standardinputoutputerror.md

Understanding Shell Script's idiom: 2>&1

A quick introduction to I/O redirection:

Simply put, redirection is the mechanism used to send the output of a command to another place.

For instance, if we just cat a file, its output will be printed in the screen, by default:

cat test.txt

But we can redirect this output to another place. Here, for example, we are redirecting it to a file called output.txt :

cat test.txt > output.txt

If we print output.txt cat output.txt we can get the contents of test.txt

here we changed the standard output (stdout) location to a file(output.txt), so it doesn’t use the screen anymore.

It’s also important to know that there are this other place, called standard error (stderr), to where programs can send their error messages.

So if we try to cat a file that doesn’t exist, like this:

cat nop.txt > output.txt

cat: nop.txt: No such file or directory

Even if we redirect the stdout to a file, we still see the error output in the screen,
because we are redirecting just the standard output, not the standard error.

quick introduction to file descriptors:

A file descriptor is nothing more that a positive integer that represents an open file. If you have 100 open files, you will have 100 file descriptors for them.

we just need to know that there are file descriptors for

Standard Output (stdout) = "1"

Standard Error (stderr) = "2"

when we redirected the output of cat test.txt to output.txt, we could rewrite the command like this:

cat test.txt 1> output.txt

This "1" is just the file descriptor for stdout.

The syntax for redirecting is

[FILE_DESCRIPTOR]>, leaving the file descriptor out is just a shortcut to 1>

We can use &1 to refer the value of the file descriptor 1 (stdout).

So when we use

2>&1

we are basically saying

“Redirect the stderr to the same place we are redirecting the stdout”.

And that’s why we can do something like this to redirect both stdout and stderr to the same place:

cat test.txt > output.txt 2>&1

cat output.txt

cat nop.txt > output.txt 2>&1

cat output.txt
cat: nop.txt: No such file or directory

We can use &1 to refer the value of the file descriptor 1 (stdout).
we can use 2>&1 and instruct the script that stderr(thrown errors/error output), also must be redirected to stdout (by specifying the &1)

command > output is just a shortcut for command 1> output

cat test.txt > output.txt is just a shortcut for cat test.txt 1> output.txt

We can use &[FILE_DESCRIPTOR] to refer a file descriptor value

using 2>&1 will redirect stderr to whatever value is set to stdout

**(and 1>&2 will do the opposite) **

For reference brianstorti.com