5/19/2023 0 Comments Blackhole devnull meaningOne last gotcha: On some systems, the /etc/aliases change may not work unless that user ( devnull) already exists on the system. You should see the correct alias returned. You can check if the alias is properly set up with the command: postmap -q /etc/postfix/virtual/dbfile It must go to a user or an email address, like this: what you can do is set up an alias user in /etc/aliases: devnull: /dev/nullĪnd then point to that in your virtual domain database file (which should be somewhere in /etc/postfix/virtual/, or it may be the /etc/postfix/virtualfile itself): you’ve made the change, run postmap /etc/postfix/virtual/dbfileand reload Postfix. With virtual domains/users, you can’t forward mail to a file. However, if you’re using Postfix virtual domains, it gets a little more complicated. The usual solution is to forward to /dev/null, and in a local-only setup you could do that in /etc/aliases: do-not-reply: /dev/null Here’s an easy way to add a Postfix virtual alias to /dev/null. Tip of the Trade: Sometimes you want to use a “do-not-reply” type of email address and you don’t want to have to mass-delete the mailbox at intervals. But sometimes you want to use a “do-not-reply” type of email address, and you don’t want to have to mass-delete the mailbox at intervals. Now let’s see what the numbers in 2>&1 represent by looking at this chart of File Descriptors.It’s poor practice to send out email from email addresses that don’t resolve, and may mean your mail ends up in a spamtrap. The >& symbol is an operator that copies the output of the first file descriptor (2) and redirects to the output of the second file descriptor (1). To break this down, we’re suppressing the error output ( stderr) of the ls -0 command, redirect it to standard output ( stdout), writing it to /dev/null thereby immediately discarding it. Notice this time, that we didn’t see any error messages. Let’s try that again with > /dev/null 2>&1: $ ls -0 > /dev/null 2>&1 $ echo $? 1 Luckily there’s a hack to do exactly what we want. However for our scripts, we want to suppress error messages. The problem with the second script is that it displays any error messages into STDERR. Now let’s look at the invalid command: $ ls -0 ls: illegal option - 0 usage: ls $ echo $? 1 This is a valid command: $ ls Applications Documents Library Music Public $ echo $? 0 Given the following two commands: the first ( ls) is a valid command while the second ( ls -0) is an invalid command because it contains an illegal option -0. By convention, an exit code of 0 indicates that the previous command was successful while anything greater indicates an error code for that specific program.įor example, if we lookup ls in the man pages, we’ll see that the exit code 1 corresponds to a minor problem. The $? symbol is a special variable that always contains the exit status of the previous command it will be overwritten every time you run a new command. $ echo 'text' > /dev/null # Upon inspection, we see that the write was successfully. Let’s see what happens when we try writing to it with the file redirection operator ( >). Colloquially it is also called the bit-bucket or the blackhole because it immediately discards anything written to it and only returns an end-of-file EOF when read. To begin, /dev/null is a special file called the null device in Unix systems. So let’s break it down with step-by-step examples. It may look confusing but it’s fairly simple to understand and a fundamental part of shell programming. Newcomers to Bash programming will sooner or later come across /dev/null and another obscure jargon: > /dev/null 2>&1.
0 Comments
Leave a Reply. |