I have a file abc.fil whose content is in the format yyyymmdd.xyz.doc. I want to cat that file abc.fil such that I get the output as 20150127.xyz.doc. Can any one please help me to resolve this problem.

Sample file:

$ cat abc.fil
o/p: yyyymmdd.xyz.doc

Output expected: Instead of yyyymmdd I want the current date to be displayed in the same order.

  • 2
    It doesn't have much to do with cat. Actually, nothing at all. You just want to replace a pattern with a current date.
    – orion
    Jan 27 '15 at 9:52

You could use sed:

sed "s/yyyymmdd/$(date '+%Y%m%d')/g" abc.fil

That replaces the string yyyymmdd with the current date formatted as desired.


If yyyymmdd is just the format of the date you want to replace, then use that command (assumes GNU sed):

sed -r "s/[12][0-9]{3}[01][0-9][0-3][0-9]/$(date '+%Y%m%d')/g" abc.fil

The long regular expression pattern means the following: The first digit can be 1 or 2 ([12]), the next three can be everything from 0 to 9 ([0-9]), that's the year. Now the month: the first digit can be 0 or 1 and the second can be everything from 0 to 9 ([01][0-9]). And at last the same with the day.

  • Why do you have so many {1}s? They do absolutely nothing.
    – Doorknob
    Jan 27 '15 at 13:40
  • @Doorknob I see, they are not necessary, I updated my answer, thanks
    – chaos
    Jan 27 '15 at 13:44
> today=$(date +%Y%m%d)
> sed "s/yyyymmdd/${today}/g" abc.fil
o/p: 20150127.xyz.doc


cat abc.fil | sed s/yyyymmdd/`date +%Y%m%d`/
  • 3
    Why the downvote? That command will work in csh/tcsh and in Bourne-like shells provided $IFS doesn't contain digits (cat is redundant though). Jan 27 '15 at 9:38
  • Not from me :-) Jan 27 '15 at 9:39
  • 1
    It's better to use $(...) instead of backticks. Otherwise, it works.
    – orion
    Jan 27 '15 at 9:54
  • Why is it better (appart from the need to escape the quotes for this forum)?
    – YoMismo
    Jan 27 '15 at 10:09
  • I'll answer my question myself. It isn't better, it only makes a difference which is that the quotes retain the meaning of the backslash while $() does not treat it specially. tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html
    – YoMismo
    Jan 27 '15 at 10:26

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