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I am struggling to convert a textfile which contains the string:

10.04.2015 12:00:15

to 

150410

The date will always be the date format but is taken from a textfile so as far as I know I can't use a date command and I think I must use awk but I don't know it well and am struggling - advice would be appreciated?

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    Note that the formats in your title are different to the ones in your input / desired output. Commented Apr 16, 2015 at 15:21
  • see stackoverflow.com/questions/6508819/… Commented Apr 16, 2015 at 15:27
  • Cheers, I'll take the advice and use the one with more options for later use. Commented Apr 16, 2015 at 15:28
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    If it's the only data in the file then e.g. awk '{ print substr($0, 7, 2) substr($0, 3, 2) substr($0, 1, 2) }' but realistically, we need to understand what else there is in the file. Show us a real representative sample input and the desired output. Commented Apr 16, 2015 at 15:33
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    If you could get rid of the dots and replace them with slashes, date -d"10/04/2015 12:00:15" "+%y%d%m" makes it. Commented Apr 16, 2015 at 15:40

3 Answers 3

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You can use bash parameter expansion to extract substrings

$ date="10.04.2015 12:00:15" $ newdate=${date:8:2}${date:3:2}${date:0:2} $ echo $newdate 150410
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4 Comments

Looks like the best of all alternatives (if the shell is bash)
...and the input is a string, rather than a file (which I don't think is the case here)
@hek2mgl, and ksh93 and zsh
Thanks, I did indeed go for this solution, the input is a string and I do use bash but ultimately I used this one as it is simply the easiest to see how it works. I am too new to sed/awk and regex's to fully understand everything but the parameter expansion is easily readable and easier to follow. Thanks, thanks for your help too Tom.
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The following sed command should be good enough:

sed -r 's/([0-9]+)\.([0-9]+)\.[0-9]{2}([0-9]{2}) ([0-9]+:?){3}/\3\2\1/g' input

or awk:

awk -F'[.: ]' '{print substr($3,3),$2,$1}' OFS='' input

After all I prefer the latter.

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I ultimately went for the above siolution as it is simply easier to use for a beginner - note I tested these and they do the job nicely too. Thanks for the help/advice.
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alternative answer, using awk:

DATE="10.04.2015 12:00:15" echo $DATE | awk '{print substr($0,9,2) substr($0,4,2) substr($0,1,2)}'

Some explanation if you want to do modifications later:

  • $0 is the input text on which substr operates
  • the the first digit is where the substring starts
  • the second digit indicates the length of the substring
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be carefull about which contains the string whre your code need a first extraction before occuring if it is inside a bigger string. Mybe assume that string to treat is only with this information

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