M

MAC
Mac is an ambiguous term.

A "Mac" computer is short for Macintosh, a brand of computer from Apple Corporation. The operating system is called MacOS. The latest version, Mac OS X, runs on top of their own version of Unix called Darwin, which is a synthesis of BSD and Mach, a kernel developed at Carnegie Mellon University.

Every computer's ethernet interface card has a hardwired MAC address.

MAC also stands for Mandatory Access Control, part of the POSIX Capabilities feature set, also known as POSIX 1e. As of this writing, the standard is still experimental and is limited to high security applications. This usage is somewhat uncommon.

Mac means "son" in Irish Gaelic and Scots Gaelic, and as such, many family names start with Mac or Mc, especially in English-speaking countries. By extension, many companies and products have names that start with Mac.

MAC Address
Every computer's ethernet interface card has a MAC address, short for Media Access Control address. (This is unrelated to Macintosh computers, although Macs also have MAC addresses.) Usually this is a six byte number, in hexadecimal, segmented by bytes, such as 01.06.03.77.e6.0A. Sometimes this is called the Hardware Address or the Station Address.

The MAC address is fixed into the interface when it is manufactured. The first three bytes tell the manufacturer. Here are some examples:

00000C Cisco
00AA00 Intel
080009 Hewlett-Packard
080020 Sun
080046 Sony
08005A IBM
800010 AT&T

Do not confuse the MAC address with the IP address, which is a four byte number, segmented by bytes, but in decimal, such as 217.219.152.96. IP addresses are controlled by software. They can change, even during operation. In fact, an interface can respond to more than one IP address, for instance, when it is multihosting. A DHCP server hands out unique IP address assignments as a computer is booting up.

A MAC address is sometimes taken to be an identifying address for a computer. Unfortunately, some computers have more than one network interface card, and some have none. Often these cards can be unplugged and moved to a different computer.

man
The man command is a way to get help information that is stored on your Unix computer. Usually the topics discussed are command-line commands, subroutines or files. You must know the name of what you are looking up to use it, although sometimes there are ways to guess.

When this document says, for instance, "see also man ls", simply type that in to a command line.
$ man ls

Since we are targeting many different Unix systems, sometimes some of the entries we refer you to won't be present on your particular system. Use of the man command is always by trial and error. Pay special attention to the "SEE ALSO" section in the man pages you do read (near the end); usually this is the best way to find more man topics to read about.

Originally, the Unix programmer's manual was printed on paper, in eight sections.
Section 1: user commands (for the command line)
Section 2: system calls (for programming)
Section 3: runtime library calls (for programming)
Section 4: file formats that the system runs on
Section 5: files that the system runs on
Section 6: games
Section 7: special device files
Section 8: administration commands (for command line)

Over the years, more sections have been added, often with suffixes such as section 3MAIL, which is for runtime library calls that deal with email. Other sections have been abandoned or redefined, depending on which dialect of Unix you are running. Add-in software packages often include their own man page files. You might have to steer the man command in the event that your keyword appears in more than one section. For instance, try man 8 mount on Linux or man -s8 mount on Solaris.

Also remember that not all information is in the man pages; on GNU systems you must often use the info command. Information on how to use gui systems is often available using a separate program - check the Help menu, or the icons on the toolbar on your screen.

see also man man, info.

mode
mode is an extremely ambiguous term. Usually a mode refers to some setting or overall state. Unfortunately, everything in computers has some sort of state or information, and only in a handful of cases do people call this a 'mode'. You have to figure it out from context.

Unix files have a 16-bit set of flags called the mode bits or the 'file mode' for the file. Sometimes this is the 'mode' people are talking about if there's no other clues.

mode bits
The mode bits on a file tells permissions and also the kind of file.

Mode bits are often displayed in octal, because the bit fields break down nicely that way.
bits (octal)meaning
0170000indicates the kind of file ( regular, special, directory, etc.)
0004000the SUID bit (usually 0)
0002000the SGID bit (usually 0)
0001000the Sticky bit (usually 0)
0000700permissions for file owner (user's userid = file's userid)
0000070permissions for group (user a member of file's groupid, but is not the owner)
0000007permissions for world (user is not owner nor member of group)

permissions for owner, group or world
bitmeaning
4Read permission
2Write permission
1Execute/Search permission

For instance, a file with mode 0100750 would be a regular file (0100000), with permissions 7 for the owner of the file, 5 for people in the same group, and 0 (no access) for anybody else.

