20 Linux System Monitoring Tools Every SysAdmin Should Know

Need to monitor Linux server performance? Try these built-in command
and a few add-on tools. Most Linux distributions are equipped with tons
of monitoring. These tools provide metrics which can be used to get
information about system activities. You can use these tools to find
the possible causes of a performance problem. The commands discussed
below are some of the most basic commands when it comes to system
analysis and debugging server issues such as:

1. Finding out bottlenecks.
2. Disk (storage) bottlenecks.
3. CPU and memory bottlenecks.
4. Network bottlenecks.

#1: top - Process Activity Command

The top program provides a dynamic real-time view of a running
system i.e. actual process activity. By default, it displays the most
CPU-intensive tasks running on the server and updates the list every
five seconds.

Fig.01: Linux top command

Commonly Used Hot Keys

The top command provides several useful hot keys:

Hot Key	Usage
t	Displays summary information off and on.
m	Displays memory information off and on.
A	Sorts the display by top consumers of various system resources. Useful for quick identification of performance-hungry tasks on a system.
f	Enters an interactive configuration screen for top. Helpful for setting up top for a specific task.
o	Enables you to interactively select the ordering within top.
r	Issues renice command.
k	Issues kill command.
z	Turn on or off color/mono

#2: vmstat - System Activity, Hardware and System Information

The command vmstat reports information about processes, memory, paging, block IO, traps, and cpu activity.
# vmstat 3

Sample Outputs:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------

 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0      0 2540988 522188 5130400    0    0     2    32    4    2  4  1 96  0  0


 1  0      0 2540988 522188 5130400    0    0     0   720 1199  665  1  0 99  0  0
 0  0      0 2540956 522188 5130400    0    0     0     0 1151 1569  4  1 95  0  0
 0  0      0 2540956 522188 5130500    0    0     0     6 1117  439  1  0 99  0  0



 0  0      0 2540940 522188 5130512    0    0     0   536 1189  932  1  0 98  0  0
 0  0      0 2538444 522188 5130588    0    0     0     0 1187 1417  4  1 96  0  0
 0  0      0 2490060 522188 5130640    0    0     0    18 1253 1123  5  1 94  0  0

Display Memory Utilization Slabinfo

# vmstat -m

Get Information About Active / Inactive Memory Pages

# vmstat -a

#3: w - Find Out Who Is Logged on And What They Are Doing

w command displays information about the users currently on the machine, and their processes.
# w username

# w vivek

Sample Outputs:

 17:58:47 up 5 days, 20:28,  2 users,  load average: 0.36, 0.26, 0.24

USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0    10.1.3.145       14:55    5.00s  0.04s  0.02s vim /etc/resolv.conf


root     pts/1    10.1.3.145       17:43    0.00s  0.03s  0.00s w

#4: uptime - Tell How Long The System Has Been Running

The uptime command can be used to see how long the server has been
running. The current time, how long the system has been running, how
many users are currently logged on, and the system load averages for
the past 1, 5, and 15 minutes.
# uptime

Output:

 18:02:41 up 41 days, 23:42,  1 user,  load average: 0.00, 0.00, 0.00

1 can be considered as optimal load value. The load can change from
system to system. For a single CPU system 1 - 3 and SMP systems 6-10
load value might be acceptable.

#5: ps - Displays The Processes

ps command will report a snapshot of the current processes. To select all processes use the -A or -e option:
# ps -A

Sample Outputs:

  PID TTY          TIME CMD
    1 ?        00:00:02 init

    2 ?        00:00:02 migration/0
    3 ?        00:00:01 ksoftirqd/0
    4 ?        00:00:00 watchdog/0
    5 ?        00:00:00 migration/1


    6 ?        00:00:15 ksoftirqd/1
....
.....
 4881 ?        00:53:28 java
 4885 tty1     00:00:00 mingetty
 4886 tty2     00:00:00 mingetty
 4887 tty3     00:00:00 mingetty
 4888 tty4     00:00:00 mingetty



 4891 tty5     00:00:00 mingetty
 4892 tty6     00:00:00 mingetty
 4893 ttyS1    00:00:00 agetty
12853 ?        00:00:00 cifsoplockd
12854 ?        00:00:00 cifsdnotifyd
14231 ?        00:10:34 lighttpd



14232 ?        00:00:00 php-cgi
54981 pts/0    00:00:00 vim
55465 ?        00:00:00 php-cgi
55546 ?        00:00:00 bind9-snmp-stat
55704 pts/1    00:00:00 ps

ps is just like top but provides more information.

