Generating a filename from the system date

anandg111 · 09-24-2013, 02:57 PM

set -- `date`
year=`expr $6 : '..$..$'`
echo "$2$3_$year"

can someone please explain this set of commands ?

Firerat · 09-24-2013, 03:10 PM

just seems an incredibly tedious way of doing

Code:

date +%d%b_%y

suicidaleggroll · 09-24-2013, 04:47 PM

Quote:

Originally Posted by Firerat

just seems an incredibly tedious way of doing

Code:

date +%d%b_%y

Should be

Code:

date +%b%d_%y

At least on my machine...but either way, yes, it looks like an inefficient, fragile, tedious approach when a single date command could do it better.

To answer the fundamental question - commands in a BASH script are no different than commands on the command line, so why not just run them and see what they do? You'll learn much more than asking us to interpret code. For example, anything in back ticks is a command, and the output of which is stuck in its place.

So for the first command, "set -- `date`". First "date" is run, and the output is passed to "set --". So to test it, just run "date" on the command line, look at what the output is. Then run "set -- `date`". The next few lines are referencing $2, $3, and $6, clearly these are variables set up by the set command, so just print them out and see what they are. Now that you know what $6 is, run the expr command to see what it's doing and what it's assigning to the "year" variable. Finally, print out the final line to see all of it:

Code:

$ date
Tue Sep 24 15:52:53 MDT 2013
$ set -- `date`
$ echo $2
Sep
$ echo $3
24
$ echo $6
2013
$ expr $6 : '..\(..\)'
13
$ year=`expr $6 : '..\(..\)'`
$ echo $year
13
$ echo $2$3_$year
Sep24_13

frankbell · 09-24-2013, 09:29 PM

A long time ago, in my first (and so far only) significant foray into BASH scripting (to automate mysql dumps in this case), I set a directory name using a date.

I'm sure this is very clumsy, but it worked. I extended it to use hours and minutes because, in testing the script, I wanted to be able to run it multiple times in the same day.

Code:

#Define now as today's day-month-year-hour-minute.

NOW=$(date +%m-%d-%Y-%H-%M)

#Make the directory to hold the backup file.

mkdir /root/backups/sql/$NOW

Firerat · 09-25-2013, 01:26 AM

ahh, yes my bad...
in my defence I was resisting YYYY-MM-DD , which in my opinion is the true way to represent dates

Code:

date +%Y-%m-%d

suicidaleggroll · 09-25-2013, 08:35 AM

Quote:

Originally Posted by Firerat

ahh, yes my bad...
in my defence I was resisting YYYY-MM-DD , which in my opinion is the true way to represent dates

Code:

date +%Y-%m-%d

Agreed, that way alphabetical = chronological order.