How To Use Date Command In Bash Scripting In Linux

When I started writing bash scripts, I realized that one operation I am often dealing with every script is handling date and time. I have done many operations like naming a file/directory with date, fetching API result, and transforming the epoch value to date/time suitable for the application, manipulating date column in CSV files, etc. If you are a beginner for Bash scripting, this article will help in understanding how to use date command in Bash scripting, how to work with date and time using date command in shell scripts, how to manipulate them and do various operations with it in Linux.

You should have a basic understanding of how a system date and time is set. This part will mostly be taken care of by the server admin in your work.

But if you are an individual user and using Linux in your personal machine, then you should have an understanding of how date and time are set, ways to synchronize them, and how to modify it.

There are two clocks used by Linux to keep the time sync.

• Real-Time Clock - This is a clock integrated into your system motherboard and will run even if you shut down the machine.
• System Clock - This will be handled by the Linux kernel and during the system boot kernel will get the initial time from the real-time clock.

I am not going to go through the process of setting up a date and time, that’s a separate topic to discuss. Let’s start looking into the core part of this article on how to use the date command in Bash scripting in Linux.

Getting Help

First, you can check the binary location of the date command by running the following command.

$ which date
/usr/bin/date

The date command is not a shell builtin but an external program. You can get that information by running the type command.

$ type -a date
date is /usr/bin/date
date is /bin/date

There are many options to work with and remembering everything is not possible and this is where the help and man page will come in handy.

Run the following command to access the man page of date command.

$ man date

Run the following command to access the help message.

$ date --help

Print Current Date And Time

Launch your terminal and simply run the date command. You will get the current date along with the time and timezone. In my case, I have set the timezone as IST during my OS installation.

$ date
Sunday 13 February 2022 11:27:17 AM IST

You can also get the date and time in the UTC timezone by using the -u flag.

$ date -u
Sunday 13 February 2022 05:57:36 AM UTC

TimeZone Conversion

You can get the output of the date command in different timezones by setting the environmental variable TZ. I have set the TZ to Moscow and Singapore time zone in the below example.

$ Z="Europe/Moscow" date
Sunday 13 February 2022 08:59:03 AM MSK

$ TZ="Asia/Singapore" date
Sunday 13 February 2022 01:59:37 PM +08

You can get the list of timezones by running the "timedatectl" command.

$ timedatectl list-timezones

$ timedatectl list-timezones | grep -i singapore
Asia/Singapore
Singapore

$ timedatectl list-timezones | grep -i moscow
Europe/Moscow

Last Modified Time Of A File

Normally, we use the stat command to check the modified time of a file. The same can be checked with the date command using the -r flag.

$ stat ~/.bashrc
  File: /home/karthick/.bashrc
  Size: 4342          Blocks: 16         IO Block: 4096   regular file
Device: 806h/2054d    Inode: 27787278    Links: 1
Access: (0644/-rw-r--r--)  Uid: ( 1000/karthick)   Gid: ( 1000/karthick)
Access: 2022-02-13 09:06:45.713721080 +0530
Modify: 2022-02-11 20:18:52.485178931 +0530
Change: 2022-02-11 20:18:52.485178931 +0530
 Birth: -

$ date -r ~/.bashrc
Friday 11 February 2022 08:18:52 PM IST

You can see the stat command output format is different from the date command, but both serve the same purpose.

Different Formatting Options

The date command supports formatting options and using these options you can format your date command output to be in any format you desire or you can grab only the particular value from the output.

Open the date command's man page and look for the section as shown in the below image.

There are a lot of formatting options, but not all will be useful. We will see some of the important formatting options and understand how it works.

By default, the date command will display the date and time. What if you wish to print only the date or time?

$ date
Sunday 13 February 2022 11:27:17 AM IST

To get only the date, you can use the %F flag or %D flag.

$ date +%F
2021-11-16

$ date +%D
02/13/22

When you are using formatter flags, you should prefix it with + symbol as I did in the above examples.

• The output from the %F flag will be in YYYY-MM-DD format.
• The output from the %D flag will be in MM/DD/YY format.

You can grab the day, month, and year individually and create your own new formatted output.

$ date +%Y
2022

$ date +%m
02

$ date +%d
13

To create your own formatted output, just combine all the flags a shown below.

