# First direct standard error output 2 to standard output 1,
# At this time, standard output 1 points to the console, so standard error output 2 points to the console,
# After that, point standard output 1 to the file /tmp/t.txt, but standard error output 2 still points to the console.
ls non_exists 2> & 1> /tmp/t.txt # So, the error message is printed on the screen,
ls: cannot access non_exists: No such file or directory
cat /tmp/t.txt # There is no content in /tmp/t.txt.
# First point the standard output 1 to the file /tmp/t.txt,
# Then point standard error output 2 to standard output 1,
# This way both standard error output 2 and standard output 1 point to the file /tmp/t.txt
ls non_exists> /tmp/t.txt 2> & 1 # The error message no longer appears on the screen,
cat /tmp/t.txt # Error messages are saved in the file /tmp/t.txt
ls: cannot access non_exists: No such file or directory
