Linux Tutorials

date command formatting with practical examples in Linux / Unix

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Date Command in Linux is very extensive and dynamic, provides very rich date formatting and is greatly customizable for working with scripts which depend on time based invocations.

Linux date command can also be used to set the system date and it requires the root permission.

Lets run date command and examine the output.

[vamshi@node02 log]$ date
Wed Apr 1 13:52:21 UTC 2020

Now lets examine some of the most useful options that comes with the date command.

Firstly date command along with -s or --set option can take for following format to set the new system time and date.

How to set the system date in Linux using date command?

[vamshi@node02 log]$ sudo date -s 'Apr 01 2020 13:52:59 UTC'
Wed Apr 1 13:52:59 UTC 2020

The date can also be setup in shot hand notation as follows,but it is more cryptic

[vamshi@node02 log]$ sudo date 040113522020.50
Wed Apr 1 13:52:50 UTC 2020
$ sudo date mmddHHMMyyyy.SS

The format is month of the Year(mm),day of the month(dd),Hour of the day(HH),minute of the Hour(MM) and the Year(yyyy),and the Seconds of the minute(.SS)
Now, Lets dive deep and get to know the date options and Demonstration practical examples in this tutorial:

Another Important Option is -d or –date=”String” which can display the time described
Lets see some examples as follows:

By running the date command, we get an elaborate time and date format along with the TimeZone information.
To covert the Epoc time to human readable date, we can use date command as follows:

[vamshi@node02 log]$ date -d"@1585749164"
Wed Apr 1 13:52:44 UTC 2020

If you want to get a future date then use:

[vamshi@linuxcent ~]$ date -d "+130 days"
Sun Aug 16 02:07:35 UTC 2020

Date command offers a great flexibility to extract past and future dates as we will show below:

$ date "+ %F" -d “+30 days”
$ date "+ %F" --date “+30 days”

To get the date in history; go back to a date some days ago in Linux

[vamshi@node02 log]$ date -d "17 days ago"
Sun Mar 15 13:52:45 UTC 2020

Here we present some of the more useful Format options:

Date Format Command Explanation Result
date +”%a” Prints the Abbreviated Day of the Week Sat-Sun Wed
date +”%A” Prints the Day of the Week Saturday-Sunday Wednesday
date +”%b” Prints Abbreviated Month Jan-Dec Apr
date +”%B” Print un-abbreviated month January-December April
date +”%c” Prints Full Current Date and time format Wed Apr 1 13:52:43 UTC 2020
date +”%D” Prints dd/mm/yy date format 04/01/2020
date +”%d” Prints day of the month (01-31) 01
date +”%D” Prints Date in MM/DD/YY 04/01/20
date +”%e” Prints the Day of the month 01
date +”%F” Prints only the Full date as YYYY-MM-DD 2020-04-01
date +”%H” Prints the hour 00-23 13
date +”%I” Prints the hour in 00-12 01
date +”%j” Prints Julian day of the Year(001-366) 092
date +”%M” Prints the Minute of the hour 00-59 52
date +”%m” Prints the month of the year 01-12 04
date +”%n” Prints the newline character Newline/Empty line
date +”%N” Prints the nanoseconds counts 036416306
date +”%P” Prints AM/PM in the day PM
date +”%r” Get only time in AM/PM notation 13:52:43 PM
date +”%S” Get the current seconds count in the minute (00-60) 43
date +”%s” Get the number of seconds since 1st January 1970 (Epoch time) 1585749164
date +”%T” Time in 24 Hour format HH:MM:YY 13:52:43
date +”%u” Get  current day of the week
1-7		 3 for Wednesday
date +”%U” Get the current week of the Year considering Sunday as first week 13
date +”%V” Get the current week of the Year considering Monday as first week 14
date +”%y” or date +”%g” Prints only the last two digits of Year 20
Date +“%Y” or date +”%F” Prints Year in YYYY format 2020
Date +“%z” Prints the current Timezone difference from UTC 00 – for UTC
date +”%Z” Prints Alphabetic time zone abbreviation UTC

 

How to write the current system time to the Machine’s hardware clock ?

