How to write log to file

Question

I m trying to write to a log file with Go   I have tried several approaches  all of which have failed  This is what I have tried   func TestLogging t  testing T        if  FileExists  logfile             CreateFile  logfile             f  err    os Open  logfile       if err    nil           t Fatalf  error   v   err                attempt  1     log SetOutput io MultiWriter os Stderr  f       log Println  hello  logfile           attempt  2     log SetOutput io Writer f       log Println  hello  logfile           attempt  3     log SetOutput f      log Println  hello  logfile      func FileExists name string  bool       if    err    os Stat name   err    nil          if os IsNotExist err                return false                     return true    func CreateFile name string  error       fo  err    os Create name      if err    nil           return err           defer func             fo Close               return nil     The log file gets created  but nothing ever gets printed or appended to it  Why

User · Answer

I usually print the logs on screen and write into a file as well  Hope this helps someone   f  err    os OpenFile   tmp orders log   os O RDWR os O CREATE os O APPEND  0666  if err    nil       log Fatalf  error opening file   v   err    defer f Close   wrt    io MultiWriter os Stdout  f  log SetOutput wrt  log Println   Orders API Called

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I prefer the simplicity and flexibility of the 12 factor app recommendation for logging  To append to a log file you can use shell redirection  The default logger in Go writes to stderr  2      app 2 gt  gt  logfile   See also  http   12factor net logs

User · Answer

os Open   must have worked differently in the past  but this works for me   f  err    os OpenFile  testlogfile   os O RDWR   os O CREATE   os O APPEND  0666  if err    nil       log Fatalf  error opening file   v   err    defer f Close    log SetOutput f  log Println  This is a test log entry     Based on the Go docs  os Open   can t work for log SetOutput  because it opens the file  for reading       func Open      func Open name string   file  File  err error  Open opens the named   file for reading  If successful  methods on the returned file can be   used for reading  the associated file descriptor has mode O RDONLY  If   there is an error  it will be of type  PathError    EDIT  Moved defer f Close   to after if err    nil check

User · Answer

Building on Allison and Deepak s answer  I started using logrus and really like it   var log   logrus New    func init             log to console and file     f  err    os OpenFile  crawler log   os O RDWR os O CREATE os O APPEND  0666      if err    nil           log Fatalf  error opening file   v   err            wrt    io MultiWriter os Stdout  f       log SetOutput wrt      I have a defer f Close   in the main function

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I m writing logs to the files  which are generate on daily basis  per day one log file is getting generated   This approach is working fine for me     var       serverLogger  log Logger    func init            set location of log file     date    time Now   Format  2006-01-02       var logpath   os Getenv constant XDirectoryPath    constant LogFilePath   date   constant LogFileExtension     os MkdirAll os Getenv constant XDirectoryPath  constant LogFilePath  os ModePerm      flag Parse       var file  err1   os OpenFile logpath  os O RDWR os O CREATE os O APPEND  0666       if err1    nil           panic err1            mw    io MultiWriter os Stdout  file      serverLogger   log New mw  constant Empty  log LstdFlags      serverLogger Println  LogFile       logpath        LogServer logs to server s log file func LogServer logLevel enum LogLevel  message string           file  no  ok    runtime Caller 1      logLineData     logger server go      if ok           file   shortenFilePath file          logLineData   fmt Sprintf file   constant ColonWithSpace   strconv Itoa no    constant HyphenWithSpace            serverLogger Println logLineData   logLevel String     constant HyphenWithSpace   message        ShortenFilePath Shortens file path to a b c d go tp d go func shortenFilePath file string  string       short    file     for i    len file  - 1  i  gt  0  i--           if file i     constant ForwardSlash               short   file i 1               break                     file   short     return file      shortenFilePath    method used to get the name of the file from full path of file  and  LogServer    method is used to create a formatted log statement  contains   filename  line number  log level  error statement etc

User · Answer

If you run binary on linux machine you could use shell script   overwrite into a file    binaryapp  gt  binaryapp log   append into a file    binaryapp  gt  gt  binaryapp log   overwrite stderr into a file    binaryapp  amp  gt  binaryapp error log   append stderr into a file    binaryapp  amp  gt  gt  binalyapp error log   it can be more dynamic using shell script file

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The default logger in Go writes to stderr  2   redirect to file   import         syscall       os      func main      fErr  err   os OpenFile  Errfile   os O APPEND os O WRONLY os O CREATE  0600    syscall Dup2 int fErr Fd     1     -- stdout      syscall Dup2 int fErr Fd     2     -- stderr

User · Answer

To help others  I create a basic log function to handle the logging in both cases  if you want the output to stdout  then turn debug on  its straight forward to do a switch flag so you can choose your output   func myLog msg    interface          defer func     r    recover    if r    nil   fmt Print  Error detected logging    r            if conf DEBUG           fmt Println msg        else           logfile  err    os OpenFile conf LOGDIR     conf AppName   log   os O RDWR   os O CREATE   os O APPEND 0666          if  checkErr err                log SetOutput logfile              log Println msg                    defer logfile Close

User · Answer

maybe this will help you  if the log file exists use it  if it does not exist create it   package main  import        quot flag quot       quot log quot       quot os quot      Se declara la variable Log  Esta ser   usada para registrar los eventos  var       Log  log Logger   Loggerx      func Loggerx    log Logger       LOG FILE LOCATION    os Getenv  quot LOG FILE LOCATION quot             En el caso que la variable de entorno exista  el sistema usa la configuraci  n del docker      if LOG FILE LOCATION     quot  quot            LOG FILE LOCATION    quot    logs  quot    APP NAME    quot  log quot        else           LOG FILE LOCATION   LOG FILE LOCATION   APP NAME    quot  log quot            flag Parse             Si el archivo existe se rehusa  es decir  no elimina el archivo log y crea uno nuevo      if    err    os Stat LOG FILE LOCATION   os IsNotExist err            file  err1    os Create LOG FILE LOCATION          if err1    nil               panic err1                              si no existe se crea uno nuevo          return log New file   quot  quot   log Ldate log Ltime log Lshortfile        else                     si existe se rehusa          file  err    os OpenFile LOG FILE LOCATION  os O CREATE os O APPEND os O WRONLY  0666          if err    nil               panic err                    return log New file   quot  quot   log Ldate log Ltime log Lshortfile           For more detail  https   su9 co 9BAE74B

User · Answer

This works for me   created a package called logger go  package logger  import      flag     os     log     go build     var     Log       log Logger     func init            set location of log file     var logpath   build Default GOPATH     src chat logger info log      flag Parse      var file  err1   os Create logpath      if err1    nil         panic err1             Log   log New file      log LstdFlags log Lshortfile        Log Println  LogFile       logpath       import the package wherever you want to log e g main go  package main  import       logger     const      VERSION    0 13      func main             time to use our logger  print version  processID and number of running process     logger Log Printf  Server v s pid  d started with processes   d   VERSION  os Getpid   runtime GOMAXPROCS runtime NumCPU

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Declare up top in your global var so all your processes can access if needed   package main  import        log       os    var       outfile      os Create  path to my log      update path for your needs     l        log New outfile      0     func main         l Println  hello  log

[go] How to write log to file

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