Reading from a file

Let’s assume you have a file lyrics.txt which contains the following data:

summer has come and passed
the innocent can never last
wake me up when september ends

Read the entire file at a time

By using file:read_file(File), you can read the entire file. It’s an atomic operation:

1> file:read_file("lyrics.txt").
{ok,<<"summer has come and passed\r\nthe innocent can never last\r\nWake me up w
hen september ends\r\n">>}

Read one line at a time

io:get_line reads the text until the newline or the end of file.

1> {ok, S} = file:open("lyrics.txt", read).
{ok,<0.57.0>}
2> io:get_line(S, '').
"summer has come and passed\n"
3> io:get_line(S, '').
"the innocent can never last\n"
4> io:get_line(S, '').
"wake me up when september ends\n"
5> io:get_line(S, '').
eof
6> file:close(S).
ok

Read with the random access

file:pread(IoDevice, Start, Len) reads from Start as much as Len from IoDevice.

1> {ok, S} = file:open("lyrics.txt", read).
{ok,<0.57.0>}
2> file:pread(S, 0, 6).
{ok,"summer"}
3> file:pread(S, 7, 3).
{ok,"has"}       

Writing to a file

Write one line at a time

Open a file with write mode and use io:format/2:

1> {ok, S} = file:open("fruit_count.txt", [write]).
{ok,<0.57.0>}
2> io:format(S, "~s~n", ["Mango 5"]).
ok
3> io:format(S, "~s~n", ["Olive 12"]).
ok
4> io:format(S, "~s~n", ["Watermelon 3"]).
ok
5>

The result will be a file named fruit_count.txt with the following contents:

Mango 5
Olive 12
Watermelon 3

Note that opening a file in write mode will created it, if not already existent in the file system.

Note also that using the write option with file:open/2 will truncate the file (even if you don’t write anything into it). To prevent this, open the file in [read,write] or [append] mode.

Write the entire file at once

file:write_file(Filename, IO) is the simplest function for writing a file at once. If the file already exists, it will overwritten, otherwise it will be created.

1> file:write_file("fruit_count.txt", ["Mango 5\nOlive 12\nWatermelon 3\n"
]).
ok
2> file:read_file("fruit_count.txt").
{ok,<<"Mango 5\nOlive 12\nWatermelon 3\n">>}
3>

Write with random access

For random access writing, file:pwrite(IoDevice, Location, Bytes) is used. If you want to replace some string in the file, this method is useful.

Let’s assume you want to change “Olive 12” to “Apple 15” in the file created above.

1> {ok, S} = file:open("fruit_count.txt", [read, write]).
{ok,{file_descriptor,prim_file,{#Port<0.412>,676}}}
2> file:pwrite(S, 8, ["Apple 15\n"]).
ok
3> file:read_file("fruit_count.txt").
{ok,<<"Mango 5\nApple 15\nWatermelon 3">>}
4> file:close(S).
ok
5>