The command hwclock can do that for us.
[/code] # sudo hwclock [OPTIONS][/code]

Lets see a practical example where our Hardware clock was 1 hour and 13 mins behind the actual system time .

[vamshi@node02 ~]$ sudo hwclock
Wed 01 Apr 2020 07:35:05 AM UTC -0.454139 seconds
[vamshi@node02 ~]$ date
Wed Apr 01 08:43:13 UTC 2020

Setting the hardware clock time to system time with option -w or --systohc as seen below.

[vamshi@node02 ~]$ sudo hwclock -w

Confirm it with hwclock command as follows:

[vamshi@node02 ~]$ sudo hwclock
Wed 01 Apr 2020 08:44:05 AM UTC -0.538163 seconds

Most of the times the hardware clock will be out of sync with the system time and its a good practice to set the hardware clock in sync and comes in real handy during the system reboots.

Setting hostname in Linux

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In the systemd Environment there has been an architectural change and the systemd daemon controlling all the essential processes

root 1 0 0 Apr15 ? 00:00:12 /usr/lib/systemd/systemd --switched-root --system --deserialize 32

We can make use of the hostnamectl command to immediately reflect the system hostname change.


Command to get the current hostname on Linux:

[vamshi@server02 ~]$ sudo hostnamectl 
   Static hostname: server02.linuxcent.com
   Pretty hostname: SERVER02
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 2338f55840d640689fc5ac4b356b160c
           Boot ID: 418256281d2f4e11822809dde7c1b09e
    Virtualization: kvm
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0-1062.18.1.el7.x86_64
      Architecture: x86-64

As you can see the current hostname is set to SERVER02

The file /etc/hostname also used to have the same effect but on the cloud systems this is dynamically generated and doesn’t hold true on cloud and tends to get overwritten after reboot.
On the general DataCenter or PC environment we can use the static files to populate the hostnames and make them permanent:

$ cat /etc/sysconfig/network
HOSTNAME=node2.linuxcent.com

But only /etc/hostname file is given importance since the systemd version and updating this ensures persistence.

$ cat /etc/hostname 
node02.linuxcent.com

 

Process to set or change the hostname in Linux Commandline:

The systemd provides a sophisticated command hostnamectl to set the hostname, It will also update the static file /etc/hostname and ensure the changes are permanent across reboots.

$ sudo hostnamectl set-hostname node02.Linuxcent

To get the hostname along with FQDN

[vamshi@server02 ~]$ hostname --fqdn
node02.linuxcent.com
[vamshi@node02 ~]$ sudo hostnamectl status 
   Static hostname: node02.linuxcent.com
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 2338f55840d640689fc5ac4b356b160c
           Boot ID: 33619e39ea4c4900bd848e13d2a6239e
    Virtualization: kvm
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0-1062.18.1.el7.x86_64
      Architecture: x86-64

Changing hostname is ubuntu and Debian systems can be done through hostnamectl command

vamshi@debian:~$ hostnamectl 
Static hostname: debian
Icon name: computer-vm
Chassis: vm
Machine ID: b4adcdb84c724856b577524ebbfa0003
Boot ID: 67e1bf27946a4770b8e939f2420d06fd
Virtualization: oracle
Operating System: Debian GNU/Linux 10 (buster)
Kernel: Linux 4.19.0-5-amd64
Architecture: x86-64

How to Shutdown or Reboot a remote Linux Host from commandline

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The Shutdown process in a Linux system is an intelligent chain process where in the system ensures the dependent process have successfully terminated.

TL;DR:

Difference between the Halt and Poweroff in Linux?
What is a Cold Shutdown and Warm Shutdown?
Linux System System Halt : The Halt process instructs the hardware to Stop the functioning of the CPU. Can be referred as a Warm Shutdown.
Linux System Poweroff/Shutdown : The Poweroff function sends the ACPI(Advanced Configuration and Power Interface) to power down the system. Can be referred as a Cold Shutdown.

