How does exactly pipelining work with 2>&1?

Question

Just started learning UNIX so the question might seem really newbie but would appreciate the answer, as I've been trying to work it out on my own for an hour already with google's help, with no success however.

cat /etc/shadow 2>&1 | wc -l

What would be the effect of this command? My guess is:

1. The command prints the line count of “/etc/shadow”, if there is a standard error, it will be redirected to standard output and the error lines will be counted.
2. The command prints the files of “/etc/shadow”, if there is a standard error, it will be redirected to standard output and the error's lines will be counted.

Answer 1

X>&Y is for file descriptor redirection: this means that all output to fd X is actually going into Y. 2>&1 throws STDERR's output into STDOUT.

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wc -l writes the number of input lines to STDOUT.

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Together, the command cat /etc/shadow 2>&1 | wc -l returns the number of lines in /etc/shadow, as well as the number of error lines.

If you don't want to count those error lines, just use cat /etc/shadow | wc -l.

Answer 2

This is not a direct answer.

What is preventing you from doing some experimenting?

If you did experiments, then you may have answered your own question.

Run various combinations of the commands.

cat /etc/shadow
cat /etc/shadows

cat /etc/shadow  2>&1
cat /etc/shadows 2>&1

and then pipe each one to wc

cat /etc/shadow    |  wc

etc. etc.