Some Unix Commands

Introduction

Here is what's on this page:

1. Using the Shell to Enter Commands
   • Continuation Lines and Background Commands
   • I/O Redirection
2. Some Basic Commands
   • I/O Redirection Examples
     • man: View on-line documentation.
     • lp: Print on the printer.
   • Some Other Commands
     • more: View text a screenful at a time.
     • egrep: Search files for text.
     • find: Search for files by name.
3. File Names and Pathnames
   • Home Directory Names
4. Creating and Using Directories
5. Copying, Moving or Renaming, and Deleting Files
6. Wildcards

1. Using the Shell to Enter Commands

All Unix systems provide a basic shell called the Bourne Shell, which is in /bin/sh. An extension to the Bourne Shell is the Korn Shell, which is in /bin/ksh. (These two shells are named for the people who wrote them.) An alternate to sh and ksh that is based on certain features of the C language is called the "C Shell," which is in /bin/csh. Finally, the C Shell has been extended, notably to make better use of the arrow keys during command entry, in the T-C Shell, which is in /bin/tcsh.

It's the tcsh that you will be using to enter commands when you use your Unix account.

It's possible to change your default shell, but it's not a good idea because we will assume you are using tcsh in all the examples we give in this course. However, if you want to experiment with the other shells you can execute them just like any other Unix command. For example, to try the Korn shell, just type "ksh" at a shell prompt. You exit a shell by typing the exit command.

The shell is the program that issues a prompt string at the beginning of a line, waits for you to enter a command, executes the command, and issues the prompt for you to enter the next command. The prompt string you will see is "qcunix1> " and, like everything else, can be changed to suit your preferences. For the examples of shell commands we will use "$ " to mean whatever prompt string you have set up.

Each command line begins with the name of a command, optionally followed by command line arguments, and optionally followed by various operators. Spaces are used to separate the elements of the command line.

When you are typing a command to the tcsh, you can use the up and down arrow keys to scroll through the history list of commands you have previously entered that the shell maintains. Use the left and right arrows to move around within a command line, use <Backspace> to erase characters, or just type new characters into the middle of the line, and press <Enter> to run the command.

A command name is either the name of an executable file or the name of a command that is executed directly by the shell itself. (You can set up aliases for command names, but we will ignore that for now.) When you type in a command line, the shell reads it and examines the first word. If it is the name of a built-in command, the shell executes it directly, and then issues a prompt for you to type another command. If it is not the name of a built-in command, the shell searches a number of standard directories until it finds an executable file with the same name, and then executes the file. When the file finishes executing, the shell issues a prompt for another command. The message "command not found" means the command name was neither a shell built-in command nor the name of an executable file in any of the standard directories.

The list of directories that the shell searches to find executable files is called the search path. Setting up the search path is an example of a built-in command. It's one of the commands that is executed automatically when you log in and the shell starts executing. The shell processes all the command lines in the file ~/.cshrc whenever it starts running, and also processes the commands in ~/.login when you first log in. See Pathname Syntax for an explanation of these file names.

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Continuation Lines and Background Commands

Note: If you log out of your account while a command is running in the background, it continues to run forever! This will not endear you with the people who administer qcunix1, to say the least. The jobs command (a tcsh built-in command) will tell you if you have any background commands running, and the kill command can be used to get rid of background commands.

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I/O Redirection

If you already know about I/O redirection and pipes in DOS, you already know about I/O redirection and pipes in Unix, since DOS adopted the same syntax for the same concepts. Even if you already know about I/O redirection, it's a good idea to read the material here, since tcsh includes some features you might not be familiar with.

Unix programs typically read input from the keyboard and write output to the terminal screen. You can tell the shell that you want the program to read its standard input from a file rather than from the keyboard by redirecting stdin, and you can tell the shell that you want to write its standard output to a file rather than to the screen by redirecting stdout. Putting a < character followed by a file name on the command line redirects stdin, and putting a > character followed by a filename redirects stdout. You can connect stdout from one program to stdin of another program by putting a | character between two commands on the same line, which is called a pipe. Any number of commands can be piped together on one command line.

The shell will not allow you to redirect stdout to a file that already exists. (There is an option to override this behavior.) However, you can redirect stdout so that it will be appended to the end of an existing file by using >> instead of > as the redirection symbol.

Programs normally write error messages to the stderr output stream rather than to stdout so that the messages will be seen by the user even if stdout is redirected to a file or a pipe. The shell will let you redirect stderr if you want to, though. The syntax is to use >& instead of > before the output file name.

In the C++ programs that you write, the cout output stream is connected to stdout, cerr and clog go to stderr, and cin comes from stdin.

You can see some examples of I/O redirection when you read the section on Basic Commands.

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2. Some Basic Commands

I/O Redirection Examples

The first Unix command you should learn is called man. All the Unix commands are documented in an on-line manual, and the man command is used to look at pages in that manual.