Show Long Format Output

# ps -Al

To turn on extra full mode (it will show command line arguments passed to process):
# ps -AlF

To See Threads ( LWP and NLWP)

# ps -AlFH

To See Threads After Processes

# ps -AlLm

Print All Process On The Server

# ps ax

# ps axu

Print A Process Tree

# ps -ejH

# ps axjf

# pstree

Print Security Information

# ps -eo euser,ruser,suser,fuser,f,comm,label

# ps axZ

# ps -eM

See Every Process Running As User Vivek

# ps -U vivek -u vivek u

Set Output In 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

Display Only The Process IDs of Lighttpd

# ps -C lighttpd -o pid=

OR
# pgrep lighttpd

OR
# pgrep -u vivek php-cgi

Display The Name of PID 55977

# ps -p 55977 -o comm=

Find Out The Top 10 Memory Consuming Process

# ps -auxf | sort -nr -k 4 | head -10

Find Out top 10 CPU Consuming Process

# ps -auxf | sort -nr -k 3 | head -10

#6: free - Memory Usage

The command free displays the total amount of free and used physical
and swap memory in the system, as well as the buffers used by the
kernel.
# free 

Sample Output:

            total       used       free     shared    buffers     cached

Mem:      12302896    9739664    2563232          0     523124    5154740
-/+ buffers/cache:    4061800    8241096
Swap:      1052248          0    1052248

#7: iostat - Average CPU Load, Disk Activity

The command iostat report Central Processing Unit (CPU) statistics
and input/output statistics for devices, partitions and network
filesystems (NFS).
# iostat 

Sample Outputs:

Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)  06/26/2009


avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           3.50    0.09    0.51    0.03    0.00   95.86



Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              22.04        31.88       512.03   16193351  260102868
sda1              0.00         0.00         0.00       2166        180



sda2             22.04        31.87       512.03   16189010  260102688
sda3              0.00         0.00         0.00       1615          0

#8: sar - Collect and Report System Activity

The sar command is used to collect, report, and save system activity information. To see network counter, enter:
# sar -n DEV | more

To display the network counters from the 24th:
# sar -n DEV -f /var/log/sa/sa24 | more

You can also display real time usage using sar:
# sar 4 5

Sample Outputs:

Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)   06/26/2009


06:45:12 PM       CPU     %user     %nice   %system   %iowait    %steal     %idle
06:45:16 PM       all      2.00      0.00      0.22      0.00      0.00     97.78


06:45:20 PM       all      2.07      0.00      0.38      0.03      0.00     97.52
06:45:24 PM       all      0.94      0.00      0.28      0.00      0.00     98.78
06:45:28 PM       all      1.56      0.00      0.22      0.00      0.00     98.22



06:45:32 PM       all      3.53      0.00      0.25      0.03      0.00     96.19
Average:          all      2.02      0.00      0.27      0.01      0.00     97.70

#9: mpstat - Multiprocessor Usage

The mpstat command displays activities for each available processor,
processor 0 being the first one. mpstat -P ALL to display average CPU
utilization per processor:
# mpstat -P ALL

Sample Output:

Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)   06/26/2009


06:48:11 PM  CPU   %user   %nice    %sys %iowait    %irq   %soft  %steal   %idle    intr/s
06:48:11 PM  all    3.50    0.09    0.34    0.03    0.01    0.17    0.00   95.86   1218.04


06:48:11 PM    0    3.44    0.08    0.31    0.02    0.00    0.12    0.00   96.04   1000.31
06:48:11 PM    1    3.10    0.08    0.32    0.09    0.02    0.11    0.00   96.28     34.93
06:48:11 PM    2    4.16    0.11    0.36    0.02    0.00    0.11    0.00   95.25      0.00



06:48:11 PM    3    3.77    0.11    0.38    0.03    0.01    0.24    0.00   95.46     44.80
06:48:11 PM    4    2.96    0.07    0.29    0.04    0.02    0.10    0.00   96.52     25.91
06:48:11 PM    5    3.26    0.08    0.28    0.03    0.01    0.10    0.00   96.23     14.98



