While watching a video, I saw the following:

% more tinyUF.txt
4 3
3 8
6 5

What is this % sign before more command?

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    export PS1=% ?? – Roman Gräf May 3 '17 at 13:51
  • @RomanGräf, what's that? – Max Koretskyi aka Wizard May 3 '17 at 13:58
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    You can change the normal $ sign using export PS1=... where ... is the replacement. – Roman Gräf May 3 '17 at 13:59
  • @RomanGräf, I see, thanks for the suggestion – Max Koretskyi aka Wizard May 3 '17 at 14:17
  • @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 4 '17 at 5:29

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 while sh-shells (like bash and ksh93) ordinarily uses 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 are 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) happens to coincide with the common shell comment character. Copying and pasting a command as root would render the pasted command inoperable, if the shell prompt was also copied. 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 uses 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.

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    Also 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 3 '17 at 7:22
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    Also 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 3 '17 at 7:25
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    a 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 3 '17 at 9:14
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    @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? – Henning Makholm May 3 '17 at 11:59
  • @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 3 '17 at 12:31

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