As you may be aware the Linux runtime environment is a duo combination of processes running in User space and the Kernel space, All the major system activities and resources are initiated and governed and terminated by Kernel space.
So we got the Kernel space and the User space, The kernel space is where all the reseurce related processes run, which follows a finite behaviour, and the the userspace where the processes are dependent on the user actions, most of the userspace programs depend on the kernel space and make a context switch to get the CPU scheduling and such..
So, In the sequence of Shutdown on a linux machine, the userspace processes are first terminated in a systematic fashion through the scripts triggered by the core systemd processes which ensures clean exit and termination all the processes.

The Linux system provides us quite a few commands to enforce fast shutdown or a graceful shutdown of the operating system, each having their own consequences.

Firstly the init or the systemd which is the pid 1 process is what controls the runlevel of the system and it determines which processes are launched and running in that runlevel

The init is a powerful command which executes the runlevel it is told to.
Here the init 0 proceeds to Power-off the machine

$ sudo init 0

Here the init 6 proceeds to Reboot the machine

$ sudo init 6

These commands are real quick as it triggers the kernel space shutdown invocation process.. most often resulting in unclean termination of processes resulting system recovery and journaling while booting.

The following commands Shutdown the machine in seconds after issuing the command But tend to follow the kill sequence and clean exit of the processed.

$ sudo shutdown
$ sudo poweroff
$ sudo systemctl poweroff

Prints a wall message to all users.
All the processes are killed and the volumes are unmounted or converted to be in Read-Only mode while system power off is in progress.
Puts the system into a complete poweroff mode cutting out power supply to the machine completely.

$ sudo halt
$ sudo systemctl halt

Prints a message of “System halted” and Puts the machine in halt mode
If the --force or -f when specified twice the operation is immediately executed without terminating any processes or unmounting any file systems and resulting in data loss

The servers can only be brought back online through physical poweron or Remote Power manager console such as IPMI or ILOM.

To reboot or [/code]systemctl kexec[/code] will to restart the operating system which is one power cycle or equivalent of shutdown followed by the startup.

$ sudo reboot

$ sudo systemctl kexec

$ sudo systemctl reboot

If the --force or -f when specified twice the operation is immediately executed without terminating any processes or unmounting any file systems and resulting in data loss

 

It is important to understand that the commands are all softlinks to systemctl shutdown command. and ensure in proper shutdown sequence process

[vamshi@linuxcent cp-command]$ ls -l /usr/sbin/halt
lrwxrwxrwx. 1 root root 16 Jan 13 14:41 /usr/sbin/halt -> ../bin/systemctl
[vamshi@linuxcent cp-command]$ ls -l /usr/sbin/reboot
lrwxrwxrwx. 1 root root 16 Jan 13 14:41 /usr/sbin/reboot -> ../bin/systemctl
[vamshi@linuxcent cp-command]$ ls -l /usr/sbin/poweroff
lrwxrwxrwx. 1 root root 16 Jan 13 14:41 /usr/sbin/poweroff -> ../bin/systemctl

It is important to observe that all the commands are softlink to the systemctl process, When issuing a shutdown or reboot

The best practice to poweroff the system by enabling broadcast the notification message to all the users connected actively either through the Pseudo remote connection terminal or TTY terminals, Demonstrated as follows:

$ sudo systemctl poweroff

# this writes an entry into the journal, the wtmp and broadcasts the shutdown message to all the users connected through PTS and TTY terminals

What is the difference between systemctl poweroff and systemctl halt ?

The Linux System when put to a Halt state, stops the all the applications and ensures they’re safely exited, filesystems and volumes are unmounted and it is taken into a halted state where in the power connection is still active. And Can only be brought  online with a power reset effectively with a simple reset.
The Halt process instructs the hardware to Stop the functioning of the CPU.
Commonly can be referred as a Warm Shutdown.

Below is the screenshot to demonstrate the same
systemctl halt command in linux

The Poweroff function sends the ACPI(Advanced Configuration and Power Interface) to power down the system.
The Linux System when put to a Poweroff state it becomes completely offline following the systematical clean termination of processes.. and power input is cut off to the external peripherals, which is also sometimes called as cold shutdown, and the startup cold start.
Commonly can be referred to as a Cold Shutdown.

