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Bash Scripting

Error Logging

Programming of any kind will always have errors, and it is sometimes difficult to pinpoint where in the coding it occurred.

A way to save errors to a file is done with redirection.

A good habit to consider.

One feature bash has is something called the bit bucket. It is one of the several pseudo-devices available to programmers.

The one we will be discussing here is /dev/null.

/dev/null is a special virtual device of no data or content. If data is sent to it (redirected), it's discarded. If data is read from it (redirected), it is null/empty.

It essentially suppresses program output.

So, what good is a device that throws everything away we send to it, or provides us with nothing to use?

Good question. /dev/null is used in cases where you don't need or want to actually see the response, but do need to act on it.

In a directory that contains files or directories, issue the command:

ls > /dev/null

You should NOT see any output.

echo "Hey stupid" > /dev/null

Same

However, if the command produced an error, you would see this result:

ls ThisFileDoesNotExist > /dev/null
ls: ThisFileDoesNotExist: No such file or directory

We did NOT redirect STDERR to /dev/null, so we need one of these file descriptors.

Whenever you execute a program, the operating system always opens three files, standard input, standard output, and standard error.

Logging errors to a file is done using 2 redirects:

one to the bit bucket

one to a file descriptor

The file descriptors are the digits (0, 1, 2)

0 standard input (STDIN)
1 standard output (STDOUT)
2 standard error (STDERR)
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.

The way we can send error messages to a file requires the use of the file descriptor 2 and the special character &:

& means whatever follows is a file descriptor, not a filename.

ls ThisFileDoesNotExist
ls: ThisFileDoesNotExist: No such file or directory

ls ThisFileDoesNotExist > /dev/null
ls: ThisFileDoesNotExist: No such file or directory

ls ThisFileDoesNotExist > /dev/null 2>&1

The last example above used redirection to the bit bucket AND redirection of STDERR and STDOUT as well - so nothing was seen.

Be careful with the 2>&1 construction:

The 2>&1 have to appear together with no spaces

A good reference to this is found at Medium.

And every time we execute a command, it returns an exit status, which the system understands.

If the result of a command is successful, the exit status is 0, otherwise it is something greater than 0.

We can check the value of the exit status by using the variable $?. It holds the exit value of the last program the shell executed.

echo $?
1