glossary.htm

the unix power tools

CLASS="glossterm"
NAME="GLOSS-DEFAULT"
>default</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>In a program that gives you more than one choice, the default choice is
the one you get by not choosing.
The default is usually the most common choice.
As an example, the default file for many UNIX programs is the standard
input.
If you don't give a filename on the command line, a program will
read its standard input.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-DOT-FILES"
>dot (.) files (.cshrc, .login, .profile</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Files that are read when you start a program (including when you log in and
start a shell).
These set up your environment and run any other UNIX commands (for
instance,
<EM
CLASS="emphasis"
>tset</EM
>).
If your account uses the C shell, it will read <EM
CLASS="emphasis"
>.cshrc</EM
> and <EM
CLASS="emphasis"
>.login</EM
>.
Accounts that use the Bourne shell and shells like it read <EM
CLASS="emphasis"
>.profile</EM
>.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch02_02.htm"
TITLE="Shell Setup Files-Which, Where, and Why "
>2.2</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-DOUBLE-QUOTE"
>double quote</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>The <CODE
CLASS="literal"
>&quot;</CODE
> character. This isn't the same as two single quotes (<CODE
CLASS="literal"
>''</CODE
>)
together.
The <CODE
CLASS="literal"
>&quot;</CODE
> is used around a part of a UNIX command line where
the shell should do variable and command substitution (and, on the C
shell, history substitution), but no other interpretation.
<EM
CLASS="emphasis"
>See also</EM
> articles
<A
CLASS="xref"
HREF="ch08_14.htm"
TITLE="Bourne Shell Quoting "
>8.14</A
>
and
<A
CLASS="xref"
HREF="ch08_15.htm"
TITLE="Differences Between Bourne and C Shell Quoting "
>8.15</A
>,
<A
CLASS="xref"
HREF="glossary.htm#GLOSS-SINGLE-QUOTE"
TITLE=""
>single quote</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-ESCAPE"
>escape</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>When you <EM
CLASS="emphasis"
>escape</EM
> a character or a string of characters, you change
the way it is interpreted.
Escaping something can take away its special meaning, as in
<SPAN
CLASS="link"
>shell quoting (<A
CLASS="linkend"
HREF="ch08_14.htm"
TITLE="Bourne Shell Quoting "
>8.14</A
>)</SPAN
>-
or can add special meaning, as in
<SPAN
CLASS="link"
>terminal escape sequences (<A
CLASS="linkend"
HREF="ch05_08.htm"
TITLE="Terminal Escape Sequences "
>5.8</A
>)</SPAN
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-FLAG"
>flag</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>In programming, a <EM
CLASS="emphasis"
>flag variable</EM
> is set to signal that some
condition has been met or that something should be done.
For example, a flag can be set (&quot;raised&quot;) if the user has entered something
wrong; the program can test for this flag and not continue until
the problem has been fixed.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-FLAME"
>flame</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A heated or irrational statement.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-FRAGMENT"
>fragment</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>In the BSD &quot;fast filesystem,&quot; a fragment is a portion of a disk
block&nbsp;- usually one-eighth of a block, but possibly one-quarter or 
one-half of a block. If the last portion of a file doesn't occupy a 
full disk block, the filesystem will allocate one or more fragments 
rather than an entire block. Don't confuse &quot;fragments&quot; with 
&quot;fragmentation.&quot; Fragments allow the BSD filesystem to use 
larger block sizes without becoming inefficient.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-FREE-SOFTWARE-FOUNDATION"
>Free Software Foundation, FSF</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A group that develops the freely available GNU software.
Their address is: 675 Massachusetts Avenue, Cambridge, MA 02139 USA.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-FULL-DUPLEX"
>full-duplex</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Communications between a terminal and a computer where data flows in both
directions at the same time.
<EM
CLASS="emphasis"
>Half-duplex</EM
> communications, where data flows in only one direction
at a time, are unusual these days.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch41_02.htm"
TITLE="stty and All That Stuff "
>41.2</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-GNU"
>GNU</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Gnu's Not Unix, a system of software planned to
eventually be a freely available substitute for UNIX.
<EM
CLASS="emphasis"
>See also</EM
>
<A
CLASS="xref"
HREF="glossary.htm#GLOSS-FREE-SOFTWARE-FOUNDATION"
TITLE=""
>Free Software Foundation, FSF</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-GOTCHA"
>gotcha</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A &quot;catch,&quot; difficulty, or surprise in the way that a program works.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-HARDCODED"
>hardcoded</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>In general, a value that can't be changed.
For example, in a shell script with the command <CODE
CLASS="literal"
>grep jane</CODE
>, the
value <CODE
CLASS="literal"
>jane</CODE
> is hardcoded; <EM
CLASS="emphasis"
>grep</EM
> will always search for
<EM
CLASS="emphasis"
>jane</EM
>.
But in the command <CODE
CLASS="literal"
>grep&nbsp;$USER</CODE
>, the text that <EM
CLASS="emphasis"
>grep</EM
> searches for
is not hardcoded; it's a variable value.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-HASH-TABLE"
>hash table</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
><EM
CLASS="emphasis"
>Hashing</EM
> data into the format of a hash table lets
specially designed programs search for data quickly.
A hash table assigns a special search code to each piece of data.
