How to read write from to file using Go

Question

I ve been trying to learn Go on my own  but I ve been stumped on trying read from and write to ordinary files   I can get as far as inFile       os Open INFILE  0  0   but actually getting the content of the file doesn t make sense  because the read function takes a   byte as a parameter   func  file  File  Read b   byte   n int  err Error

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This is good version   package main  import      io ioutil          func main       contents      ioutil ReadFile  plikTekstowy txt     println string contents     ioutil WriteFile  filename   contents  0644

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With newer Go versions  reading writing to from file is easy  To read from a file   package main  import        fmt       io ioutil     func main         data  err    ioutil ReadFile  text txt       if err    nil           return           fmt Println string data       To write to a file   package main  import  os   func main         file  err    os Create  text txt       if err    nil           return           defer file Close        file WriteString  test nhello       This will overwrite the content of a file  create a new file if it was not there

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Just looking at the documentation it seems you should just declare a buffer of type   byte and pass it to read which will then read up to that many characters and return the number of characters actually read  and an error    The docs say     Read reads up to len b  bytes from the File  It returns the number of bytes read and an Error  if any  EOF is signaled by a zero count with err set to EOF    Does that not work   EDIT  Also  I think you should perhaps use the Reader Writer interfaces declared in the bufio package instead of using os package

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byte is a slice  similar to a substring  of all or part of a byte array  Think of the slice as a value structure with a hidden pointer field for the system to locate and access all or part of an array  the slice   plus fields for the length and capacity of the slice  which you can access using the len   and cap   functions   Here s a working starter kit for you  which reads and prints a binary file  you will need to change the inName literal value to refer to a small file on your system   package main import        fmt        os     func main         inName     file-rw bin       inPerm     0666      inFile  inErr    os Open inName  os O RDONLY  inPerm       if inErr    nil           inBufLen    16          inBuf    make   byte  inBufLen           n  inErr    inFile Read inBuf           for inErr    nil               fmt Println n  inBuf 0 n                n  inErr   inFile Read inBuf                       inErr   inFile Close

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The Read method takes a byte parameter because that is the buffer it will read into  It s a common Idiom in some circles and makes some sense when you think about it   This way you can determine how many bytes will be read by the reader and inspect the return to see how many bytes actually were read and handle any errors appropriately   As others have pointed in their answers bufio is probably what you want for reading from most files    I ll add one other hint since it s really useful  Reading a line from a file is best accomplished not by the ReadLine method but the ReadBytes or ReadString method instead

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Let s make a Go 1-compatible list of all the ways to read and write files in Go  Because file API has changed recently and most other answers don t work with Go 1  They also miss bufio which is important IMHO  In the following examples I copy a file by reading from it and writing to the destination file  Start with the basics package main  import        quot io quot       quot os quot     func main            open input file     fi  err    os Open  quot input txt quot       if err    nil           panic err               close fi on exit and check for its returned error     defer func             if err    fi Close    err    nil               panic err                            open output file     fo  err    os Create  quot output txt quot       if err    nil           panic err               close fo on exit and check for its returned error     defer func             if err    fo Close    err    nil               panic err                            make a buffer to keep chunks that are read     buf    make   byte  1024      for              read a chunk         n  err    fi Read buf          if err    nil  amp  amp  err    io EOF               panic err                    if n    0               break                       write a chunk         if    err    fo Write buf  n    err    nil               panic err                     Here I used os Open and os Create which are convenient wrappers around os OpenFile  We usually don t need to call OpenFile directly  Notice treating EOF  Read tries to fill buf on each call  and returns io EOF as error if it reaches end of file in doing so  In this case buf will still hold data  Consequent calls to Read returns zero as the number of bytes read and same io EOF as error  Any other error will lead to a panic  Using bufio package main  import        quot bufio quot       quot io quot       quot os quot     func main            open input file     fi  err    os Open  quot input txt quot       if err    nil           panic err               close fi on exit and check for its returned error     defer func             if err    fi Close    err    nil               panic err                           make a read buffer     r    bufio NewReader fi          open output file     fo  err    os Create  quot output txt quot       if err    nil           panic err               close fo on exit and check for its returned error     defer func             if err    fo Close    err    nil               panic err                           make a write buffer     w    bufio NewWriter fo          make a buffer to keep chunks that are read     buf    make   byte  1024      for              read a chunk         n  err    r Read buf          if err    nil  amp  amp  err    io EOF               panic err                    if n    0               break                       write a chunk         if    err    w Write buf  n    err    nil               panic err                       if err   w Flush    err    nil           panic err           bufio is just acting as a buffer here  because we don t have much to do with data  In most other situations  specially with text files  bufio is very useful by giving us a nice API for reading and writing easily and flexibly  while it handles buffering behind the scenes   Note  The following code is for older Go versions  Go 1 15 and before   Things have changed  For the new way  take a look at this answer  Using ioutil package main  import        quot io ioutil quot     func main            read the whole file at once     b  err    ioutil ReadFile  quot input txt quot       if err    nil           panic err                write the whole body at once     err   ioutil WriteFile  quot output txt quot   b  0644      if err    nil           panic err           Easy as pie  But use it only if you re sure you re not dealing with big files

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Try this   package main  import      io          func main       contents      io ReadFile  filename      println string contents      io WriteFile  filename   contents  0644

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New Way Starting with Go 1 16  use os ReadFile to load the file to memory  use os WriteFile to write to a file from memory  Be careful with the os ReadFile because it reads the whole file into memory  package main  import  quot os quot   func main         b  err    os ReadFile  quot input txt quot       if err    nil           log Fatal err                 data  contains everything your file does        This writes it to the Standard Out     os Stdout Write data          You can also write it to a file as a whole     err   os WriteFile  quot destination txt quot   b  0644      if err    nil           log Fatal err

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Using io Copy  package main  import        io       log       os     func main             open files r and w     r  err    os Open  input txt       if err    nil           panic err            defer r Close        w  err    os Create  output txt       if err    nil           panic err            defer w Close           do the actual work     n  err    io Copy w  r      if err    nil           panic err            log Printf  Copied  v bytes n   n      If you don t feel like reinventing the wheel  the io Copy and io CopyN may serve you well  If you check the source of the io Copy function  it is nothing but one of the Mostafa s solutions  the  basic  one  actually  packaged in the Go library  They are using a significantly larger buffer than he is  though

[file] How to read/write from/to file using Go?

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