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Here's one thing that I find difficult to remember how to do in shell: run a command, have stdout go to a file (or /dev/null or whatever), but pipe stderr to m

• pwg 7y ago
  The syntax for that is present in the manual: Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output and standard error to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error was duplicated from the standard output before the standard output was redirected to dirlist. Does it look a little arcane, yes, esp. until one memorizes it. Can it be memorized? Yes, because it is just a single 'incantation': "2>&1". Just put that redirection operator before the redirection of the standard output to the file, and the result is stdout goes to the file, stderr goes to the pipe.
  • dllthomas 7y ago
    It's actually a pretty simple model. Every process has an array of open file objects. File descriptors are indexes into that array. "Redirection" copies the underlying entries, and are processed left-to-right. "Redirecting streams" winds up being confusing - it's all just `dup2`.
  • Legogris 7y ago
    Just in case someone's confused by this, it becomes clear if you know how pipes work. `2>&1` means "redirect stderr to whatever stdout is currently pointed to" (redirection by copy, not by reference). More recent version of bash manual[0] has a less confusing wording in the same paragraph IMO: Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist. [0]: https://www.gnu.org/software/bash/manual/html_node/Redirections.html https://www.gnu.org/software/bash/manual/html_node/Redirecti...
  • spc476 7y ago
    Okay, but can you pipe stdout to one string of commands, and stderr to a different string of commands? That's something I feel should be possible but how Unix shells handle redirection is just ... alien to my way of thinking. Yes, I know that underneath it's all calls to `pipe()` and `dup2()`, which I can do (and have done) in a language other than shell. It's the shell redirection syntax (for anything more complex than simple redirection or a pipe) that just doesn't make sense to me.
    • pwg 7y ago
      Yes you can (provided you have Bash and a system that supports process substitution): command 2>&1 > >(cat - | cat - > fd1) | cat - | cat - > fd2 Note, the extra 'cats' are just to show a "string of commands". How to read this: When Bash sets up the file descriptors for 'command' it initially makes descriptor 1 refer to the pipe, and descriptor 2 refer to the terminal. So, the dup operator (2>&1) copies the fd in descriptor 1 (which is the pipe) into descriptor 2 (so after the dup operator is processed both stderr and stdout for command reference the pipe). Next descriptor 1 is replaced (the > operator) by a reference to a fifo created by the process substitution operator (the >(...) operator). So now command's descriptor 1 refers to the fifo created by >() (which itself contains a "string of commands"). Then, because stderr for command was made to be a copy of what was previously stdout (before modifying stdout) it continues to refer to the pipe Bash setup (the | operator), and stderr now flows out over the pipes to another "string of commands".
• dredmorbius 7y ago
  ( echo "stdout"; echo "stderr" 1>&2 ) 2>&1 1>/dev/null
  • echelon 7y ago
    Practically alien runes.