RH033 Unit11 Investigating and Managing Processes
Objectives
Upon completion of this unit, you should be able to:
- Explain what a process is
- Describe how to manage process
- Use job control tools
What is a Process?
1) A process is a set of instructions loaded into memory
- Numeric Process ID (PID) used for identification
- UID, GID and SELinux content determines filesystem access. Normally inherited from the executing user
Listing Processes
1) View Process information with ps
- Shows the processes from the current terminal by default
- -a includes processes on all terminals
- -x includes processed 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
Finding Processes
1) Most flexible: ps options | other commands
- ps axo comm, tty | grep ttyS0
2) By predefined patterns: pgrep
- pgrep �CU root
- pgrep �CG student
3) By exact program name: pidof
- pidof bash
Signals
1) 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) �C Terminate clearly
- Signal 9, KILL �C Terminate immediately
- Signal 1, HUP �C Re-read configuration files
- man 7 signal shows complete list
Sending Signals to Processes
1) By PID: kill [signal] pid …
2) By Name: killall [signal] comm …
3) By Pattern: pkill [-signal] pattern
Scheduling Priority
1) Scheduling priority determines access to the CPU
2) Priority is affected by a process’s nice value
3) Value range from �C20 to 19 but default to 0. Lower nice value means high CPU priority
4) Viewed with ps �Co comm, nice
Altering Scheduling Priority
1) Nice values may be altered…
- when starting a process: $ nice �Cn 5 command
- After starting: $ renice 5 PID
2) Only root may decrease nice values
Interactive Process Management Tools
1) CLI: top
2) GUI: gnome-system-monitor
3) Capabilities:
- Display real time processed information
- Allow sorting, killing and re-nicing
Job Control
1) Run a process in the background: Append an ampersand to the command line: firefox &
2) Temporarily halt a running program: Use Ctrl+z or send signal 17 (STOP)
3) 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]
Scheduling a Process To Execute Later
1) One-time job use at, recurring jobs use crontab
| Create | at time | crontab -e |
| List | at �Cl | crontab -l |
| Details | at �Cc jobnum | N/A |
| Remove | at �Cd jobnum | crontab �Cr |
| Edit | N/A | crontab �Ce |
2) Non-redirected output is mailed to the user
3) root can modify job for other users
Crontabe File Format
1) Entry consists of five space-delimited fields followed by a command line. One entry one line, no limit to line length
2) Fields are minute, hour, day of month, month, and day of week
3) Comment lines begin with #
4) See man 5 crontab for details
Grouping Commands
1) Two way to group commands:
- Compound: date; who | wc �Cl (commands run back-to-back)
2) Subshell: (date; who | wc �Cl) >> /tmp/trace
- All output is sent to a single STDOUT and STDERR
Exit Status
1) 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
Conditional Execution Operators
1) Commands can be run conditionally based on exit status
- && reprents conditional AND THEN (the second runs only when the first succeeds)
- || reprents conditional OR ELSE (the second runs only when the first fails)
The test Command
1) Evaluates boolean statements for use in conditional execution
- Return 0 for true
- Reture 1 for false
2) Examples in long form:
$ test “$A” = "$B” && echo “Strings are equal”
$ test “$A” -eq "$B” && echo “Integers are equal”
3) Examples in shorthand notation
$ [ “$A” = "$B” ] && echo “Strings are equal”
$ [ “$A” -eq "$B”] && echo “Integers are equal”
File Tests
1) 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 regular directory
- -x tests to see if a file exists and is a regular executable
Scripting: if Statements
1) Execute instructions based on the exit status of a command
if ...; then …
elif …; then …
else …
fi
End of Unit11
1) Questions and Answers
2) Summary:
- A process is any set of instructions in memory
- Processes are managed with: ps, kill, top, gnome-system-monitor
- Suspend jobs with Ctrl+z, manage with fg, bg