If you found the article worth your time, Please share your inputs in the comments section and share your experiences with shutdown and reboot issues

How to create user account in Linux

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The Linux system provides a couple of command line utilities to create new users on the system

As we are aware, the Linux login has the essential fields listed as follows:

  • A unique system wide username,
  • A Strong password,
  • The home directory and
  • A login shell.

These are the mandatory fields to enable account creation.

The other fields are the UID and GID numbers associated with User an Group name numerical IDs which will be generated sequentially allocated by the Linux Kernel

We can do a broad categorization of login accounts into 2 types, those are the Privileged and the normal user.

The Absolute Privileged account is root which comes by default in all the linux machines.

The normal account can be enabled with root Privileged by assigning user to certain groups and providing elevated access in the scope.

What is the Process to create a User account in Linux?

The user creation has to be done with root privileges using useradd command.

$ sudo useradd newuser

Now it’s time to enter the password

$ sudo passwd newuser

How to check if the userid is present and active on the system?

The new user details will be updated to /etc/passwd file and the login information updated to /etc/shadow

Now let’s check if the user account is created and has a valid shell

vamshi@node03:/$ grep vamshi /etc/passwd

vamshi:x:1001:1001::/home/vamshi:/bin/bash

How to Add the user to new groups in Linux?

Usermod command line linux utility enables to add user to groups and the ability to add an existing user to new groups additionally or overwrite the group membership

$ usermod -aG dockerroot wheel vamshi

The option -a: appends the user to two new groups called dockerroot and wheel with out overwriting the existing user assigned groups, violating this option will restrict the newuser to be part of only the mentioned groups in the command

How to check and verify if the user is a member of group in Linux?

[vamshi@node02 Linux-blog]$ id vamshi
uid=1001(vamshi) gid=1001(vamshi) groups=1001(vamshi),0(root),10(wheel),992(dockerroot)

How to Verify the Login Confirmation in Linux?

From the root user account run the command: su - newuser to check the new login account environment.

How to find the group names assigned to the user

The user can list of his active membership groups by running the linux command groups

The user can run the groups command to list the groups with active membership

[vamshi@linuxcent ~]$ groups
vamshi root wheel dockerroot

Login to the server remotely using SSH

You may now use the ssh command to login with the new username and enter your password at login prompt.

$ ssh vamshi@linuxcent.com

How to connect to server with SSH running on non-standard port like 2202?

[vamshi@linuxcent ~]$ ssh localhost -p 2202
Last login: Mon Mar 13 17:57:56 2020 from 10.100.0.1

How to create a useraccount in Linux using useradd command?

The usercreation can also be done with parametrized command as demonstrated below:

$ sudo useradd vamshi -b /home/ -m -s /bin/bash

Alternatively you can be more elaborate as mentioned below:

$ sudo useradd vamshi -c "Vamshi's user account" -d /home/vamshi -m -s /bin/bash -G dockerroot

The useradd command-utility options describes as follows:

-b or --base-dir : base directory of new user home directory.

-c or --comment : Description about the user Or as A Standard Practice can be used to Mention the Current User’s Full name.

-d or --home-dir : create the user’s home directory

-m or --create-home :  create the user’s home directory as per -d option.

-s or --shell : Type of Login Shell.

-u or --uid : is the Unique UID on linux machine

-G or --groups : list of secondary groups to be assigned

-k or --skel : determines the default parameters if no options are passed while account creation. Present at /etc/default/useradd

With the skel properties finely tunes, you can proceed to use adduser command which is based on the default skel behavior as shown below:

$ sudo adduser vamshi

How to using the SSH key pair to login:
Use the -i followed by the /path/to/id_rsa private key file

$ ssh -i ~/.ssh/id_rsa vamshi@linuxcent.com
$ ssh -i ~/.ssh/id_rsa -l linuxcent.com

-l : using the login name

-i : is the identity file; rsa the private key file

 

Troubleshooting the SSH connection in Verbose mode printing Debug information

Using -v option with the ssh command will print the debug information while logging

The verbosity levels -v can be concatenated from one to Nine; eg -v to -vvvvvvvvv

$ ssh -i ~/.ssh/id_rsa vamshi@linuxcent.com -vvvvvvvvv

Linux Copy File Command for Files and Directories – cp Command Examples

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Linux copy files command: cp is generally used for organizing the data on the Linux operating system, It copies the files and directories.

