I have a file of the following structure:

cat .bash_history 

i.e. timestamp followed by a text line. I want to process every timestamp line using shell's date (something like date -d ${1/\#/@} '+%F %T'), merge it with the next line and wrap with table tags, i.e. the output would be the following:

<tr> <td>2019-03-11 18:12:15</td> <td><pre>ls</pre></td> </tr>
<tr> <td>2019-03-11 18:12:39</td> <td><pre>date</pre></td> </tr>
<tr> <td>2019-03-11 18:13:00</td> <td><pre>cat .bash_history</pre></td> </tr>

I understand it would be something like awk 'NR % 2 {print | " <something here>" } !(NR % 2) {p=$0}' input_file - but I can't find the way to implement all the necessary transformations in one command.

  • GNU awk, available on most systems with bash, can do this timestamp conversion internally without running the date program, and is probably 10 to 100 times faster. – dave_thompson_085 Mar 13 at 9:27

Beware the following code lacks error-handling. Explanations are embedded by way of inline comments

awk '{

    #strip leading "#", run through date and read into $0
    gsub(/^#/, ""); "date -d @"$0" \"+%F %T\"" | getline; 
    #wrap with table tags and print
    printf "<tr> <td>%s</td>", $0;
    #read the subsequent "non-date" line
    #wrap with table tags and print
    printf " <td><pre>%s</pre></td> </tr>\n", $0

}' file
  • thank you so much!!! (it took me a while to understand how your magic code works...) – Vasily A Mar 13 at 5:49

With GNU Awk (gawk) you could use the built-in strftime:

gawk '
    NR%2 {ts = strftime("%F %T",substr($0,2)); next} 
         {printf("<tr> <td>%s</td> <td><pre>%s</pre></td> </tr>\n",ts,$0)}
' input_file 
<tr> <td>2019-03-11 18:12:15</td> <td><pre>ls</pre></td> </tr>
<tr> <td>2019-03-11 18:12:39</td> <td><pre>date</pre></td> </tr>
<tr> <td>2019-03-11 18:13:00</td> <td><pre>cat .bash_history </pre></td> </tr>
  • cool, that's even better! thank you! – Vasily A Mar 13 at 12:48

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