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sources/10 basic examples of Linux ps command.md
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10 basic examples of Linux ps command
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================================================================================
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### Linux ps command ###
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The ps command on linux is one of the most basic commands for viewing the processes running on the system. It provides a snapshot of the current processes along with detailed information like user id, cpu usage, memory usage, command name etc. It does not display data in real time like top or htop commands. But even though being simpler in features and output it is still an essential process management/monitoring tool that every linux newbie should know about and learn well.
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In this post we are going to revise the basics of using the ps command to check the processes and filter and sort them in different ways to suit better.
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### Note on syntax ###
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The ps command comes with an unusual set of 2 syntax styles. That is BSD and UNIX both. New users are often confused with and mis-interpret the two styles. So here is some basic info to get it clear before moving on.
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> Note : "ps aux" is not the same as "ps -aux". For example "-u" is used to show process of that user. But "u" means show detailed information.
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BSD style - The options in bsd style syntax are not preceded with a dash.
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ps aux
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UNIX/LINUX style - The options in linux style syntax are preceded by a dash as usual.
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ps -ef
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It is okay to mix both the syntax styles on linux systems. For example "ps ax -f".
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But in this post we shall mostly focus on the unix style syntax.
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### How to use ps command ###
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#### 1. Display all processes ####
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The following command will give a full list of processes
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$ ps ax
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$ ps -ef
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Pipe the output to "less" to make it scrollable.
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Use the "u" option or "-f" option to display detailed information about the processes
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$ ps aux
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$ ps -ef -f
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> Why is the USER column not displaying my username, but showing others like root, www-data etc ?
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For all usernames (including yours) if the length is greater than 8 characters then ps will fall back to show only the UID instead of username.
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#### 2. Display process by user ####
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To filter the processes by the owning user use the "-u" option followed by the username. Multiple usernames can be provided separated by a comma.
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$ ps -f -u www-data
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UID PID PPID C STIME TTY TIME CMD
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www-data 1329 1328 0 09:32 ? 00:00:00 nginx: worker process
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www-data 1330 1328 0 09:32 ? 00:00:00 nginx: worker process
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www-data 1332 1328 0 09:32 ? 00:00:00 nginx: worker process
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www-data 1377 1372 0 09:32 ? 00:00:00 php-fpm: pool a.localhost
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www-data 1378 1372 0 09:32 ? 00:00:00 php-fpm: pool a.localhost
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www-data 4524 2359 0 10:03 ? 00:00:00 /usr/sbin/apache2 -k start
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www-data 4527 2359 0 10:03 ? 00:00:00 /usr/sbin/apache2 -k start
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www-data 4528 2359 0 10:03 ? 00:00:00 /usr/sbin/apache2 -k start
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#### 3. Show process by name or process id ####
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To search the processes by their name or command use the "-C" option followed by the search term.
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$ ps -C apache2
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PID TTY TIME CMD
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2359 ? 00:00:00 apache2
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4524 ? 00:00:00 apache2
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4525 ? 00:00:00 apache2
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...
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To display processes by process id, use the "-p" option and provides the process ids separated by comma.
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$ ps -f -p 3150,7298,6544
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The "-C" must be provided with the exact process name and it cannot actually search with a partial name or wildcard. To search the process list more flexibly, the usual grep command has to be used
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$ ps -ef | grep apache
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#### 4. Sort process by cpu or memory usage ####
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System administrators often want to find out processes that are consuming lots of memory or CPU. The sort option will sort the process list based on a particular field or parameter.
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Multiple fields can be specified with the "--sort" option separated by a comma. Additionally the fields can be prefixed with a "-" or "+" symbol indicating descending or ascending sort respectively. There are lots of parameters on which the process list can be sorted. Check the man page for the complete list.
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$ ps aux --sort=-pcpu,+pmem
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Display the top 5 processes consuming most of the cpu.
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$ ps aux --sort=-pcpu | head -5
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USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
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root 1 2.6 0.7 51396 7644 ? Ss 02:02 0:03 /usr/lib/systemd/systemd --switched-root --system --deserialize 23
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root 1249 2.6 3.0 355800 30896 tty1 Rsl+ 02:02 0:02 /usr/bin/X -background none :0 vt01 -nolisten tcp
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root 508 2.4 1.6 248488 16776 ? Ss 02:02 0:03 /usr/bin/python /usr/sbin/firewalld --nofork
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silver 1525 2.1 2.3 448568 24392 ? S 02:03 0:01 /usr/bin/python /usr/share/system-config-printer/applet.py
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#### 5. Display process hierarchy in a tree style ####
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Many processes are actually forked out of some parent process, and knowing this parent child relationship is often helpful. The '--forest' option will construct an ascii art style tree view of the process hierarchy.
