Write to a particular line in a file

General Usage
1

Is there a native Julia command to write to a particular line of a file? I know how to open a file, namely using open, but it seems like the options are to overwrite the entire file (with the “w” and write commands/options), or write to its end (with the “a” and write commands/options). I haven’t found an option to specify a line to add content to.

I know that if all else fails, I could use:

run(`sed -i "$line i What I want to write" $file`)

But I’m wondering whether there’s another way that just uses Julia commands and no external commands.

2 Likes
2

Here is one way to do this:

julia> function skiplines(io::IO, n)
           i = 1
           while i <= n
              eof(io) && error("File contains less than $n lines")
              i += read(io, Char) === '\n'
           end
       end
skiplines (generic function with 1 method)

julia> # write 3 lines to test.foo
       write("test.foo", "foo\nbar\nbaz\n")
12

julia> f = open("test.foo", "r+")
IOStream(<file test.foo>)

julia> skiplines(f, 2)

julia> # seek stream back before the last '\n'
       skip(f, -1)
IOStream(<file test.foo>)

julia> # mark current stream position, so we can seek back later
       mark(f)
7

julia> buf = IOBuffer()
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1)

julia> # write rest of file into buf
       write(buf, f)
5

julia> seekstart(buf)
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=5, maxsize=Inf, ptr=1, mark=-1)

julia> # go back to the end of the 2nd line we marked earlier
       reset(f)
7

julia> print(f, "bara")

julia> # now write rest of file from buf back into f
       write(f, buf)
5

julia> close(f)

julia> Text(read("test.foo", String))
foo
barbara
baz
4 Likes
3

I tried copy-pasting your code into a function, editL. Here it is:

function editL(file, string, no)
    write(file, string);
    f = open(file, "r+");
    skiplines(f, no);
    skip(f, -1)
    mark(f)
    buf = IOBuffer()
    write(buf, f)
    seekstart(buf)
    reset(f)
    write(f, buf)
    close(f)
end

and it doesn’t do what I expected, it actually deletes the whole file and adds string (the contents of that third argument, not the word string) to it.

4

Delete the first line. The first write is just to create the file test.foo for this demonstration.

5

Turns out your code also solves Writing LaTeX from Julia.