Question

Error Redirection Command output is sometimes mixed up with command errors since they are both sent to the screen by default. Run the following command: cp What did you get displayed? Is that output or error? Now run the command: cp > cpfile What happened? Since the same message got displayed on the screen and was not sent to file cpfile then it must not be output. It is error. To understand how to redirect errors, we should learn about file descriptors. There are three file descriptors used by programs to specify input, output, and error. Standard input has file descriptor 0 Standard output has file descriptor 1 Standard error has file descriptor 2 There is no need to use the file descriptors 0 and 1 when redirecting input and output respectively since they use two different characters namely < and >. To redirect error we need to use the (>) character so to distinguish it from redirecting error, we must specify the file descriptor before the > character as follows: cp 2> cpfile What happened now? Check the contents of file cpfile. What did you find? Redirecting output and error to different places may be very useful especially when dealing with commands that produce both at the same time. Try the following command: find / -name passwd -print What did you get? Was that output or error? Now run the command as follows: find / -name passwd -print 2> errors What did you get now? Check file errors content. Now run the command as follows: find / -name passwd -print > output 2> error What happened? Check both files output and error. To append errors use (2>>).

Error Redirection
Command output is sometimes mixed up with command errors since they are both sent to
the screen by default. Run the following command:
cp
What did you get displayed?
Is that output or error?
Now run the command:
cp > cpfile
What happened?
Since the same message got displayed on the screen and was not sent to file cpfile then it
must not be output. It is error.
To understand how to redirect errors, we should learn about file descriptors. There are
three file descriptors used by programs to specify input, output, and error.
Standard input has file descriptor 0
Standard output has file descriptor 1
Standard error has file descriptor 2
There is no need to use the file descriptors 0 and 1 when redirecting input and output
respectively since they use two different characters namely < and >.
To redirect error we need to use the (>) character so to distinguish it from redirecting
error, we must specify the file descriptor before the > character as follows:
cp 2> cpfile
What happened now?
Check the contents of file cpfile. What did you find?
Redirecting output and error to different places may be very useful especially when
dealing with commands that produce both at the same time. Try the following command:
find / -name passwd -print
What did you get? Was that output or error?
Now run the command as follows:
find / -name passwd -print 2> errors
What did you get now?
Check file errors content.
Now run the command as follows:
find / -name passwd -print > output 2> error
What happened?
Check both files output and error.
To append errors use (2>>).

Added by Vicente E.

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