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The following command will search for processes by the name apache2 and construct a tree and display detailed information.
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$ ps -f --forest -C apache2
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UID PID PPID C STIME TTY TIME CMD
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root 2359 1 0 09:32 ? 00:00:00 /usr/sbin/apache2 -k start
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www-data 4524 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start
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www-data 4525 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start
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www-data 4526 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start
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www-data 4527 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start
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www-data 4528 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start
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> Try not to use any sorting with the tree style display, as they both effect the order of display in different ways.
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#### 6. Display child processes of a parent process ####
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Here is an example of finding all forked apache processes.
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$ ps -o pid,uname,comm -C apache2
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PID USER COMMAND
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2359 root apache2
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4524 www-data apache2
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4525 www-data apache2
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4526 www-data apache2
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4527 www-data apache2
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4528 www-data apache2
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[term]
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The first process that is owned by root is the main apache2 process and all other apache2 processes have been forked out of this main process. The next command lists all child apache2 processes using the pid of the main apache2 process
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[term]
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$ ps --ppid 2359
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PID TTY TIME CMD
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4524 ? 00:00:00 apache2
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4525 ? 00:00:00 apache2
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4526 ? 00:00:00 apache2
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4527 ? 00:00:00 apache2
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4528 ? 00:00:00 apache2
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#### 7. Display threads of a process ####
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The "-L" option will display the threads along with the processes. It can be used to display all threads of a particular process or all processes.
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The following command shall display all the threads owned by the process with id 3150.
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$ ps -p 3150 -L
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#### 8. Change the columns to display ####
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The ps command can be configured to show a selected list of columns only. There are a large number of columns to to show and the full list is available in the man pages.
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The following command shows only the pid, username, cpu, memory and command columns.
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$ ps -e -o pid,uname,pcpu,pmem,comm
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It is possible to rename the column labels
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$ ps -e -o pid,uname=USERNAME,pcpu=CPU_USAGE,pmem,comm
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PID USERNAME CPU_USAGE %MEM COMMAND
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1 root 0.0 0.0 init
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2 root 0.0 0.0 kthreadd
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3 root 0.0 0.0 ksoftirqd/0
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4 root 0.0 0.0 kworker/0:0
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5 root 0.0 0.0 kworker/0:0H
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7 root 0.0 0.0 migration/0
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8 root 0.0 0.0 rcu_bh
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9 root 0.0 0.0 rcuob/0
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10 root 0.0 0.0 rcuob/1
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Quite flexible.
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#### 9. Display elapsed time of processes ####
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The elapsed time indicates, how long the process has been running for. The column for elapsed time is not shown by default, and has to be brought in using the "-o" option
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$ ps -e -o pid,comm,etime
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#### 10. Turn ps into an realtime process viewer ####
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As usual, the watch command can be used to turn ps into a realtime process reporter. Simple example is like this
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$ watch -n 1 'ps -e -o pid,uname,cmd,pmem,pcpu --sort=-pmem,-pcpu | head -15'
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The output on my desktop is something like this.
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Every 1.0s: ps -e -o pid,uname,cmd,pmem,pcpu --... Sun Dec 1 18:16:08 2013
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PID USER CMD %MEM %CPU
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3800 1000 /opt/google/chrome/chrome - 4.6 1.4
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7492 1000 /opt/google/chrome/chrome - 2.7 1.4
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3150 1000 /opt/google/chrome/chrome 2.7 2.5
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3824 1000 /opt/google/chrome/chrome - 2.6 0.6
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3936 1000 /opt/google/chrome/chrome - 2.4 1.6
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2936 1000 /usr/bin/plasma-desktop 2.3 0.2
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9666 1000 /opt/google/chrome/chrome - 2.1 0.8
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3842 1000 /opt/google/chrome/chrome - 2.1 0.8
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4739 1000 /opt/google/chrome/chrome - 1.8 1.0
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3930 1000 /opt/google/chrome/chrome - 1.7 1.0
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3911 1000 /opt/google/chrome/chrome - 1.6 0.6
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3645 1000 /opt/google/chrome/chrome - 1.5 0.4
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3677 1000 /opt/google/chrome/chrome - 1.5 0.4
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3639 1000 /opt/google/chrome/chrome - 1.4 0.4
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The output would be updated every 1 second to refresh the stats. However do not think that this is similar to top.
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You would notice that the output of top/htop command changes much more frequently compared to the above ps command.
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This is because the top output sorts on a value that is a mix of cpu usage and memory usage. But the above ps command sorts in a more simpler manner, taking 1 column at a time (like school maths). So it would not update rapidly like top.
