS, LOOK AT THE SCRIPTS IN /etc/init.d/*

Get updated when I create new content.
Unsubscribe whenever. Never any spam.

Note

I'm Tama. I turn down every ad, affiliate, and sponsor request I get. I write free resources that help people. If you enjoy my content, please consider supporting what I do.

Support my work

1. Process

Listing Process

Process Status

Finding Process

2. Signals

Sending Signals to Process

3. Scheduling Priority

Altering Scheduling Priority

Process Management Tools

CLI - top, htop

GUI - gnome-system-monitor

4. Job Control

Background Process

Suspended Running Program

Manage Background Or Suspended Jobs

5. Scheduling Process

at command

crontab command

crontab in details

Crontab File Format

The Cron Access Control

System Crontab Files

Default Daily Cron Jobs

A) The tmpwatch:

B) The logrotate:

The Anacron System

How Anacron Works

6. Grouping Commands

Compound

Subshell

7. Exit Status

8. Conditional Execution Operators

9. test command

File Tests

10. Scripting If Statements

Newsletter

Note