Using and manipulating date variable inside AWK

Question

I assume that there may be a better way to do it but the only one I came up with was using AWK.

I have a file with name convention like following:

testfile_2016_03_01.txt

Using one command I am trying to shift it by one day testfile_20160229.txt

I started from using:

file=testfile_2016_03_01.txt IFS="_" arr=($file) datepart=$(echo ${arr[1]}-${arr[2]}-${arr[3]} | sed 's/.txt//') date -d "$datepart - 1 days" +%Y%m%d

the above works fine, but I really wanted to do it in AWK. The only thing I found was how to use "date" inside AWK

new_name=$(echo ${file##.*} | awk -F'_' ' { "date '+%Y%m%d'" | getline date; print date }') echo $new_name

okay so two things happen here. For some reason $4 also contains .txt even though I removed it(?) ##.*

And the main problem is I don't know how to pass the variables to that date, the below doesn't work

`awk -F'_' '{"date '-d "2016-01-01"' '+%Y%m%d'" | getline date; print date}')

ideally I want 2016-01-01 to be variables coming from the file name $2-$3-$4 and substract 1 day but I think I'm getting way too many single and double quotes here and my brain is losing..

Remove from end: ${file%%.*} instead of ## (or just one % as there is just one . anyway). — Benjamin W.
– Benjamin W., Commented Mar 4, 2016 at 21:55
Thanks BenjaminW, it works (though I don't know why ## didn't, I do have it in different scripts). @EdMorton I do have gawk yes — Moseleyi
– Moseleyi, Commented Mar 4, 2016 at 22:03
## removes matching from the start, you'd use it to get, e.g., an extension or basename: var='/abc/def/file.txt'; echo "${var#*.}"; echo "${var##*/}" returns txt and file.txt. — Benjamin W.
– Benjamin W., Commented Mar 4, 2016 at 22:51

anubhava · Accepted Answer · 2016-03-04 22:03:38Z

Equivalent awk command:

file='testfile_2016_03_01.txt' echo "${file%.*}" | awk -F_ '{cmd="date -d \"" $2"-"$3"-"$4 " -1 days\"" " +%Y%m%d"; cmd | getline date; close(cmd); print date}' 20160229

Ed Morton · Accepted Answer · 2016-03-04 22:22:26Z

WIth GNU awk for time functions:

$ file=testfile_2016_03_01.txt $ awk -v file="$file" 'BEGIN{ split(file,d,/[_.]/); print strftime(d[1]"_%Y%m%d."d[5],mktime(d[2]" "d[3]" "d[4]" 12 00 00")-(24*60*60)) }' testfile_20160229.txt

potong · Accepted Answer · 2016-03-05 11:39:18Z

This might work for you:

file='testfile_2016_03_01.txt' IFS='_.' read -ra a <<< "$file" date -d "${a[1]}${a[2]}${a[3]} -1 day" "+${a[0]}_%Y%m%d.${a[4]}"

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