3 ms·

stderr is not for errors! (at least, not necessarily) It's for messaging things to the user. These are often errors, but there are some examples like curl that

stderr is not for errors! (at least, not necessarily) It's for messaging things to the user. These are often errors, but there are some examples like curl that use stderr for progress. Leave it up to the CLI to decide what colors to use.

lbotos 8y ago
why does curl used stderr over say stdout? (and happy to read any links you have, trying to learn)
- dickeytk 8y ago
  factor #4 in my article: https://medium.com/@jdxcode/12-factor-cli-apps-dd3c227a0e46 https://medium.com/@jdxcode/12-factor-cli-apps-dd3c227a0e46
- black-tea 8y ago
  Because stdout is used for the output of the program, ie. the stuff downloaded from the URL.
- deleted 8y ago
  [deleted]
v_lisivka 8y ago
Standard output for errors is for errors. If you need one more stream with your own rules, just open it. #include <stdio.h> int main(void) { /* Open third stream. Stream must be opened in shell using 3>... */ FILE *user = fdopen (3, "w"); /* If third stream is not open, then print to first stream. */ if (!user) user = fdopen(1, "w"); printf("Output.\n"); fprintf(stderr, "Error!\n"); fprintf(user, "Important message to user!\n"); fclose(user); return 0; } $ gcc test.c -o test $ ./test Output. Error! Important message to user! $ ./test 1>out.txt 2>error.log 3>/dev/tty Important message to user!
- dickeytk 8y ago
  Well first of all the docs say it's for "diagnostic output" http://pubs.opengroup.org/onlinepubs/9699919799/functions/stderr.html http://pubs.opengroup.org/onlinepubs/9699919799/functions/st... not "errors". Secondly, it doesn't matter what's right or not, the reality is CLIs (that you didn't write) generally use stderr for progress so you can't assume everything on it is an error.
  - v_lisivka 8y ago
    Generally, CLI turns out progress and colors when output is not connected to tty, so I can safely assume, in most cases. that everything on stderr is about errors. If not, I can always send my patch to upstream.
  - v_lisivka 8y ago
    Actually, Linux manual says: DESCRIPTION Under normal circumstances every UNIX program has three streams opened for it when it starts up, one for input, one for output, and one for printing diagnostic or error messages. These are typically attached to the user's terminal (see tty(4)) but might instead refer to files or other devices, depending on what the parent process chose to set up. (See also the "Redirection" section of sh(1).) The input stream is referred to as "standard input"; the output stream is referred to as "standard output"; and the error stream is referred to as "standard error". These terms are abbreviated to form the symbols used to refer to these files, namely stdin, stdout, and stderr.
    - dickeytk 8y ago
      Right, as I said: stderr is not only for errors
TeMPOraL 8y ago
In Common Lisp, you have following streams defined in the spec: *debug-io* - bidirectional, for interactive debugging *error-output* - output, for warnings and non-interactive error messages *query-io* - bidirectional, for asking user questions and reading answers *standard-input* - stdin *standard-output* - stdout *trace-output* - for tracing functions and timing execution You can redefine each of them independently. I think this granularity is much better than our usual stdin/stdout/stderr split we're used to. Note e.g. the query-io being separate from stdin/stdout, which means a properly defined way for a program to handle interactivity while simultaneously having data piped into stdin and out of stdout. It was IMO a good idea, and I wonder how the world ended up adopting just three standards streams in the end.

lbotos 8y ago

why does curl used stderr over say stdout? (and happy to read any links you have, trying to learn)

dickeytk 8y ago
factor #4 in my article: https://medium.com/@jdxcode/12-factor-cli-apps-dd3c227a0e46 https://medium.com/@jdxcode/12-factor-cli-apps-dd3c227a0e46
black-tea 8y ago
Because stdout is used for the output of the program, ie. the stuff downloaded from the URL.
deleted 8y ago
[deleted]

v_lisivka 8y ago

Standard output for errors is for errors. If you need one more stream with your own rules, just open it. #include <stdio.h> int main(void) { /* Open third stream. Stream must be opened in shell using 3>... */ FILE *user = fdopen (3, "w"); /* If third stream is not open, then print to first stream. */ if (!user) user = fdopen(1, "w"); printf("Output.\n"); fprintf(stderr, "Error!\n"); fprintf(user, "Important message to user!\n"); fclose(user); return 0; } $ gcc test.c -o test $ ./test Output. Error! Important message to user! $ ./test 1>out.txt 2>error.log 3>/dev/tty Important message to user!

dickeytk 8y ago
Well first of all the docs say it's for "diagnostic output" http://pubs.opengroup.org/onlinepubs/9699919799/functions/stderr.html http://pubs.opengroup.org/onlinepubs/9699919799/functions/st... not "errors". Secondly, it doesn't matter what's right or not, the reality is CLIs (that you didn't write) generally use stderr for progress so you can't assume everything on it is an error.
- v_lisivka 8y ago
  Generally, CLI turns out progress and colors when output is not connected to tty, so I can safely assume, in most cases. that everything on stderr is about errors. If not, I can always send my patch to upstream.
- v_lisivka 8y ago
  Actually, Linux manual says: DESCRIPTION Under normal circumstances every UNIX program has three streams opened for it when it starts up, one for input, one for output, and one for printing diagnostic or error messages. These are typically attached to the user's terminal (see tty(4)) but might instead refer to files or other devices, depending on what the parent process chose to set up. (See also the "Redirection" section of sh(1).) The input stream is referred to as "standard input"; the output stream is referred to as "standard output"; and the error stream is referred to as "standard error". These terms are abbreviated to form the symbols used to refer to these files, namely stdin, stdout, and stderr.
  - dickeytk 8y ago
    Right, as I said: stderr is not only for errors

TeMPOraL 8y ago

In Common Lisp, you have following streams defined in the spec: *debug-io* - bidirectional, for interactive debugging *error-output* - output, for warnings and non-interactive error messages *query-io* - bidirectional, for asking user questions and reading answers *standard-input* - stdin *standard-output* - stdout *trace-output* - for tracing functions and timing execution You can redefine each of them independently. I think this granularity is much better than our usual stdin/stdout/stderr split we're used to. Note e.g. the query-io being separate from stdin/stdout, which means a properly defined way for a program to handle interactivity while simultaneously having data piped into stdin and out of stdout. It was IMO a good idea, and I wonder how the world ended up adopting just three standards streams in the end.