Shell File Descriptors

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 redirection.

The 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

Valid indices for file descriptors range from 3 to 9.

To open a file descriptor for reading, try:

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

Mostly only useful for 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<&-

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