Go - Logging to File

Problem: You want to log events to a logfile instead of standard error.

Solution: Use the SetOutput function to set the log to write to a file.

You use SetOutput to redirect the output to a file.

file ,   err   :=   os . OpenFile ( "app.log" ,   os . O_APPEND | os . O_CREATE | os . O_WRONLY ,   0644 ) 
if   err   !=   nil   { 
      log . Fatal ( err ) 
} 
defer   file . Close ()

You call SetOutput with the file as the parameter, then continue to log:

log . SetOutput ( file )
log . Println ( "Some event happened" )

The output will be written to the app.log file instead of the screen.

As you probably realize from the code, setting up the logs to be written to file should be done once. What if you want to write to the screen and the file simultaneously? You could reset the log output each time (don’t do this) or maybe create two loggers, one for standard error and another for a file, then call the loggers in separate statements.
Or you can use Go’s MultiWriter function in the io package, which creates a writer that duplicates its writes to all the provided writers:

file ,   err   :=   os . OpenFile ( "app.log" ,   os . O_APPEND | os . O_CREATE | os . O_WRONLY ,   0644 ) 
if   err   !=   nil   { 
      log . Fatal ( err ) 
} 
defer   file . Close () 
writer   :=   io . MultiWriter ( os . Stderr ,   file ) 
log . SetOutput ( writer ) 
log . Println ( "Some  event  happened" ) 
log . Println ( "Another  event  happened" )

Doing this will write to both the screen and the file simultaneously. In fact, you can write to more than two writers!