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clarify some language; insert spaces in args to confuse beginners less (#400)
* clarify some language; insert spaces in args to confuse beginners less
* fix 2>&1 footgun, explain it and how to avoid it
* add cadence and AstroSnail to credits
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blog/fun-with-redirection-2021-09-22.markdown
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blog/fun-with-redirection-2021-09-22.markdown
···
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feed the `cut` command's standard input using the `<` operator:
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```console
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$ cut -d' ' -f2 < uname.txt
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$ cut -d ' ' -f 2 < uname.txt
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shachi
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chrysalis
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kos-mos
···
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write it like this:
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```sh
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cat uname.txt | cut -d' ' -f2
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cat uname.txt | cut -d ' ' -f 2
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```
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[The mnemonic we use for remembering the `cut` command is that fields are
···
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This will get you the exact same output:
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```console
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$ cat uname.txt | cut -d' ' -f2
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$ cat uname.txt | cut -d ' ' -f 2
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shachi
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chrysalis
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kos-mos
···
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example, if you wanted to sort them you could pipe the result to `sort`:
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```console
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$ cat uname.txt | cut -d' ' -f2 | sort
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$ cat uname.txt | cut -d ' ' -f 2 | sort
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chrysalis
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kos-mos
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ontos
···
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go to the same file. You can do this with a command like this:
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```
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$ rustc foo.rs > foo.log 2>&1
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```
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This tells the shell to point standard out to `foo.log`, and then standard
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error to standard out (which is now `foo.log`). There's a footgun here though;
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the order of the redirects matters. Consider the following:
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```
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$ rustc foo.rs 2>&1 > foo.log
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error: expected one of `!` or `::`, found `main`
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--> foo.rs:1:5
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|
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1 | fun main() {}
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| ^^^^ expected one of `!` or `::`
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error: aborting due to previous error
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$ cat foo.log
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$ # foo.log is empty, why???
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```
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This tells the shell to point standard error to standard out and then the
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combined output to `foo.log`. There's a short form of this too:
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We wanted to redirect stderr to `foo.log`, but that didn't happen. Why? Well,
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the shell considers our redirects one at a time from left to right. When the
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shell sees `2>&1`, it hasn't considered `> foo.log` yet, so standard out (`1`)
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is still our terminal. It dutifully redirects stderr to the terminal, which is
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where it was already going anyway. Then it sees `1 > foo.log`, so it redirects
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standard out to `foo.log`. That's the end of it though. It doesn't
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retroactively redirect standard error to match the new standard out, so our
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errors get dumped to our terminal instead of the file.
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Confusing right? Lucky for us, there's a short form that redirects both at the
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same time, making this mistake impossible:
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```
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$ rustc foo.rs &> foo.log
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```
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[Where can I expect to use that?](conversation://Mara/hmm)
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This will put standard out and standard error to `foo.log` the same way that
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`> foo.log 2>&1` will.
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[It's a bourne shell extension, but I've tested it in `zsh` and `fish`. You can
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also do `&|` to pipe both standard out and standard error at the same time in
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the same way you'd do `2>&1 | whatever`.](conversation://Cadey/enby)
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[Will that work in every shell?](conversation://Mara/hmm)
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That will put standard out and standard error to `foo.log` the same way that
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`2>&1 > foo.log` will. You can also use this with `>>`:
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[It's a bourne shell (`bash`) extension, but I've tested it in `zsh` and `fish`.
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You can also do `&|` to pipe both standard out and standard error at the same
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time in the same way you'd do `2>&1 | whatever`.](conversation://Cadey/enby)
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You can also use this with `>>`:
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```
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$ rustc foo.rs &>> foo.log
···
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[How do I redirect standard in to a file?](conversation://Mara/hmm)
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The answer there is not directly! There is a workaround in the form of a tool
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called `tee` which outputs its standard in to both standard out and a file. For
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example:
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Well, you don't. Standard in is an input, so you can change where it comes
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_from_, not where it goes.
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But, maybe you want to make a copy of a program's input and send it somewhere
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else. There is a way to do _that_ using a command called `tee`. `tee` copies
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its standard input to standard output, but it also writes a second copy to a
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file. For example:
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```console
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$ dmesg | tee dmesg.txt | grep 'msedge'
···
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---
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Thanks to violet spark for looking over this post and fact-checking as well as
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helping mend some of the brain dump and awkward wording into more polished
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sentences.
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Thanks to violet spark, cadence, and AstroSnail for looking over this post and
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fact-checking as well as helping mend some of the brain dump and awkward
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wording into more polished sentences.