06:48:11 PM    6    4.00    0.10    0.34    0.01    0.00    0.13    0.00   95.42      3.75
06:48:11 PM    7    3.30    0.11    0.39    0.03    0.01    0.46    0.00   95.69     76.89

#10: pmap - Process Memory Usage

The command pmap report memory map of a process. Use this command to find out causes of memory bottlenecks.
# pmap -d PID

To display process memory information for pid # 47394, enter:
# pmap -d 47394

Sample Outputs:

47394:   /usr/bin/php-cgi
Address           Kbytes Mode  Offset           Device    Mapping

0000000000400000    2584 r-x-- 0000000000000000 008:00002 php-cgi
0000000000886000     140 rw--- 0000000000286000 008:00002 php-cgi


00000000008a9000      52 rw--- 00000000008a9000 000:00000   [ anon ]
0000000000aa8000      76 rw--- 00000000002a8000 008:00002 php-cgi
000000000f678000    1980 rw--- 000000000f678000 000:00000   [ anon ]
000000314a600000     112 r-x-- 0000000000000000 008:00002 ld-2.5.so



000000314a81b000       4 r---- 000000000001b000 008:00002 ld-2.5.so

000000314a81c000       4 rw--- 000000000001c000 008:00002 ld-2.5.so

000000314aa00000    1328 r-x-- 0000000000000000 008:00002 libc-2.5.so



000000314ab4c000    2048 ----- 000000000014c000 008:00002 libc-2.5.so

.....
......
..
00002af8d48fd000       4 rw--- 0000000000006000 008:00002 xsl.so
00002af8d490c000      40 r-x-- 0000000000000000 008:00002 libnss_files-2.5.so



00002af8d4916000    2044 ----- 000000000000a000 008:00002 libnss_files-2.5.so

00002af8d4b15000       4 r---- 0000000000009000 008:00002 libnss_files-2.5.so



00002af8d4b16000       4 rw--- 000000000000a000 008:00002 libnss_files-2.5.so

00002af8d4b17000  768000 rw-s- 0000000000000000 000:00009 zero (deleted)
00007fffc95fe000      84 rw--- 00007ffffffea000 000:00000   [ stack ]


ffffffffff600000    8192 ----- 0000000000000000 000:00000   [ anon ]
mapped: 933712K    writeable/private: 4304K    shared: 768000K

The last line is very important:

• mapped: 933712K total amount of memory mapped to files
• writeable/private: 4304K the amount of private address space
• shared: 768000K the amount of address space this process is sharing with others

#11 and #12: netstat and ss - Network Statistics

The command netstat displays network connections, routing tables,
interface statistics, masquerade connections, and multicast
memberships. ss command is used to dump socket statistics. It allows
showing information similar to netstat. See the following resources
about ss and netstat commands:

#13: iptraf - Real-time Network Statistics

The iptraf command is interactive colorful IP LAN monitor. It is an
ncurses-based IP LAN monitor that generates various network statistics
including TCP info, UDP counts, ICMP and OSPF information, Ethernet
load info, node stats, IP checksum errors, and others. It can provide
the following info in easy to read format:

• Network traffic statistics by TCP connection
• IP traffic statistics by network interface
• Network traffic statistics by protocol
• Network traffic statistics by TCP/UDP port and by packet size
• Network traffic statistics by Layer2 address

Fig.02: General interface statistics: IP traffic statistics by network interface

Fig.03 Network traffic statistics by TCP connection

#14: tcpdump - Detailed Network Traffic Analysis

The tcpdump is simple command that dump traffic on a network.
However, you need good understanding of TCP/IP protocol to utilize this
tool. For.e.g to display traffic info about DNS, enter:
# tcpdump -i eth1 'udp port 53'

To display all IPv4 HTTP packets to and from port 80, i.e. print only
packets that contain data, not, for example, SYN and FIN packets and
ACK-only packets, enter:
# tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

To display all FTP session to 202.54.1.5, enter:
# tcpdump -i eth1 'dst 202.54.1.5 and (port 21 or 20'

To display all HTTP session to 192.168.1.5:
# tcpdump -ni eth0 'dst 192.168.1.5 and tcp and port http'

Use wireshark to view detailed information about files, enter:
# tcpdump -n -i eth1 -s 0 -w output.txt src or dst port 80

#15: strace - System Calls

Trace system calls and signals. This is useful for debugging webserver and other server problems. See how to use to trace the process and see What it is doing.

