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I have a stream of info from a serial input (GPS Antennae) and wish to output that info into a text file on every input (every second in this case) but instead of appending it to the end of the file as > would do after the initial overwrite I would like it to overwrite it every second so only the latest info is displayed.

I have tried \r which achieves the effect in bash but not the output file.

cat /dev/ttyACM0 | grep --line-buffered  -E "GNGGA" | awk 'BEGIN {FS=","};{printf "%s%s\t\t%s%s\t\t%s%s\t%s%s","Time= ",$2,"Lat= ",$3,"Lon= " ,$5,"Alt= " ,$10; fflush(stdout) }' > somefiles.txt

This includes the initial input, a grep to focus on one line and awk to get the specific parts of the info I need, they don't affect the overwrite issue as far as I know. 

Time= 155325.00     Lat= 7428.77433     Lon= 82845.15963    Alt= 21.5

This is the output that starts by overwriting the somefiles.txt but then appends until you stop and run the command again.

So is there a way to make only the latest input appear as one line in the text file?

Thanks

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You can print or printf straight to a file within awk, and close it after every write. That would make awk reopen and truncate it on the next print. 

awk -vfile=test.out '{print $0 > file; close(file)}'

(Strictly speaking you get a race condition here, another process might try to read the file just between the truncate and write, so it would seem empty (or worse but less likely, partial).)

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  • Thanks for that, it works as needed. The race condition is something I'll check when the txt file is accessed to make sure it's there and complete.
    – pingualoty
    Apr 7, 2017 at 9:28

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