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CS 497C – Introduction to UNIX Lecture 4: Understanding the UNIX Command

CS 497C – Introduction to UNIX Lecture 4: Understanding the UNIX Command. Chin-Chih Chang chang@cs.twsu.edu. The PATH: Locating Commands. UNIX obtains the list of directories that has to be searched from of an environment variable – PATH .

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CS 497C – Introduction to UNIX Lecture 4: Understanding the UNIX Command

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  1. CS 497C – Introduction to UNIXLecture 4: Understanding the UNIX Command Chin-Chih Changchang@cs.twsu.edu

  2. 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, you’ll find a directory list separated by colons: $ echo $PATH /bin:/usr/bin:/usr/X11R6/bin:/usr/local/bin:.

  3. 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. $ typels ls is /bin/ls

  4. 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.

  5. 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 won’t 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.

  6. 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.

  7. 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

  8. 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 file’s attributes. The –t option sorts files according to the time of modification. The -a option lists the hidden files.

  9. 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 won’t let you combine options in the way you did just now. tar –cv –f /dev/fd0 –b 18

  10. 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 won’t work: tar –cvfb 18 /dev/fd0

  11. Exceptions and Variations • All commands don’t compulsorily use options and arguments. • Commands like clear don’t accept any argument. • The who and date commands may or may not be specified with arguments.

  12. 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 chap01chap02) • 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.

  13. 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.

  14. 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 > (or ?). $ echo “This is a two-line > text message” • In C shell you have to enter a \ (backslash).

  15. 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.

  16. 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

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