I/O Redirection

Idio defines some reader operators for I/O redirection, the familiar > etc.. Remember that, in Idio, whitespace is important as > can be part of a valid symbol.

These are implemented by changing the current input, output or error handle from or to some entity. When we reach the point of trying to invoke an external command the current input, output and error handles will be transformed into suitable file descriptors.

Note

Whilst the operators are limited to manipulating stdin, stdout and stderr you can effect any other changes with calls to libc/dup2 etc. directly.

Standard Redirection

< expr > expr >> expr 2> expr 2>> expr

Here, expr can evaluate to:

  • a handle (fd or string)

  • a string to be opened for input or output

  • #n as a shorthand for the string "/dev/null"

standard-io.idio
fn := "foo.txt"

;; overwrite a file
echo "hello" > fn

fh := open-file fn "a"

;; append via a file handle
echo "world" >> fh
close-handle fh

cat fn

rm fn

$ idio standard-io
hello
world

Using Handles

Using the redirection operators on a handle does not implicitly rewind the handle. All operators are effectively continuing from the existing file/string position.

I/O Duplication

<& expr >& expr 2>& expr

Here, whatever expr evaluates to must already exist, so:

  • a handle (fd or string)

  • a C int (or fixnum) representing the file descriptor to call libc/dup2 with

io-dup.idio
fn := "foo.txt"

fh := open-output-file fn
hprintf fh "hello\n"

;; append via a file handle
echo "world" >& fh

close-handle fh

cat fn
rm fn

$ idio io-dup
hello
world

Last built at 2024-05-21T06:11:41Z+0000 from 77077af (dev) for Idio 0.3