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--------------------------------------------------------------------------------
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via: http://www.binarytides.com/linux-ps-command/
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译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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142
sources/How To Display And Set Hostname in Linux.md
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sources/How To Display And Set Hostname in Linux.md
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How To Display And Set Hostname in Linux
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================================================================================
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With more and more computer connected to the network, computer need to have an attribute to make it different to each other. Same as human in the real world, computer also hava an attribute which named hostname.
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### What is hostname ###
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From its manual page, hostname is used to display the system’s DNS name and to display or set its hostname or NIS domain name. So hostname is related to DNS (Domain Name System) or NIS (Network Information System).
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### How to display hostname ###
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Hostname is a pre-installed command in every Linux distribution. You can display your machine hostname by typing hostname in your console. Here’s a sample command and the output :
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$ hostname
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ubuntu
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The above command will tell you that the computer name is **ubuntu**.
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### How to set a hostname ###
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Hostname is set when you install you Linux at the first time. There is a step in your installation procedure that your Linux will ask you to fill hostname information. However, **you can do it later** if you want.
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To set it your hostname, you can use this command :
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# hostname dev-machine
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$ hostname
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dev-machine
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You **need to be root** user or equal to set / change your hostname machine. The # sign is indicated that you are a root user. The above command is telling your computer to set its hostname into **dev-machine**. If you don’t receive any error message then your hostname is changed. Again, you check it using hostname command to see the result.
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Setting hostname using hostname command **is not permanent**. When you reboot your computer, your setting will gone. **To make it permanent**, you must manually edit hostname configuration files.
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**On Debian / Ubuntu based Linux**
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You will find it in these folders, **/etc/hostname** and **/etc/hosts**.
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Here’s the content of each files.
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**/etc/hostname**
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# vi /etc/hostname
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dev-machine
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**/etc/hosts**
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# vi /etc/hosts
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127.0.0.1 localhost
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127.0.0.1 dev-machine
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You will found it **active immediately without restarting** your Linux.
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**On RedHat / CentOS based Linux**
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You will find it in these folders, **/etc/hosts** and and **/etc/sysconfig/networks**
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Here’s the content of each files.
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**/etc/hosts**
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127.0.0.1 localhost.localdomain localhost dev-machine
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::localhost 127.0.0.1
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/etc/sysconfig/network
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NETWORKING=yes
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NETWORKING_IPV6=no
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HOSTNAME=dev-machine
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### How to display dnsdomainname ###
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From hostname definition above, hostname can also display a dnsname for your Linux. If hostname command will display your hostname, then dnsndomainname command will show your domain name. Let’s see the sample.
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$ dnsdomainname
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bris.co.id
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On this article, the result of dnsdomainname command is **bris.co.id**.
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If you see the result is (**none**), then your machine **is not configured in FQDN** (Fully Qualified Domain Name). Dnsdomainname command will grab information from **/etc/hosts** file. You should configure it in FQDN format. Here’s the sample :
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**/etc/hosts**
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127.0.0.1 localhost.localdomain localhost dev-machine
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::localhost 127.0.0.1
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192.168.0.104 dev-machine.bris.co.id dev-machine
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To display it more detail, you can use parameter **-v**
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$ dnsdomainname -v
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gethostname()=’dev-machine.bris.co.id’
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Resolving ‘dev-machine.bris.co.id’ …
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Result: h_name=’dev-machine.bris.co.id’
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Result: h_aliases=’dev-machine’
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Result: h_addr_list=’192.168.0.104’
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### How to display hostname with more detail information ###
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Hostname command comes with some parameters and some aliases such as dnsdomainname command. Here’s some parameter that may useful on day-to-day operation. The results of the commands below is based on **/etc/hosts** configuration above
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**Display the IP Address**
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$ hostname -i
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192.168.0.104
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**Display the domain**
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$ hostname -d
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bris.co.id
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**Display the the short name**
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$ hostname -s
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dev-machine
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*This command will produce the same result with only typing hostname*
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**Display with FQDN format**
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$ hostname -f
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dev-machine.bris.co.id
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**Display with detail information**
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All the parameters mentioned above can be ‘**summarized**’ by the parameter **-v and -d**. Let’s take a look the sample
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$ hostname -v -d
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gethostname()=’dev-machine.bris.co.id’
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Resolving ‘dev-machine.bris.co.id’ …
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Result: h_name=’dev-machine.bris.co.id’
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Result: h_aliases=’dev-machine’
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Result: h_addr_list=’192.168.0.104’
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bris.co.id
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Feel familiar? Yes, this result is the same with **dnsdomainname -v** command that also mentioned above.
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--------------------------------------------------------------------------------
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via: http://linoxide.com/linux-command/display-set-hostname-linux/
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译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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