We shall take a deeper look at Linux cp command-utility in the section

In order to copy files and directories, you must have read permissions on the source file(s) and write permissions on the destination directory

How do I copy files under Linux operating systems?

How do I make a 2nd copy of a file on a Linux bash shell?

How can I copies files and directories on a Linux

Linux Copy File command Syntax

cp sourcefile destinationfile
cp sourcefile DESTDIR
cp sourcefile1 sourcefile2 DESTDIR
cp [OPTION] SOURCE DESTFILE
cp [OPTION] SOURCE DESTDIR

How to Copy a Directory if the destination does not exist?

To achieve this we can make use of the following cp command options -R or -r: Copy directories recursively.

Linux cp command Syntax with -R option:

cp -R SOURCE DESTINATION

If the destination doesn’t exist, it will be created.

It can also be used to Copy the contents Recursively

Lets see the demonstration as follows:

[vamshi@linuxcent ]$ cp -R dir1/ dir1-copy
[vamshi@linuxcent ]$ ls -l 
total 0
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:35 dir1
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:37 dir1-Recursive

Using the verbose Option -v to print the copy activity information onto the output screen.

Let’s use the -v flag to print the verbose information onto the screen.

How to Preserve the Source file and Directory permission?

Linux Copy command Syntax with -p option:

-p option preserves the mode, ownership and timestamps from the source to the destination

cp -p file1 file1-copy

Lets us see the Demonstration as Below

[vamshi@node02 cp-command]$ cp -Rp dir1/ dir1-copy
[vamshi@node02 cp-command]$ ls -ld dir1*
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:35 dir1
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:35 dir1-copy
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:37 dir1-Recursive

From the out we can conclude the the Linux copy command with -p Option preserves the original timestamps information and copies it to the destination

Linux cp command with Force copy -f Option, It forcefully overwrites the destination content
Sample Syntax:

cp -f file1 file1-copy

How to Copy Multiple files at once ?

Asterisk / wildcard (*) character is used to copy files multiple files with same pattern.

[vamshi@linuxcent ]$ cp -varpf file* DEST/
‘file10.txt’ -> ‘DEST/file10.txt’
‘file1.txt’ -> ‘DEST/file1.txt’
‘file2.txt’ -> ‘DEST/file2.txt’
‘file3.txt’ -> ‘DEST/file3.txt’
‘file4.txt’ -> ‘DEST/file4.txt’
‘file5.txt’ -> ‘DEST/file5.txt’
‘file6.txt’ -> ‘DEST/file6.txt’
‘file7.txt’ -> ‘DEST/file7.txt’
‘file8.txt’ -> ‘DEST/file8.txt’
‘file9.txt’ -> ‘DEST/file9.txt’

The options -p or -d enables preserving the links and can be used in conjunction with -R option to copy contents Recursively from the source directory.

How to Copy Files and Folders on Linux Using the cp Command recursively to Destination Directory

How to preserve the links with cp command?

Using the Options -p preserves the links and -r Option copies the content recussively same as -R Option and -v prints the verbose information