For example, the C shell uses a hash table to locate commands
more quickly; the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>rehash</EM
> (<A
CLASS="linkend"
HREF="ch04_02.htm"
TITLE="A bin Directory for Your Programs and Scripts "
>4.2</A
>)</SPAN
>
command rebuilds the hash table after you add a new command.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-IO"
>I/O</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Input/output of text from software or hardware.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-INODE"
>inode</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A data structure that describes a file. Within any filesystem, the
number of inodes, and hence the maximum number of files, is set when
the filesystem is created.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch01_22.htm"
TITLE="How UNIX Keeps Track of Files: Inodes "
>1.22</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-I-NUMBER"
>i-number</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A UNIX file has a name (for people to identify it with) and an i-number
(for UNIX to identify it with).
Each file's i-number is stored in a directory, along with the filename, to
let UNIX find the file that you name.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch01_22.htm"
TITLE="How UNIX Keeps Track of Files: Inodes "
>1.22</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-JOB"
>job</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>One UNIX command. It is easy to be sloppy and use the terms job,
process, and program interchangeably. I do it and I'm sure you do,
too. Within UNIX documentation, though, the word &quot;job&quot; is usually
used to mean one, and only one, command line. Note that one command
line can be complex. For example:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
><CODE
CLASS="userinput"
><B
>pic a.ms | tbl | eqn | troff -ms</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>is one command, and hence one job, that is formed from four processes.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-JOB-NUMBER"
>job number</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Shells with job control assign a job number to every command that is stopped or
that is running in the
<SPAN
CLASS="link"
>background (<A
CLASS="linkend"
HREF="ch01_26.htm"
TITLE="What a Multiuser System Can Do for You "
>1.26</A
>)</SPAN
>.
You can use job numbers to refer
to your own commands or groups of commands. Job numbers are
generally easier to use than process IDs; they are much smaller
(typically between 1 and 10), and therefore easier to remember.
The C shell <EM
CLASS="emphasis"
>jobs</EM
> command displays job numbers.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch12_01.htm#UPT-ART-0439"
TITLE="Job Control: Work Faster, Stop Runaway Jobs "
>12.1</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-KERNEL"
>kernel</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>The part of the UNIX operating system that provides memory
management, I/O services, and all other low-level services. The
kernel is the &quot;core&quot; or &quot;heart&quot; of the operating system.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch01_14.htm"
TITLE="The Kernel and Daemons "
>1.14</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-KLUDGE"
>kludge</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A program or a solution to a problem that isn't written carefully, doesn't
work as well as it should, doesn't use good programming style, and so on.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-LIBRARY-FUNCTION"
>library function</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Packages of system calls (and of other library functions) for programmers
in C and other languages.
In general (though not always), a library function is a &quot;higher-level
operation&quot; than a system call.
<EM
CLASS="emphasis"
>See also</EM
>
<A
CLASS="xref"
HREF="glossary.htm#GLOSS-SYSTEM-CALL"
TITLE=""
>system call</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-LOAD-AVERAGE"
>load average</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>A measure of how busy the CPU is. The load average is useful, though
imprecise. It is defined as the average number of jobs in the run
queue plus the average number of jobs that are blocked while waiting for
disk I/O.
The
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>uptime</EM
> (<A
CLASS="linkend"
HREF="ch39_07.htm"
TITLE="Checking System Load: uptime "
>39.7</A
>)</SPAN
>
command shows the load average.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-.LOGIN-FILE"
>.login file</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
><EM
CLASS="emphasis"
>See</EM
> 
<A
CLASS="xref"
HREF="glossary.htm#GLOSS-DOT-FILES"
TITLE=""
>dot (.) files (.cshrc, .login, .profile</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-MODE"
>mode</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>In UNIX, an
octal number
that describes what access a file's owner, group,
and others have to the file.
<EM
CLASS="emphasis"
>See also</EM
> article
<A
CLASS="xref"
HREF="ch01_23.htm"
TITLE="File Access Permissions "
>1.23</A
>.</P
></DD
><DT
CLASS="glossterm"
><A
CLASS="glossterm"
NAME="GLOSS-MODULO"
>modulo</A
></DT
><DD
CLASS="glossdef"
><P
CLASS="para"
>Think back to your fourth grade arithmetic.
When you divide two
numbers, you have a <EM
CLASS="emphasis"
>dividend</EM
> (the number on top), a
<EM
CLASS="emphasis"
>divisor</EM
> (the number on the bottom), a <EM
CLASS="emphasis"
>quotient</EM
> (the
answer), and a <EM
CLASS="emphasis"
>remainder</EM
> (what's left over).
In computer science, this kind of division is very important.
However, we're usually more interested in the remainder than in the quotient.
When we're interested in the remainder, we call the operation a
<EM
CLASS="emphasis"
>modulus</EM
> (or <EM
CLASS="emphasis"
>modulo</EM
>, or <EM
CLASS="emphasis"
>mod</EM
>).

For instance, one of the examples on your fourth grade arithmetic text