$ date +%Y_%m_%d
2022_02_13

Similar to date, you can also get the time alone using %r(12 hour format) or %T(24 hour format) flags.

$ date +%r # 12 hour format
11:24:12 PM IST

$ date +%T # 24 hour format
23:24:11

You can grab hours, minutes, seconds individually and create your own formatted output.

$ date +%H
12

$ date +%M
24

$ date +%S
03

Combine all the above flags and create custom output.

$ date +%H_%M_%S
12_26_53

Now where do these custom formatting options help? For me, when I create any files as part of the script, I will name it with today’s date. So It will be useful for me to take a look at it when needed.

Till now we have seen how to work with date and time individually. Sometimes, you may want to display both date and time. In that case, you can combine the date and time like below or create your own formatted output.

$ date "+%F %r"
2021-11-16 11:29:06 PM IST

Run Script Only On Certain Days

In some cases, you may want to run your script only on a certain day of the week. For example, if I want my script to run only on Friday, then I can use %A(Friday) or %a(Fri) flags and write logic accordingly.

if [[ $(date +%A) = Friday ]]
then
  echo "Running the code"
else
  echo "Today is not Friday, Exiting the script"
fi
OUTPUT:
Today is not Friday, Exiting the script

There are more formatting options available. Refer to the man page and explore them all.

Storing Output To A Variable

When writing scripts it is a best practice to store the output of the date command to a variable and use the variable in the script. One such scenario will be creating the file name with the date and time.

Take a look at the below example where today's date will be stored in variable TDY and the same is used to create a file name "disk_usage_${TDY}.txt" which will store the output of the "df -h" command.

$ TDY=$(date +%F)

$ df -h > ~/disk_usage_${TDY}.txt

$ ls -l *.txt
-rw-rw-r-- 1 karthick karthick 474 Feb 13 23:40 disk_usage_2022-02-13.txt

$ cat disk_usage_2022-02-13.txt 
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           1.6G  154M  1.5G  10% /run
/dev/sda7        93G   27G   61G  31% /
tmpfs           7.8G   82M  7.7G   2% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           4.0M     0  4.0M   0% /sys/fs/cgroup
/dev/sda9       261G  130G  118G  53% /nextdrive
/dev/sda6       348G  110G  221G  34% /home
/dev/sda5       2.0G  234M  1.8G  12% /boot/efi
tmpfs           1.6G  180K  1.6G   1% /run/user/1000

Using -d or --date Flag

This is an important feature I like the most with the date command. You can pass the string as an argument to -d or --date flag and you will get the output in the default format.

Take a look at the below examples. I have passed string arguments as "today", "yesterday", "tomorrow", "last Thursday", "Today (+,-) 20 years" to the --date flag. Date command will interpret these strings and convert the output to its default format.

$ date --date "Today"
Sunday 13 February 2022 01:01:26 PM IST

$ date --date "now"
Sunday 13 February 2022 01:01:34 PM IST

$ date --date "yesterday"
Saturday 12 February 2022 01:01:39 PM IST

$ date --date "Tomorrow"
Monday 14 February 2022 01:01:45 PM IST

$ date --date "Last Thursday"
Thursday 10 February 2022 12:00:00 AM IST

$ date --date "Today - 20 years"
Wednesday 13 February 2002 01:01:56 PM IST

$ date --date "Today + 20 years"
Thursday 13 February 2042 01:02:01 PM IST

You can pass more relative string arguments other than what I have shown in the above example.

Epoch Time

Epoch time can be converted to the current date and time format using the -d or --date flag. The %s flag output will be in epoch format.

$ date +%s
1644737777

To convert it to standard human-readable format, add @ followed by epoch time like as shown below.

$ date --date @1644737777
Sunday 13 February 2022 01:06:17 PM IST

In all the above examples, you can see the default output is displayed. You can combine the formatters along with the -d or --date flag to get custom output.

$ date --date "Today + 20 years" +%Y
2042

$ date --date "Today + 21 years" +%s
2275890194

$ date --date @2275890194 +%Y
2042

Conclusion

In this article, we discussed how to use the date command in Bash scripting with examples. By this time, you should be comfortable in using the date command in your scripts. There are a lot of formatting options available, test and get familiar with them.

If you have any feedback or suggestions about this article, let us know via the comment section below.