Often, only the last nine bits, the permission bits, are discussed because the rest are predictable. One would say that our example file has mode 750.

Mode bits are also often displayed in 'rwx' format, where the previous file's mode bits become - rwx r-x ---. The first character tells the file type: hyphen is a regular file, d is a directory, and so on. The last nine characters represent the last nine permission bits. A letter means the bit is 1, and a hyphen means 0. This format was popularized by the ls command.

The command line command chmod changes the permission bits for a file. It of course cannot transmute the type of a file.

There are minor variations in the rules from one platform to another, so you might want to do a man 2 stat or man chmod for more information.


Modify Time
The modify time (or date) on a file is the time at which it was last written to. Along with the access time and the header time, it is one of the main timestamps on a Unix file.

The modify time for a regular file is set whenever it is written to or appended to. For a directory, it is set whenever a file is added, removed, or renamed in its list, but not for other kinds of changes to the files inside.

When you edit or modify a file, and then save the changes, sometimes the original file rewritten from the beginning. Sometimes, the original file is deleted, and a new file is created with the same name. Sometimes, the original file is renamed to an alternate name (such as filename~), and the new version is written out under the original filename. Although the resulting file is the same, the different methods can result in different access times, header times and modify times for the resulting file, and also for the directory it is in. Even viewing a file might result in its modification, depending on what program you are using. This is usually considered bad form, as this might risk damaging the data, and also needlessly changes the modify time, but it occasionally happens.

You should do some experimentation with your software, on your volume, before you trust the times, especially access times, that you see.

Mount
To mount a volume means to connect your computer to the volume data, somehow, so that it appears in your computer's filesystem, as one or more directories full of files. The location in your filesystem that it appears is called the Mount Point.

Mount Point
The place in the overall filesystem that a volume is rooted. In Interrogator, you can see all of your mount points by looking at the Host Window (Open menu).

For example, a Linux system might have this arrangement:

disk hda5 mounted on /
disk hda6 mounted on /usr
disk hda7 mounted on /usr/local

The hda codes actually refer to the block devices in the /dev directory. Each refers to a large expanse of bytes (perhaps 2 gigabytes) in a disk partition on disk /dev/hda. Each one has been formatted so that it can be a volume on Linux.

The first volume has a subdirectory named usr. There are no files in this directory, which is fortunate because nobody can see them. If you open a Directory window in Interrogator for the directory /usr, you will instead see files that are on the root directory of the second volume. This is because /usr is the mount point for the volume.

In this /usr directory, there are several directories, but we want to pay attention to the one named local. Opening a Directory window for /usr/local, you will see files that are in the root directory for the third volume, because /usr/local is the mount point for that volume.

Now let's imagine that the superuser unmounts hda7 from /usr/local, and he finds a directory /home/low-cal to mount it on. The Host window will show this:

disk hda7 mounted on /home/low-cal

The previous mount point, /usr/local, will go back to being a normal directory on the /usr volume. The previous contents, if any, will be visible. The original contents of /home/low-cal, however, will now be invisible. (The fact that hda7 is now rooted on hda5 instead of hda6 is hardly important, except that you can't dismount /usr with /usr/local being used.)

If you have a CD or other removable disk, this will also show up in the Host window A cd might show up as this:

disk hdb mounted on /cdrom

Notice that the device has no number - the CD is not partitioned the way the hard disk was. hdb in Linux means slave IDE disk - it's just the name of the plug that the CD drive is plugged into.

Mount points are unique to Unix. In MacOS and MS Windows, all disk volumes are rooted at the top level (My Computer in Windows, or the desktop on MacOS). Unix instead has one tree hierarchy. The root volume has its mount point at the top, and other volumes have their mount points further down in the tree. Also on Unix, the name of the top level directory comes from its mount point, not the disk's own name, as in MacOS. Unix has no drive letters as with MS Windows.


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