While browsing through the web and local documents, I often see expressions like this:  See apt(8) for more information about the available commands.  My question is, what does the number here stand for?
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So, for example, when I type man ls I see LS(1). But if I type man apachectl I see APACHECTL(8) and if I type man cd I end up with cd(n).  I'm wondering what the significance of the numbers in the parentheses are, if they have any.
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What is a , and how do I use them on Ubuntu?
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This may be a simple question, but how do I use files starting with "-" with command line programs?   For example,  I am trying to use pdfgrep on a file named -2013-01-01.pdf but it complains that there are no options defined for 2, 0, 1, 3 and so on.  Changing the filename to something that does not start with "-" solves it, but this is not an option since the files aren't created by me. I simply want to check for a specific change I know is coming.
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What is the difference between absolute / canonical / relative paths?  I guess that absolute starts with a /. Can it contain /../?  Is a canonical path different from an absolute path in that it can't contain /../?  Is foo a relative path?  Googling posix canonical path didn't turn up much... What are the POSIX references for these terms?
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Setting up keys for a local repository on a new ubuntu 20.10 virtual machine, I got a message that apt-key add was deprecated and I should read the apt-key(8) man page. The apt-key(8) man page is a collection of words strung together, but if it contains information I can't winkle it out. Can anyone tell me what, exactly, I should type on my terminal instead of: apt-key add name-of-file The command does apparently still work after honking at me, so I was able to proceed, but would like to know what I'll need to do in the future.
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When I run:  ls -n  I get something like:   -rwxrwxrwx  1 1000 1000   765528 2009-10-15 18:41 file1 drwxr-xr-x 10 1000 1000     4096 2010-12-07 20:50 dir1 drwxr-xr-x  3 1000 1000     4096 2010-10-24 16:57 dir2   What does the second column (the number) mean?
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Given the following command:  gzip -dc /cdrom/cdrom0/file.tar.gz | tar xvf –   What does the - at the end of the command mean? Is it some kind of placeholder?
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What would be a more straight forward readable way of these commands?  And why exit 255 and not exit 1?  [ $# -eq 3 ] || { echo error ; exit 255 ; }  grep -q "string" &amp;&amp; exit 0
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I was browsing the html of my favorite site...ahem...and I saw this in the markup:  &lt;link href="/Content/all.min.css?d=20090107" rel="stylesheet" type="text/css" /&gt;   what does "?d=20090107" do? I'm assuming it's a date of some kind, but I'm not sure why it's in the path to the file. Any ideas?
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I've noticed that the majority of Ubuntu help websites present apt commands in the following format:  sudo add-apt-repository ppa:[ppa-address] sudo apt-get update sudo apt-get install [application-name]   But I know that apt commands like this one work too:  sudo add-apt-repository ppa:[ppa-address] &amp;&amp; sudo apt-get update &amp;&amp; sudo apt-get install [application-name]   Is there a particular reason that most sites present in the first format? Are there known issues with chaining commands with &amp;&amp;, or to put it another way are there any benefits/drawbacks to using either method?  Further, other then loss of internet, are there any circumstances in which an apt command might fail?
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Sometimes I see chmod commands that use four octal digits instead of three -- what is the optional first digit for? For example, chmod 777 is equivalent to chmod a+rwx; what's the same command for chmod 2777?
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Simple question, but I'm not sure where to look and google doesn't respond to periods and slashes.  I'm just trying to count the # of files &amp; directories in the current directory (not including subfolders/files) and was trying to differentiate ls -1 | wc -l and ls | wc -l since they seem identical.  A site I was looking at said "Keep in mind that this is also counting the ./ and ../ directories." regarding the one with ls -1, and I'm not sure if that means it includes the directories before or something (which I don't want), but it didn't seem to do that from testing.  Could someone confirm which one of those would be most adequate for counting # of files &amp; directories in the current directory only (not sub) and what they mean by ./ and ../ directories?
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Sometimes I see a command like  find . -name * -exec ls -a {} \;   I was asked to execute this.  What does {} \; mean here?
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Often, I read something like command time(1) or printf(3) in the manpages.  What do the numbers in the brackets stand for?
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Is there a standard idiom for comparing version numbers?  I can't just use a straight String compareTo because I don't know yet what the maximum number of point releases there will be.  I need to compare the versions and have the following hold true:  1.0 &lt; 1.1 1.0.1 &lt; 1.1 1.9 &lt; 1.10
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I am a new user, who has just been told to run / execute a command. The answer has some text with a grey background like this text here or  sometimes this text here.   However, I don't know what "run" or "command" means in relation to Ubuntu.  How do I do this?    This is intended for use as a Q&amp;A you can link to when you tell someone to run a command.
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The output of ls -l command yields the following result:    What is the number field between file permission and owner? i.e. what are those 1, 1, 1, and 2? I checked the --help but that doesn't explain it.  [EDIT]  I thought it was the number of files in a directory but it isn't. See image. "tempFolder" has 3 files but still show a "2"