#16: /Proc file system - Various Kernel Statistics

/proc file system provides detailed information about various hardware devices and other Linux kernel information. See Linux kernel /proc documentations for further details. Common /proc examples:

# cat /proc/cpuinfo

# cat /proc/meminfo

# cat /proc/zoneinfo

# cat /proc/mounts

17#: Nagios - Server And Network Monitoring

Nagios is a
popular open source computer system and network monitoring application
software. You can easily monitor all your hosts, network equipment and
services. It can send alert when things go wrong and again when they
get better. FAN is
"Fully Automated Nagios". FAN goals are to provide a Nagios
installation including most tools provided by the Nagios Community. FAN
provides a CDRom image in the standard ISO format, making it easy to
easilly install a Nagios server. Added to this, a wide bunch of tools
are including to the distribution, in order to improve the user
experience around Nagios.

18#: Cacti - Web-based Monitoring Tool

Cacti is a complete network graphing solution designed to harness
the power of RRDTool's data storage and graphing functionality. Cacti
provides a fast poller, advanced graph templating, multiple data
acquisition methods, and user management features out of the box. All
of this is wrapped in an intuitive, easy to use interface that makes
sense for LAN-sized installations up to complex networks with hundreds
of devices. It can provide data about network, CPU, memory, logged in
users, Apache, DNS servers and much more. See how to install and configure Cacti network graphing tool under CentOS / RHEL.

#19: KDE System Guard - Real-time Systems Reporting and Graphing

KSysguard is a network enabled task and system monitor application
for KDE desktop. This tool can be run over ssh session. It provides
lots of features such as a client/server architecture that enables
monitoring of local and remote hosts. The graphical front end uses
so-called sensors to retrieve the information it displays. A sensor can
return simple values or more complex information like tables. For each
type of information, one or more displays are provided. Displays are
organized in worksheets that can be saved and loaded independently from
each other. So, KSysguard is not only a simple task manager but also a
very powerful tool to control large server farms.

Fig.05 KDE System Guard {Image credit: Wikipedia}

See the KSysguard handbook for detailed usage.

#20: Gnome System Monitor - Real-time Systems Reporting and Graphing

The System Monitor application enables you to display basic system
information and monitor system processes, usage of system resources,
and file systems. You can also use System Monitor to modify the
behavior of your system. Although not as powerful as the KDE System
Guard, it provides the basic information which may be useful for new
users:

• Displays various basic information about the computer's hardware and software.
• Linux Kernel version
• GNOME version
• Hardware
• Installed memory
• Processors and speeds
• System Status
• Currently available disk space
• Processes
• Memory and swap space
• Network usage
• File Systems
• Lists all mounted filesystems along with basic information about each.

Fig.06 The Gnome System Monitor application

Bounce: Additional Tools

A few more tools:

• nmap - scan your server for open ports.
• lsof - list open files, network connections and much more.
• ntop
  web based tool - ntop is the best tool to see network usage in a way
  similar to what top command does for processes i.e. it is network
  traffic monitoring software. You can see network status, protocol wise
  distribution of traffic for UDP, TCP, DNS, HTTP and other protocols.
• Conky -
  Another good monitoring tool for the X Window System. It is highly
  configurable and is able to monitor many system variables including the
  status of the CPU, memory, swap space, disk storage, temperatures,
  processes, network interfaces, battery power, system messages, e-mail
  inboxes etc.
• GKrellM
  - It can be used to monitor the status of CPUs, main memory, hard
  disks, network interfaces, local and remote mailboxes, and many other
  things.
• vnstat
  - vnStat is a console-based network traffic monitor. It keeps a log of
  hourly, daily and monthly network traffic for the selected interface(s).
• htop - htop is an enhanced version of top, the interactive process viewer, which can display the list of processes in a tree form.
• mtr - mtr combines the functionality of the traceroute and ping programs in a single network diagnostic tool.