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Linux Process Management (ps, top, renice, kill)

This article introduces some of the commands and utilities used to manage processes on Linux, with specific reference to the information needed for the RHCSA EX200 and RHCE EX300 certification exams.

Remember, the exams are hands-on, so it doesn't matter which method you use to achieve the result, so long as the end product is correct.

ps

The ps command produces a report of the current processes on the system. The man page is very comprehensive, including the options to perform some useful common tasks.

To see every process on the system using standard syntax:
  ps -e
  ps -ef
  ps -eF
  ps -ely

To see every process on the system using BSD syntax:
  ps ax
  ps axu

To print a process tree:
  ps -ejH
  ps axjf

To get info about threads:
  ps -eLf
  ps axms

To get security info:
  ps -eo euser,ruser,suser,fuser,f,comm,label
  ps axZ
  ps -eM

To see every process running as root (real & effective ID) in user
format:
  ps -U root -u root u

To see every process with a user-defined format:
  ps -eo pid,tid,class,rtprio,ni,pri,psr,pcpu,stat,wchan:14,comm
  ps axo stat,euid,ruid,tty,tpgid,sess,pgrp,ppid,pid,pcpu,comm
  ps -eopid,tt,user,fname,tmout,f,wchan

Print only the process IDs of syslogd:
  ps -C syslogd -o pid=

Print only the name of PID 42:
  ps -p 42 -o comm=

If there are a lot of processes on the system, you will probably want to page through or limit them using one of the filtering options, or the grep command.

# Shows a page at a time.
ps -ef | more

# Returns only those lines containing the string "ora".
ps -ef | grep ora

# Returns only those lines containing the string "ora",
# with a case-insensitive search.
ps -ef | grep -i ora

# Returns only those lines containing the string "ora", 
# omitting the grep line using "grep -v".
ps -ef | grep ora | grep -v grep

# Returns only those lines containing the string "ora",
# omitting the grep line using a regular expression.
ps -ef | grep [o]ra 

top

The top command is probably the most well know utility for displaying the most resource intensive processes on the system. For the most part you can get away with just running top and looking at the output.

# top
top - 17:14:40 up  6:49,  1 user,  load average: 0.00, 0.00, 0.00
Tasks: 104 total,   1 running, 103 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.1%us,  0.1%sy,  0.0%ni, 99.6%id,  0.2%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   2050836k total,   459132k used,  1591704k free,    44956k buffers
Swap:  4128760k total,        0k used,  4128760k free,   163880k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
 3414 root      20   0 15084 1088  824 R  2.0  0.1   0:00.01 top                
    1 root      20   0 19396 1500 1192 S  0.0  0.1   0:00.71 init               
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthreadd           
    3 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0                                      
...

If you want to add or remove columns from the display, press the "f" key and toggle the letters in the "Current Fields" string. Capital letters mean the column is displayed. When you are satisfied press the return key and you will see the displayed columns will have changed.

Current Fields:  NAEHIOQTWKMbcdfgjplrsuvyzX  for window 1:Def
Toggle fields via field letter, type any other key to return 

* N: %MEM       = Memory usage (RES)
* A: PID        = Process Id
* E: USER       = User Name
* H: PR         = Priority
* I: NI         = Nice value
* O: VIRT       = Virtual Image (kb)
* Q: RES        = Resident size (kb)
* T: SHR        = Shared Mem size (kb)
* W: S          = Process Status
* K: %CPU       = CPU usage
* M: TIME+      = CPU Time, hundredths
  b: PPID       = Parent Process Pid
  c: RUSER      = Real user name
...

To alter the order of the columns displayed, press the "o" key and use the upper case and lower case letters corresponding to each column to move then left and right of the "Current Fields" string.

The sorting of the data can be altered using the following keys:

The "r" and "k" keys are used to renice and kill sessions. These commands will be discussed below.

People often get a little confused about the memory information summary in the top part of the screen because the free memory listed seems very low to them. Linux uses free physical memory for the file system cache to improve I/O performance. If more memory is needed for processes, it is released from the file system cache. Linux only starts to use swap when all physical memory has been used. So the actual free memory on the system is free+buffers+cached. If your concern is free memory, it is easier to use the free command to monitor it. The "Mem:" line gives the values presented by top, while the "-/+ buffers/cache:" line gives the memory used by processes, ignoring the file system cache.

# free
             total       used       free     shared    buffers     cached
Mem:       2050836     459248    1591588          0      44988     163880
-/+ buffers/cache:     250380    1800456
Swap:      4128760          0    4128760
#

renice

The renice command is used to alter the scheduling priority of one or more processes. The "-h" flag displays the cut down usage notes.

# renice -h

Usage:
 renice [-n] priority [-p|--pid] pid  [... pid]
 renice [-n] priority  -g|--pgrp pgrp [... pgrp]
 renice [-n] priority  -u|--user user [... user]
 renice -h | --help
 renice -v | --version

#

The priority can be changed to an absolute value or a value relative to the current setting. The following example changes the priority of the process with the PID of 8 to the value 10, then adds 1 to the priority.

# renice 10 8
8: old priority 0, new priority 10
# 

# renice +1 8
8: old priority 10, new priority 11
# 

With no flags set, it is assumed you are using "renice -n priority -p pid", so you are only targeting processes by the pid. The following are equivalent.

# renice -n +1 -p 8 9 
# renice +1 8 9

You can also target multiple processes based on their group or user.

# renice -n +1 -g 500
# renice -n +1 -u oracle

You can also combine flags to target more processes.

Before you start altering the priorities you need to understand the inter-dependencies between processes. It is possible that by slowing down one process you will adversely affect the performance of other processes that depend on it.

The renice command can also be performed interactively in the top command, or from the "System Monitor" GUI.

kill

Not surprisingly, the kill command is used to kill processes. Assuming we have identified a process we want to kill and it has a process id of "1234", we may use one of the two common forms are show below.

# kill 1234
# kill -9 1234

The first example sends the "TERM" signal to the specified process, which is the preferred option. The second is more aggressive, sending the "KILL" signal. You should only use the "KILL" signal if your attempts to kill the process with the "TERM" signal have failed.

Always make sure to double check the process you are killing. Killing the wrong process can cause the system to crash.

The kill command can also be performed interactively in the top command, or from the "System Monitor" GUI.

You can kill multiple processes using a single command by combining kill with the ps and awk commands.

# kill -9 `ps -ef | grep ora | awk '{print $2}'`

System Monitor

The "Processes" tab of the "System Monitor" dialog (Applications > System Tools > System Monitor) displays information similar to the top command.

System Monitor

Clicking on the column titles causes the display to be sorted by the column. Repeated clicking toggles between ascending and descending order for the column.

Right-clicking on a process displays a popup menu, allowing you to kill or change the priority (renice) the process.

System Monitor Popup Menu

For more information see:

Hope this helps. Regards Tim...

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