Redirection of stdout and stderr to the same file
By Dmitry Kabanov
Often, when running a program in console, one needs to save all its output
to a file.
Very often, such a program writes both to the standard output (stdout)
and to the standard error output (stderr).
For example, by default, in a Python program the print function writes to
stdout and the functions of the logging module to stderr.
Therefore, one needs to redirect both stdout and stderr to the same file.
Redirection basics
Redirection of stdout is done via > syntax:
cmd >log.txt
and redirection of stderr to stdout is done with 2>&1.
Thus, one just needs to combine these two redirections together
to redirect stderr to stdout and then redirect both to log.txt.
However, the order of redirection instructions is important here!
Wrong way
This command:
cmd 2>&1 >log.txt # wrong!
writes stderr to stdout but separately writes stdout to file log.txt.
If stdout is connected to screen,
then stderr effectively will be written to screen but stdout to file.
Why this happens?
The shell executes these redirection instructions immediately,
hence, when 2>&1 redirection is implemented, it is not yet known
that stdout will be redirected to log.txt in the next instruction.
Right way
The right way to redirection is the command
cmd >log.txt 2>&1 # right!
that will write both stderr and stdout to the same file file.txt.
As a shortcut for the previous command, one can use
cmd &>log.txt
This shortcut is not only shorter but also allows to avoid the possibility of a bug by using the wrong order.