While watching a video, I saw the following:
% more tinyUF.txt
10
4 3
3 8
6 5
What is this % sign before more command?
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Gilles 'SO- stop being evil'
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Max Koretskyi
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2`export PS1=%` ?? – Roman Gräf May 03 '17 at 13:51
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@RomanGräf, what's that? – Max Koretskyi May 03 '17 at 13:58
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1You can change the normal `$` sign using `export PS1=...` where `...` is the replacement. – Roman Gräf May 03 '17 at 13:59
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@RomanGräf, I see, thanks for the suggestion – Max Koretskyi May 03 '17 at 14:17
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@Roman Gräf: You can actually do a whole lot more with the prompt than just change the character. Mine includes the machine name (since I often have xterm windows into other machines) and the last few directories in the current path. And if I'm root, it becomes bright red. – jamesqf May 04 '17 at 05:29
1 Answers
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That's the shell prompt, or more precisely, it's the shell's primary prompt (there are several). It's the shell's way of saying, "go ahead, I'm ready for input now".
The % prompt is common in csh-type shells and in the zsh shell, while sh-shells (like bash and ksh93) ordinarily use a $ as the prompt.
The prompt usually changes to # for the root user since a sufficiently powerful user should be reminded of that power by having an alternate prompt (as the POSIX standard puts it).
The primary prompt in sh-type shells is determined by the value of the shell variable PS1.
Summary of the comments below, with additions:
The # character of the root prompt (used by both sh and csh shells) coincides with the familiar shell comment character. Copying and pasting a command as root would render the pasted command inoperable if a user also copied the shell prompt. Note that # was adopted as the root prompt before the shell had a comment convention (reference: email from Doug McIlroy).
The es and rc shells of Plan 9 use the ; character as the default prompt. A consequence of this is that copying and pasting a command, including the prompt, will still mean that the pasted command is valid (and it will be executed).
A way for enabling one to have a custom, but still copy-pastable, shell prompt would be to use : something ; , where something could be the current directory, hostname or time, for example.
Kusalananda
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2Also in `zsh` (a Bourne-style shell, but with lots of features from `tcsh`). Note that Bourne-like and csh-like shells typically use `#` for users of uid 0. – Stéphane Chazelas May 03 '17 at 07:22
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1Also note that `rc`-like shells use `;` there (one can copy-paste the full line and that's valid shell code, same idea as root's `#` prompt being a comment). – Stéphane Chazelas May 03 '17 at 07:25
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2a very important choice of # for root is that if you copy a command including it you will not cause damages (as # starts a comment, so will comment your command (well, its first line)). Some people use ">" and if you copy it as well, it can clobber (empty completely) binaries if the command has a full or relative path or if you are in the right directory (and have sufficient rights, for example if you are root). I have seen this happen. – Olivier Dulac May 03 '17 at 09:14
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1@StéphaneChazelas: Didn't the #-for-root convention originate a _long_ time before terminals where cutting-and-pasting an entire line is easily possible became common? – hmakholm left over Monica May 03 '17 at 11:59
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1@HenningMakholm, yes. It originated even before csh introduced # as a comment leader. Still, it's true that `#...` lines are comments when copy-pasted even if it was not the intention. I don't know if it was the intention behind `;` in `rc` either, though I observe it's a convenient consequence. – Stéphane Chazelas May 03 '17 at 12:31
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It would be more correct to say Bourne-like than `sh`-type. The Thomson shell (`sh` until Unix v6 before it was replaced by the Bourne shell in v7) had `%`/`#` as well. I do not know why the Bourne shell changed it to `$`. Possibly to make it clear to the user that they were getting a different shell from what they would be used to before. Note that `#` as a comment leader was introduced by csh. The Bourne shell had no comments until much later (you'd use the `:` null-command as an ersatz). – Stéphane Chazelas May 03 '17 at 20:46
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It looks like `sh` in [PDP-7 Unix](https://github.com/DoctorWkt/pdp7-unix) and [Unix V1](http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/sh.s) had `@`/`#` before it was changed to `%`/`#` ([v5 had `%`](http://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s2/sh.c), but I didn't find what it was for v2, v3, v4). – Stéphane Chazelas May 03 '17 at 21:59
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IIRC, in early Unices, `@` was also the `kill` character (now ^U) and `#` was erase (now ^H or ^?), so if the early ttys had had the capability to copy-paste, that would have caused interesting behaviours – Stéphane Chazelas May 03 '17 at 22:03