Why does 2>&1 need to come before a | (pipe) but after a “> myfile” (redirect to file)?

Question

When combining stderr with stdout, why does 2>&1 need to come before a | (pipe) but after a > myfile (redirect to file)?

Answer 1

To add to ormaaj's answer:

The reason you need to specify redirection operators in the proper order is that they're evaluated from left to right. Consider these command lists:

# print "hello" on stdout and "world" on stderr
{ echo hello; echo world >&2; }

# Redirect stdout to the file "out"
# Then redirect stderr to the file "err"
{ echo hello; echo world >&2; } > out 2> err

# Redirect stdout to the file "out"
# Then redirect stderr to the (already redirected) stdout
# Result: all output is stored in "out"
{ echo hello; echo world >&2; } > out 2>&1

# Redirect stderr to the current stdout
# Then redirect stdout to the file "out"
# Result: "world" is displayed, and "hello" is stored in "out"
{ echo hello; echo world >&2; } 2>&1 > out