stdin, stdout, stderr and File Descriptors

The Three Standard Streams

Every process starts with three open file descriptors: 0 (stdin, input), 1 (stdout, normal output), and 2 (stderr, error output). Bash lets you target any of them explicitly using the N> syntax.

Example: The Three Standard Streams

#!/bin/bash
echo "this goes to stdout" 1>&1
echo "this goes to stderr" 1>&2

Sending Error Messages to stderr

Well-behaved scripts print diagnostics and errors to stderr with >&2 so that normal output on stdout stays clean and can be piped or captured separately.

Note: Use a small log_error helper like this instead of scattering >&2 everywhere.

Example: Sending Error Messages to stderr

#!/bin/bash
log_error() {
    echo "ERROR: $1" >&2
}

log_error "something went wrong"
echo "normal output continues on stdout"

Order Matters When Merging Streams

cmd > file 2>&1 first points stdout at file, then makes stderr a copy of the (now redirected) stdout, so both land in file. Reversing the order to cmd 2>&1 > file makes stderr a copy of the *old* stdout (the terminal) before stdout moves to the file.

Warning: Read redirections left to right; 2>&1 > file is a very common bug because it does not do what people expect.

Example: Order Matters When Merging Streams

#!/bin/bash
{ echo "out line"; echo "err line" >&2; } > both.log 2>&1
echo "--- both.log contents ---"
cat both.log
rm -f both.log

Checking Which Stream a Descriptor Points To

The test -t operator (or [ -t N ]) tells you whether file descriptor N is connected to a terminal. In a non-interactive script it usually is not, which is useful for deciding whether to print colored output.

Example: Checking Which Stream a Descriptor Points To

#!/bin/bash
if [ -t 1 ]; then
    echo "stdout is a terminal"
else
    echo "stdout is redirected or non-interactive"
fi