how to convert GMT to IST using awk command - bash

Question

In nginx access.log we have time format in GMT timezone. using awk command,converting logs to csv file.
awk '{print $4","$7,"$10","$15","$16","$17}' </usr/local/nginx/logs/access.log>access.csv
$4 - displays date in GMT (31/Jul/2015:16:03:07).
Kindly guide how to change it to IST and update in that csv file.

Answer 1

If you have access to GNU awk and GNU date, you could use TZ to easily get time at IST:
$ TZ='Asia/Kolkata' date -d "#1438374787"
Where the '1438374787' value is the seconds since epoch in GMT, also known as systime.
To make a systime out of a date, we could use mktime from (GNU) awk:
Convert the GMT string to epoch time directly in awk:
$ echo "a test date 31/07/2015:16:33:07" | awk '{gsub("[/:]"," ",$4); $0=$0;t=sprintf("%d %d %d %d %d %d 0",$6,$5,$4,$7,$8,$9);print(mktime(t))}'
For that to work, you need all values to be numbers (no jul, sorry). If that is not possible, you need to work out a format for an string that the command date could read correctly (sometimes not an easy task).
Placing all in one script:
#!/bin/bash --
echo "a test date 31/07/2015:16:33:07" | awk '{
split($4,A,"[:/]");
gmtdate=sprintf("%d %d %d %d %d %d",A[3],A[2],A[1],A[4],A[5],A[6]);
print gmtdate;
T1=mktime(gmtdate);
print T1;
pt = sprintf ("TZ=%s%s%s date -d %s%s%s%s","\047","Asia/Kolkata","\047","\042","#",T1,"\042");
print pt;
system(pt);
}'
And, running it:
$ ./stackoverflow-awk-time-test.sh
2015 7 31 16 33 7
1438374787
TZ='Asia/Kolkata' date -d "#1438374787"
Sat Aug 1 02:03:07 IST 2015
Several additional debug values are printed from awk (easy to remove).