I want to write a value to a text file with a timestamp. The data/value gets updated every few minutes, but I only want to record the last value and its date. The next day I want to start again with a new line but keeping the previous value. The output will then be charted showing daily values.
I can do this with an if, then, else loop, but I imagine there's a more elegant awk, sed, 'something' solution.
This will be run by a cron job.
Day 1. Every 5 minutes insert a row with $date, $value
During the same day keep updating that row
Day 2. Insert a new row and repeat
Desired output
Day 1 10am
2021-08-17, 5.20
Day 1 11pm
2021-08-17, 12.10
Day 2 10am
2021-08-17, 12.10
2021-08-18, 4.90
Day 2 11pm
2021-08-17, 12.10
2021-08-18, 13.10
One idea would be to skip/ignore/delete the line with today's date (if it exists) and then append a new line with today's date.
Sample data file:
$ cat date.log
2021-08-14, 23.1
2021-08-15, 17.3
2021-08-16, 9.3
$ today=$(date '+%Y-%m-%d')
$ echo $today
2021-08-17
$ newvalue=13.2
One sed idea to implement this logic:
$ sed -i -n -e "/^${today}, .*$/"'!p' -e '$a'"${today}, ${newvalue}" date.log
Where:
-i -n -e - -inplace update of source file, -n suppress automatic printing of pattern space, -e designate a piece of script"/^${today}, .*$/" - search for pattern that matches (start of line) + ${today} + , + rest of line; need to use double quotes so that ${today} is replaced with it's actual value'!p' - reverse pattern search and print line (ie, print everything but line that matches ^${today}, .*$); need to use single quotes since !p in double quotes will be replaced with the last history command that started with the letter p-e '$a' - another piece of script that finds end of file ($) and appends the following string; must use single quotes so that bash doesn't try to replace the literal $a with the contents of variable a"${today}, ${newvalue}" - string to append to end of fileIf we preface our sed call with set -xv (enable debug mode) we see the following is printed at the console:
+ sed -i -n -e '/^2021-08-17, .*$/!p' -e '$a2021-08-17, 13.2' date.log
And the contents of our file:
$ cat date.log
2021-08-14, 23.1
2021-08-15, 17.3
2021-08-16, 9.3
2021-08-17, 13.2 # new line appended to file
A couple more runs (after issuing set +xv to disable debug mode):
$ newvalue=32.7
$ sed -i -n -e "/^${today}, .*$/"'!p' -e '$a'"${today}, ${newvalue}" date.log
$ cat date.log
2021-08-14, 23.1
2021-08-15, 17.3
2021-08-16, 9.3
2021-08-17, 32.7 # updated (really deleted and appended)
$ newvalue=73.xxx
$ sed -i -n -e "/^${today}, .*$/"'!p' -e '$a'"${today}, ${newvalue}" date.log
$ cat date.log
2021-08-14, 23.1
2021-08-15, 17.3
2021-08-16, 9.3
2021-08-17, 73.xxx # updated (really deleted and appended)
$ today='2021-09-23'
$ newvalue=7.25
$ sed -i -n -e "/^${today}, .*$/"'!p' -e '$a'"${today}, ${newvalue}" date.log
$ cat date.log
2021-08-14, 23.1
2021-08-15, 17.3
2021-08-16, 9.3
2021-08-17, 73.xxx
2021-09-23, 7.25 # new line appended to file