Bash File Descriptors

Default Files

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

To read from a non-standard file descriptor, redirect the extra stream into STDIN. For example, to read from &3, try:

while read line; do
  :
done 0<&3