Does stdout get stored somewhere in the filesystem or in memory?

Question

I know I can save the result of a command to a variable using `last_output=$(my_cmd)` but what I'd really want is for `$last_output` to get updated every time I run a command. Is there a variable, zsh module, or plugin that I could install? I guess my question is does `stdout` get permanently written somewhere (at least before the next command)? That way I could manipulate the results of the previous command without having to re-run it. This would be really useful for commands that take a long time to run

Accepted Answer

If you run the following:

```
exec > >(tee save.txt)
# ... stuff here...
exec >/dev/tty
```

...then your stdout for everything run between the two commands will go both to stdout, and to `save.txt`.

You could, of course, write a shell function which does this for you:

```
with_saved_output() {
  "$@" \
  2> >(tee "$HOME/.last-command.err >&2) \
  | tee "$HOME/.last-command.out"
}
```

...and then use it at will:

```
with_saved_output some-command-here
```

...and zsh almost certainly will provide a mechanism to wrap interactively-entered commands. (In bash, which I can speak to more directly, you could do the same thing with a DEBUG trap).

However, even though you can, you shouldn't do this: When you split stdout and stderr into two streams, information about the exact ordering of writes is lost, even if those streams are recombined later.

Thus, the output

```
O: this is written to stdout first
E: this is written to stderr second
```

could become:

```
E: this is written to stderr second
O: this is written to stdout first
```

when these streams are individually passed through `tee` subprocesses to have copies written to disk. There are also buffering concerns created, and differences in behavior caused by software which checks whether it's outputting to a TTY and changes its behavior (for instance, software which turns color-coded output on when writing directly to console, and off when writing to a file or pipeline).

Does stdout get stored somewhere in the filesystem or in memory?

Solution

Problem