Tell me the best way to add a backup date?

I need to make a backup of a file and i would like to have a timestamp as part of the name to make it easier to distinguish

What is a copy command? how can you insert the date?

[root@mongo-test3 ~]# cp foo.txt {,.backup.`date`}
cp: target `2013}' is not a directory

[root@mongo-test3 ~]# cp foo.txt {,.backup. $((date)) }
cp: target `}' is not a directory

[root@mongo-test3 ~]# cp foo.txt foo.backup.`date`
cp: target `2013' is not a directory

This isn't working because the command date returns a string with spaces in it.

$ date
Wed Oct 16 19:20:51 EDT 2013

If you want file names such as that you'll need to wrap this string in quotes

$ touch "foo.backup.$(date)"

$ ll foo*
-rw-rw-r-- 1 saml saml 0 Oct 16 19:22 foo.backup.Wed Oct 16 19:22:29 EDT 2013

You're probably thinking of a different string to be appended would be my guess though. I usually use something similar

$ touch "foo.backup.$(date +%F_%R)"
$ ll foo*
-rw-rw-r-- 1 saml saml 0 Oct 16 19:25 foo.backup.2013-10-16_19:25

See the man page for date for more formatting codes around the output for the date & time.

Additional formats

If you want to take full control of the site if you look at the site you can do it like this

$ date +"%Y%m%d"
20131016

$ date +"%Y-%m-%d"
2013-10-16

$ date +"%Y%m%d_%H%M%S"
20131016_193655

NOTE: You can use date -I or date --iso-8601 which will produce identical output to date +"%Y-%m-%d . This switch also has the ability to take an argument to indicate various time formats.

$ date -I=?
date: invalid argument ‘=?’ for ‘--iso-8601’
Valid arguments are:
  - ‘hours’
  - ‘minutes’
  - ‘date’
  - ‘seconds’
  - ‘ns’
Try 'date --help' for more information.

You may want some examples

$ date -Ihours
2019-10-25T01+0000

$ date -Iminutes
2019-10-25T01:21+0000

$ date -Iseconds
2019-10-25T01:21:33+0000