This document discusses processes in Linux and tools for managing them. It begins by explaining that a process is a set of instructions loaded into memory that has a numeric PID for identification. It then covers commands like ps, pgrep, and pidof for listing and finding processes. The document also discusses signals for inter-process communication, scheduling priority, job control, and tools like top, at and crontab for interactive process management and scheduling jobs.

1. RedHat Enterprise Linux Essential
Unit 10: Investigating and Managing
Processes

2. Investigating and Managing Processes
Upon completion of this unit, you should be able to:
 Explain what a process is
 Describe how to manage processes
 Use job control tools

3. What is a 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
• Normally inherited from the executing user

4. Listing Processes
 View Process information with ps
 Shows processes from the current terminal by default
 a includes processes on all terminals
 x includes processes not attached to terminals
 u prints process owner information
 f prints process parentage
 o PROPERTY,... prints custom information:
• pid, comm, %cpu, %mem, state, tty, euser, ruser
 Process states – running, sleeping, uninterruptable sleep, zombie.

5. Finding Processes
 Most flexible: ps options | other commands
ps axo comm,tty | grep ttyS0
ps -ef
 By predefined patterns: pgrep
$ pgrep -U root
$ pgrep -G student
● By exact program name: pidof
$ pidof bash

6. Example with cpu
ps –Ao pcpu,pid,user,args | sort –k 1 –r |head -10

7. Signals
 Most fundamental inter-process communication
 Sent directly to processes, no user-interface required
 Programs associate actions with each signal
 Signals are specified by name or number when sent:
• Signal 15, TERM (default) - Terminate cleanly (stopping)
• Signal 9, KILL - Terminate immediately (kill)
• Signal 1, HUP - Re-read configuration files (reload)
• Kill –l list info for singal
• man 7 signal shows complete list

8. Sending Signals to Processes
 By PID: kill [signal] pid ...
 By Name: killall [signal] comm ...
 By pattern: pkill [-signal] pattern

9. Scheduling Priority
 Scheduling priority determines access to the CPU
 Priority is affected by a process‘ nice value
 Values range from -20 to 19 but default to 0
 Lower nice value means higher CPU priority
 Viewed with ps -Ao comm,nice
ex: firefox &
ps –Ao comm,nice |grep firefox

10. Altering Scheduling Priority
 Nice values may be altered...
 When starting a process:
$ nice -n 5 command
 After starting:
$ renice 5 PID
renice 5 –p PID
 Only root may decrease nice values

11. Interactive Process Management Tools
 CLI: top
 GUI: gnome-system-monitor
 Capabilities
 Display real-time process information
 Allow sorting, killing and re-nicing

12. Job Control
 Run a process in the background
 Append an ampersand to the command line: firefox &
 Temporarily halt a running program
 Use Ctrl-z or send signal 17 (STOP)
 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]

13. Scheduling a Process To Execute Later
 One-time jobs use at, recurring jobs use crontab
Create at time crontab -e
List at -l crontab -l
Details at -c jobnum N/A
Remove at -d jobnum crontab -r
Edit N/A crontab -e
 Non-redirected output is mailed to the user
 root can modify jobs for other users

14. Using at
 Setting an at job: at [options] time
 Options:
-b run command only when the system load is low
-d job# delete an at job from queue
-f filename read job from a specified file
-l list jobs for that user (all jobs for root)
-m mail user quen job completes
-q queuename send the jobs to a queue (a to z and A to Z)
 Time formats:
now, 17:00, +3 hours, +2 minutes, +2 days, +3 months, 19:15 3.12.10,
midnight, 4PM, 16:00 +3 days, mon, tomorrow …
 Show the at jobs queue of user: atq or at –l
 Deletes at jobs from the jobs queue: atrm job# [job#] …

15. Examples with at
 Create an simple at job to run in 5 minutes later
$ at now+5 minutes
echo “I am running in an at job” > /tmp/test_cron
[Ctrl-D]
 Create an at job which read commands from a file and run at
midnight
$ at –f /tmp/myjob.sh midnight
 Using echo to run multiple commands with at
$ echo „cd /tmp; ls –a > /tmp/test_cron ‟ | at now+2 minutes
 Listing all at jobs
$ at –l
$ atq

16. crontab commands
 Create/edit a user crontab: crontab –e
 Create a user crontab by reading from file:
crontab –e filename
 Display user‟s crontab file: crontab –l
 Delete user‟s crontab file: crontab –r
 Edit a user‟s crontab file (for root only):
crontab –e –u username

17. Crontab File Format
 Entry consists of five space-delimited fields followed by a
command line
01 * * * * root cd /tmp && ls -laht
 One entry per line, no limit to line length
 Fields are minute, hour, day of month, month, and day of week
 Comment lines begin with #
 See man 5 crontab for details

18. Examples of user crontabs
 Run a command every hour from 8:00 to 18:00 everyday
0 8-18 * * * <command>
 Run a command every 4 hours on the half hour (i.e 6:30, 10:30, 14:30, 16:30)
everyday
30 6-16/4 * * * <command>
 Run a command every day, Monday to Friday at 01:00, and doesn‟t report to
syslog
-0 1 * * 1-5 <command>
 Run the command every Monday and Tuesday at 12:00, 12:10, 12:20, 12:30
0,10,20,30 12 * * 1,2 <command>
 Run a command every 10 minutes
*/10 * * * * <command>
echo “00 21 * * 7 root rm -f /tmp/*.log” >> /etc/crontab
crontab -e
00 8-17 * * 1-5 du -sh $HOME >> /tmp/diskreport

19. Grouping Commands
 Two ways to group commands:
 Compound: date; who | wc -l
• Commands run back-to-back
 Subshell: (date; who | wc -l) >> /tmp/trace
• All output is sent to a single STDOUT and STDERR

20. 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 sets status to num
 Example:
$ ping -c1 -W1 localhost999 &> /dev/null
$ echo $?
2

21. Conditional Execution Operators
 Commands can be run conditionally based on exit status
 && represents conditional AND THEN
 || represents conditional OR ELSE
 Examples:
$ grep -q no_such_user /etc/passwd || echo 'No such user'
No such user
$ ping localhost &> /dev/null
> && echo “localhost is up"
> || echo ‘localhost is unreachable’
localhost is up

22. The test Command
 Evaluates boolean statements for use in conditional execution
 Returns 0 for true
 Returns 1 for false
 Examples in long form:
test "$A" = "$B" && echo "string" || echo "not equal"
$ test "$A" -eq "$B" && echo "Integers are equal“
 Examples in shorthand notation:
$ [ "$A" = "$B" ] && echo "Strings are equal"
$ [ "$A" -eq "$B" ] && echo "Integers are equal"

23. File Tests
 File tests:
 -f tests to see if a file exists and is a regular file
 -d tests to see if a file exists and is a directory
 -x tests to see if a file exists and is executable
 [ -f ~/lib/functions ] && source ~/lib/functions

24. File Tests (cont’)
Options Mean
-d file True if the file is a directory.
-e file True if the file exists.
-f file True if the file exists and is a regular file.
-h file True if the file is a symbolic link.
-L file True if the file is a symbolic link.
-r file True if the file exists and is readable by you.
-s file True if the file exists and is not empty.
-w file True if the file exists and is writable by you.
-x file True if the file exists and is executable by you.
-O file True if the file is effectively owned by you.
-G file True if the file is effectively owned by your group.

25. Scripting: if Statements
 Execute instructions based on the exit status of a command
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