CS 497C

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CS 497C Introduction to UNIXLecture 4
Understanding the UNIX Command

• Chin-Chih Changchang_at_cs.twsu.edu

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The PATH Locating Commands

• UNIX obtains the list of directories that has to
  be searched from of an environment variable
  PATH.
• If you evaluate the value of PATH, youll find a
  directory list separated by colons
• echo PATH
• /bin/usr/bin/usr/X11R6/bin/usr/local/bin.

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The PATH Locating Commands

• There are five directories in this list, and when
  you issue a command, the system will search this
  list in the sequence specified to locate and
  execute it.
• . indicates the current directory.
• To know in which directory the command is
  located, you can use the type command.
• type ls
• ls is /bin/ls

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Internal and External Commands

• The agency that actually does all this work is
  known as shell a command interpreter.
• Since ls is a file having an independent
  existence in the /bin directory (or /usr/bin), it
  is called an external command. Most commands are
  external in nature.
• The commands built in shell are known as internal
  commands.

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Internal and External Commands

• The type command itself is a shell built-in.
  Whether or not you are able to execute it depends
  on the shell you use.
• In some versions of the C shell, the type command
  wont work. In this case, which or whereis can be
  used.
• The shell uses its internal command even though
  there is an external command of the same name.

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Command Structure

• An UNIX command is composed of two parts command
  and argument.
• Commands and arguments are separated by any
  number of spaces or tabs know as whitespace.
• ls-l
• ls-l Command not found.
• The argument that begins with a symbol is
  called an option.

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Options and Filenames

• Every command has a fixed set of options.
• The command with its arguments and options is
  entered in one line that is referred to as the
  command line.
• Some commands accept a single filename, some
  accept more than one filenames.
• Wrong options will be indicated.
• An option can have its own arguments.
• pine f mail-may

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Combining Options

• Options that begin with a sign can normally be
  combined with only one sign.
• For instance, this command has three options
• ls l a -t
• The -l option provides most details of a files
  attributes. The t option sorts files according
  to the time of modification. The -a option
  lists the hidden files.

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Combining Options

• You can combine these options in this way
• ls lat
• ls -atl
• This facility reduces your typing load. The shell
  parses (break up) the option combination into
  separate options.
• Some commands wont let you combine options in
  the way you did just now.
• tar cv f /dev/fd0 b 18

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Combining Options

• There are four options here, but two of them (-f
  and b) has their own arguments. These arguments
  are called option parameters.
• We combined the c and v options here. We can
  combine the f and b options, provided that
  their own parameters are placed in the same
  sequence.
• tar cvfb /dev/fd0 18
• The following one wont work
• tar cvfb 18 /dev/fd0

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Exceptions and Variations

• All commands dont compulsorily use options and
  arguments.
• Commands like clear dont accept any argument.
• The who and date commands may or may not be
  specified with arguments.

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Exceptions and Variations

• The ls command permits more variations
• Without any argument (ls)
• With only one option (ls l)
• With only filenames (ls chap01 chap02)
• Using a combination of both (ls la chap01
  chap02)
• This text discusses commands that use options.
  The vast majority will conform to the option
  rules that have been discussed but there are some
  exceptions.

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Flexibility of Command Usage

• UNIX allows you to specify more than one command
  in the same command line. Each command is
  separated from the other by a (semicolon)
• who ls -l
• The is one of special characters that are
  understood by the shell. These special characters
  are known as metacharacters.

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A Command Line Can Overflow

• When a command has more characters than the
  terminal width, it simply overflows to the next
  line.
• You may want to spread the command into multiple
  lines. The shell issues a secondary prompt,
  usually gt (or ?).
• echo This is a two-line
• gt text message
• In C shell you have to enter a \ (backslash).

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Flexibility of Command Usage

• Subsequent commands can be entered at the
  keyboard without waiting for the prompt. There is
  a type-ahead buffer (a temporary storage area)
  which stores all these commands and passes them
  on for execution.
• A command may not behave in the way you want
  because UNIX comes in many flavors.

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Flexibility of Command Usage

• The uname can print system information.
• uname
• Linux
• The uname r command shows the version number of
  the kernel.
• uname r
• 2.2.19