[vamshi@node02 Linux-blog]$ cp -varpf Redhat-Distro/ /tmp/DEST
‘Redhat-Distro/’ -> ‘/tmp/DEST’
‘Redhat-Distro/Fedora’ -> ‘/tmp/DEST/Fedora’
‘Redhat-Distro/Fedora/fedora.txt’ -> ‘/tmp/DEST/Fedora/fedora.txt’
‘Redhat-Distro/Centos’ -> ‘/tmp/DEST/Centos’
‘Redhat-Distro/Centos/centos.txt’ -> ‘/tmp/DEST/Centos/centos.txt’
‘Redhat-Distro/Centos/CentOS-versions’ -> ‘/tmp/DEST/Centos/CentOS-versions’
‘Redhat-Distro/Centos/CentOS-versions/centos7.txt’ -> ‘/tmp/DEST/Centos/CentOS-versions/centos7.txt’
‘Redhat-Distro/Centos/CentOS-versions/centos6.1.txt’ -> ‘/tmp/DEST/Centos/CentOS-versions/centos6.1.txt’
‘Redhat-Distro/Centos/README-CentOS’ -> ‘/tmp/DEST/Centos/README-CentOS’
‘Redhat-Distro/README-Redhat-Distro’ -> ‘/tmp/DEST/README-Redhat-Distro’
‘Redhat-Distro/RHEL-Versions’ -> ‘/tmp/DEST/RHEL-Versions’
‘Redhat-Distro/RHEL-Versions/redhat5.txt’ -> ‘/tmp/DEST/RHEL-Versions/redhat5.txt’
‘Redhat-Distro/RHEL-Versions/redhat8.txt’ -> ‘/tmp/DEST/RHEL-Versions/redhat8.txt’
‘Redhat-Distro/redhat.txt’ -> ‘/tmp/DEST/redhat.txt’

How to make a symbolic link with Linux cp command to files ?

As we know that ln command us useful to create symboic links, But the Linux copy command Syntax can do that to files with -s Option which creates Symbolic links:

cp -s SOURCE DESTINATION

Linux copy command Syntax with Softlink with Demonstration:

[vamshi@linuxcent ~]$ ls -l total 0
-rw-rw-r--. 1 vamshi vamshi 0 Apr 11 06:39 file1.txt

lrwxrwxrwx. 1 vamshi vamshi 9 Apr 11 06:39 file2.txt -> file1.txt

Linux cp command with interactive prompt using -i option

Sample Syntax:

cp -i file1 file1-copy

Also you can make it a best practice to setup alias alias for cp command.
The best practice is enable options -av

cp -av SOURCE DESTINATION
export cp="cp -av"

How can i copy the hidden files ?

To Copy the hidden files we can use cp command with -a option,lets us see in a practical example.

$ cp -av source/ destination/
‘source/.config1’ -> ‘destination/source/.config1’
‘source/.config2’ -> ‘destination/source/.config2’
‘source/.config3’ -> ‘destination/source/.config3’

Generally the hidden files in Linux are prefixed with a dot . So we can also use the wildcard character *, and copy them, below is another pracctical example

[vamshi@linuxcent cp-command]$ cp -av source/.conf* destination/
‘source/.config1’ -> ‘destination/.config1’
‘source/.config2’ -> ‘destination/.config2’
‘source/.config3’ -> ‘destination/.config3’

How to Copy a File from One Location to Another With a Different Name on Linux Using the cp Command

Assuming we have a couple of users on our linux server called Alice and Bob

[alice@linuxcent ~]$ sudo cp -avrpf /home/alice/djangoproject1/ /home/bob/
‘djangoproject1/’ -> ‘/home/bob/djangoproject1’
‘djangoproject1/__init__.py’ -> ‘/home/bob/djangoproject1/__init__.py’
‘djangoproject1/asgi.py’ -> ‘/home/bob/djangoproject1/asgi.py’
‘djangoproject1/settings.py’ -> ‘/home/bob/djangoproject1/settings.py’
‘djangoproject1/urls.py’ -> ‘/home/bob/djangoproject1/urls.py’
‘djangoproject1/wsgi.py’ -> ‘/home/bob/djangoproject1/wsgi.py’
‘djangoproject1/__pycache__’ -> ‘/home/bob/djangoproject1/__pycache__’
‘djangoproject1/__pycache__/__init__.cpython-36.pyc’ -> ‘/home/bob/djangoproject1/__pycache__/__init__.cpython-36.pyc’
‘djangoproject1/__pycache__/settings.cpython-36.pyc’ -> ‘/home/bob/djangoproject1/__pycache__/settings.cpython-36.pyc’

How to backup files using cp command?

The linux cp command offer the option --backup to backup the data files, below is the command.

cp --backup source destination