Bash 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. Also note that even though the redirection for STDERR followed the redirection to STDOUT, both streams are collated and redirected correctly.

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 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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