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(Inspired by: https://discuss.python.org/t/_/107345) It was brought to my attention that it actually works to do something like: $ spam=$(printf '%1000s') $ spam=${spam// /abcdefg} $ echo $spam...
#2: Post edited
by
Karl Knechtel
·
2026-05-13T19:45:14Z (4 months ago)
- *(Inspired by: https://discuss.python.org/t/_/107345)*
- It was brought to my attention that *it actually works* to do something like:
- ```
- $ spam=$(printf '%1000s')
- $ spam=${spam// /abcdefg}
- $ echo $spam | less
- ```
- (There are other TUI programs I use where this doesn't work; whatever is received from the pipe is interpreted as if it were typed by the user, and then the program hangs if it hasn't yet received a "quit" command in the input.)
- We have multiple pages of text fed to the standard input of `less`, which is then able to show the first page and wait for the usual sort of commands from the user.
- But *how*? I thought that the program is reading those commands from standard input, but that was already redirected via a pipe. In other programs I've created myself, once standard input was attached somewhere, it would be impossible to "feed" it from somewhere else (e.g. if I have `foo | bar` already running, I can find no way for `baz` to write to `bar`'s input).
And even if the key presses were being translated into input fed to `less`'s stdin, *how would it be able to distinguish* which input comes from the keyboard vs. the pipe?
- *(Inspired by: https://discuss.python.org/t/_/107345)*
- It was brought to my attention that *it actually works* to do something like:
- ```
- $ spam=$(printf '%1000s')
- $ spam=${spam// /abcdefg}
- $ echo $spam | less
- ```
- (There are other TUI programs I use where this doesn't work; whatever is received from the pipe is interpreted as if it were typed by the user, and then the program hangs if it hasn't yet received a "quit" command in the input.)
- We have multiple pages of text fed to the standard input of `less`, which is then able to show the first page and wait for the usual sort of commands from the user.
- But *how*? I thought that the program is reading those commands from standard input, but that was already redirected via a pipe. In other programs I've created myself, once standard input was attached somewhere, it would be impossible to "feed" it from somewhere else (e.g. if I have `foo | bar` already running, I can find no way for `baz` to write to `bar`'s input).
- And even if the key presses were being translated into input fed to `less`'s stdin, *how would it be able to distinguish* which input comes from the keyboard vs. the pipe? (It clearly can't do this by inspecting the actual byte values, since `less` accepts commands that are ordinary text, such as `q`, rather than purely relying on things that would start with a control character.)
#1: Initial revision
by
Karl Knechtel
·
2026-05-13T19:43:40Z (4 months ago)
How do programs like `less` receive keyboard input as well as from a pipe?
*(Inspired by: https://discuss.python.org/t/_/107345)*
It was brought to my attention that *it actually works* to do something like:
```
$ spam=$(printf '%1000s')
$ spam=${spam// /abcdefg}
$ echo $spam | less
```
(There are other TUI programs I use where this doesn't work; whatever is received from the pipe is interpreted as if it were typed by the user, and then the program hangs if it hasn't yet received a "quit" command in the input.)
We have multiple pages of text fed to the standard input of `less`, which is then able to show the first page and wait for the usual sort of commands from the user.
But *how*? I thought that the program is reading those commands from standard input, but that was already redirected via a pipe. In other programs I've created myself, once standard input was attached somewhere, it would be impossible to "feed" it from somewhere else (e.g. if I have `foo | bar` already running, I can find no way for `baz` to write to `bar`'s input).
And even if the key presses were being translated into input fed to `less`'s stdin, *how would it be able to distinguish* which input comes from the keyboard vs. the pipe?