Commands used for System Information

DATE

The simple “date” command displays the current date and time (including the day of the week, month, time, time zone, year).

date

Date TZ

By default, “date” command uses the time zone defined in path “/etc/localtime”. Linux user can change the time zone via Terminal by using command “TZ”.

TZ=GMT date

TZ=America/New_York date

Date --set

Linux allows its user to set the current date and time of the system manually.
Syntax: date –set=”Date_in_format(YYMMDD) Time_in_format(HH:MM)”

sudo date --set="YYYYMMDD HH:MM"

$ date Thu Mar  2 07:23:38 PM EST 2023 $ TZ=GMT date Fri Mar  3 12:03:59 AM GMT 2023 $ TZ=America/New_York date Thu Mar  2 07:04:12 PM EST 2023

Example - This would set the system date to May 18, 2023 10:10 PM 

sudo date --set="20230519 22:10"

DF

The command “df” shows the amount of disk space used and disk space available on every file system containing each filesystem’s name and its path.

df

The command “df -h” shows the same result as the command “df” but now the data is in a more human-readable format.

df -h

$ df Filesystem    1K-blocks       Used  Available Use% Mounted tmpfs         1623284       3612    1619672   1% /run /dev/nvme0n1p3 491343600   18123184  452739188   4% / tmpfs            8116400     104604    8011796   2% /dev/shm tmpfs               5120          4       5116   1% /run/lock tmpfs            8116400          0    8116400   0% /run/qemu /dev/nvme0n1p2   456036     182424     239424  44% /boot /dev/nvme0n1p1    98304      57271      41033  59% /boot/efi /dev/sda1       47744748      57156   45229840   1% /tmp /dev/sda3     2787016696 1123163768 1531975216  43% /home /dev/sda2     47745772   31301948   13986020  70% /var

$ df -h Filesystem     Size  Used Avail Use% Mounted on tmpfs           1.6G  3.6M  1.6G   1% /run /dev/nvme0n1p3  469G   18G  432G   4% / tmpfs           7.8G  103M  7.7G   2% /dev/shm tmpfs           5.0M  4.0K  5.0M   1% /run/lock tmpfs           7.8G     0  7.8G   0% /run/qemu /dev/nvme0n1p2  446M  179M  234M  44% /boot /dev/nvme0n1p1  96M   56M   41M  59% /boot/efi /dev/sda1       46G   56M   44G   1% /tmp /dev/sda3       2.6T  1.1T  1.5T  43% /home /dev/sda2       46G   30G   14G  70% /var

FREE

The command “free” displays the amount of free and used memory in the complete system.

free

PS

The command “ps” which is also known as the process status command is used to provide information about the processes currently running on the system, including their respective process identification numbers (PIDs)

ps

PS AUX & PS -EF

Both list all processes of all users. In that aspect -e and ax are completely equivalent.

Where they differ is output format specifier, -f is "full", while u is "user-oriented". The displayed columns are different:

ps -ef

ps -aux

$ ps     PID TTY          TIME CMD  434765 pts/0    00:00:00 bash  441698 pts/0    00:00:00 ps

$ ps -ef UID          PID    PPID  C STIME TTY          TIME CMD root           1       0  0 Feb07 ?        00:19:03 /sbin/init splash root           2       0  0 Feb07 ?        00:00:03 [kthreadd] root           3       2  0 Feb07 ?        00:00:00 [rcu_gp] root           4       2  0 Feb07 ?        00:00:00 [rcu_par_gp] root           5       2  0 Feb07 ?        00:00:00 [slub_flushwq] root           6       2  0 Feb07 ?        00:00:00 [netns]

$ ps -aux USER  PID %CPU %MEM VSZ  RSS TTY STAT START TIME COMMAND root 1  0.0  0.0 169808 12472 ? Ss Feb07  19:03 /sbin/init splash root    2  0.0  0.0  0     0 ? S    Feb07   0:03 [kthreadd] root    3  0.0  0.0  0     0 ? I<   Feb07   0:00 [rcu_gp] root    4  0.0  0.0  0     0 ? I<   Feb07   0:00 [rcu_par_gp] root    5  0.0  0.0  0     0 ? I<   Feb07   0:00 [slub_flushwq] root    6  0.0  0.0      0     0 ?        I<   Feb07   0:00 [netns]

UPTIME

The command “uptime” provides information about how long the system has been running in one line. The result for this command includes the current time, the time duration system has been running, the number of users who are currently logged on, and the system load averages for the past 1, 5, and 15 minutes respectively.

uptime

 W

The command “w” displays detailed information about the users who are logged into the system currently.

w

PASSWD

The command “passwd” stands for password and it is used to change the password of the user.

passwd my_user

EXIT

The command “exit” as the name says it is used to exit from the system and log out from the current user.

exit

SS

The ss command is a modern replacement for the classic netstat command. You can use it on Linux to get statistics about your network connections.

Use the -ltn flags to list all listening  ports on your system.

ss -ltn

Use the following to see if a specific port is listening on your system. In this case, the source port (sport) and destination port (dport) is 80 or you can use the protocol name, http, instead.

ss -a '( dport = :80 or sport = :80 )'

ss -a '( dport = :http or sport = :http )'

$ ss -ltn State   Recv-Q Send-Q Local Address:Port Peer Address:Port Process LISTEN  0      4096   0.0.0.0:9000       0.0.0.0:*             LISTEN  0      4096   0.0.0.0:6767       0.0.0.0:*             LISTEN  0      4096   0.0.0.0:111        0.0.0.0:*             LISTEN  0      4096   0.0.0.0:80         0.0.0.0:*

$ ss -a '( dport = :80 or sport = :80 )'
Netid State     Recv-Q Send-Q    Local Address:Port     Peer Address:PortProcess
tcp   LISTEN    0      4096            0.0.0.0:http          0.0.0.0:*          
tcp   LISTEN    0      4096               [::]:http             [::]:*

SHUTDOWN

The command “shutdown” is used to shut down the system.

Note: The shutdown command needs superuser privileges. Hence, you should either be root or run the command with sudo.

Using the command with no flags will schedule a shutdown 1 minute from execution.

sudo shutdown

Use the following to IMMEDIATELY shutdown your system.

sudo shutdown now

You can schedule a shutdown in future by providing the time argument either in +t format or in hh:mm format.

For example, if you want to shutdown the system after 15 minutes, you can use this command:

sudo shutdown +15

If you want to shutdown the system at 6 PM in the afternoon, you can use it in the following manner:

sudo shutdown 18:00

Cancel a shutdown

sudo shutdown -c

Reboot a system

sudo shutdown -r

sudo reboot