= Bash File Descriptors =

<<TableOfContents>>

----



== Standard File Descriptors ==

All shells inherit the 0th (`STDIN`), 1st (`STDOUT`), and 2nd (`STDERR`) file descriptors. File descriptors are referenced by their index and an ampersand (`&`) prefix.

File descriptors can be used with [[Bash/Redirection|redirection]].

The `echo` [[Bash/BuiltinCommands#Echo|builtin]] normally outputs to `STDOUT`. To redirect that output to `STDERR`, try:

{{{
echo "Error!" >&2
}}}

Note that the `STDOUT` is redirected implicitly.

Alternatively, to combine the `STDOUT` and `STDERR` streams and redirect them to a file, try:

{{{
command example >/dev/null 2>&1
}}}

Note that the `&` can be omitted from the source file descriptor.

To read from `STDIN`, try:

{{{
while read line; do
  :
done <&0
}}}

----



== Non-standard File Descriptors ==



=== Declaration ===

Non-standard file descriptors have to be opened before they can be accessed.

To open a file descriptor for writing, try:

{{{
exec 3> myfifo
}}}

To open a file descriptor for reading, try:

{{{
exec 3< /path/to/a/temp/file
}}}

Mostly only useful for [[Linux/Networking#Unix_Sockets|sockets]], but a bidirectional file descriptor can be opened like:

{{{
exec 3<> /dev/tcp/www.example.com/80
}}}



=== Usage ===

Non-standard file descriptors, once opened, are used exactly like a standard file descriptor.

To write to one, try:

{{{
echo "Debug message" >&3
}}}

To read from one, redirect the extra stream into `STDIN`. For example, to read from `&3`, try:

{{{
while read line; do
  :
done 0<&3
}}}



=== Closing ===

An opened file descriptor must be closed. This is accomplished by either redirecting it to `-`, or by redirecting `-` into it.

{{{
exec 3>&- #or: exec 3<&-
}}}



----
CategoryRicottone