To start with a basic example,

     $ man man

) Remember, in the examples, "$ " represents the prompt that the shell displays before reading the command line you type in. In this example, the first man is the name of the command you want to run (man) and the second man is a command line argument, in this case the name of the command you want to know about.

You get to the next screenful of man output by pressing the spacebar, or you can quit man by pressing 'q'.

Now, let's use I/O redirection to save the output of a man command in a file. In this case, you would redirect stdout:

     $ man man > man.out

For an example of input redirection, we'll use the lp command , which normally prints whatever it reads from the keyboard (stdin) on the printer. You can redirect stdin to come from the a file, though:

     $ lp < man.out

Finally, you could use a pipe to print the output of a man command without bothering to build a file:

     $ man | lp

Some Other Commands

Scroll Through Text

Example: Scroll through the contents of somefile:

     $ more somefile

     $ ls /usr/users | more

Search Files by Content

Example: Search the files named alpha, beta, and gamma for the string "hello there".

     $ egrep "hello there" alpha beta gamma

Complicated Example: Let's say you have a project with several files in it and you think you sometimes spelled a variable this_thing and sometimes wrote it thisThing. The following command will find all occurrences of either string in all the files in the current directory:

     $ egrep "this_*[tT]hing" *

For more information on egrep (or any other command!) see the man page.

Search for Files by Name

     $ find $HOME -name somefile -print

     $ find . -name "*.cc" -print

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3. File Names and Pathnames

     $ touch a_new_file.that_i_touched.but_it_is empty

Unix keeps track of the time a which a file was last modified, which is the most common use for touch. If you touch a file that already exists, its modification time is changed, which can be useful sometimes when using the make utility that we will be dealing with elsewhere. You usually create new files with a text editor, not with touch. We introduced touch here so we could talk about file names without having to deal with the editor.

     $ ls -l

There is an old joke about two Unix users who were discussing how arcane the system can be. In particular, they were arguing about how many different meanings a period character has in Unix. One claimed there were 47 different uses for the period, and the second claimed there were 48. They argued for a long time until the first said in exasperation, "OK, you'll just have to list all your 48 ways." "Well," the second one said, "first, you can put one at the end of a sentence, ..." "Stop, stop," the first user said, "you're right. I forgot about that one!" Well, you are about to see your first three uses of periods in Unix.

     $ ls -la
     $ ls -l -a

• When you type a pathname in DOS you use a '\' (backslash) character to separate parts of the path. In Unix you use a '/' (forward slash) character to do the same thing.
• DOS filenames are limited to 8 characters, a period, and a 3 character extension. As you saw earlier, Unix allows long pathnames and puts no restrictions on how many periods there are in a file name or how many characters follow a period.
• DOS provides a few bits of attibute information about files: read-only, hidden, system, and archived. Unix provides read, write, and execute permission bits for the file owner, the owner's group, and others.

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Home Directory Names

All the sample code from the textbook is available in the directory CS-200/Examples in Dr. Vickery's account. You can refer to this directory using either the full pathname, /usr/users/vickery/CS-200/Examples or by using the ~ (tilde) character to get the shell to supply the pathname to Dr. Vickery's home directory for you: ~vickery/CS-200/Examples.

You can refer to your own home directory in three different ways:

• The characters ~/ at the beginning of a pathname refer to your home directory.
• The environment variable $HOME refers to your home directory. Unfortunately, setting and referencing environment variables is beyond the scope of this Web page.
• The sequence ~<user ID> refers to your home directory if you substitute your own account name for <user ID>. This is just the general case of the example given for accessing Dr. Vickery's home directory.

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4. Creating and Using Directories

You can use the pwd command to display the full pathname of your current working directory at any time. Also, tcsh can put the last component of the pathname of the current working directory in your prompt string:

     $ set prompt="$USER@qcunix1[%C]> "

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5. Copy, Move, and Delete Files

The following example copies file abc to def, moves file ghi into the parent of the current working directory without changing its name, moves file jkl to the subdirectory sub (which must already exist) with the new name mno, and finally removes all files used in the example:

     $ cp abc def
     $ mv ghi ..
     $ mv jkl sub/mno
     $ rm abc def ../ghi sub/mno

For this reason, I strongly suggest that you always use the "-i" option with the rm command, which causes it to ask you to confirm each file you ask to remove. The best way to do this is to make an alias for the rm command that includes the -i option. Type this into your ~/.cshrc file:

     alias rm "rm -i"

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6. Wildcards

Square brackets are used to list specific characters or a range of characters to substitute into potential file names. For example, the command, "ls [a-z]*" will list the names of all files in the current directory that have names starting with a lower case letter of the alphabet, and not those that start with an upper case letter, a digit, or any other character. (Note that the asterisk as a wildcard matches zero or more occurrences of any character, but as a pattern match character in egrep it matches zero or more occurrences of the character that immediately precedes it.)

Wildcards work across subdirectories. The following example will copy all source files in all subdirectories that have names starting with a lower case letter into the directory BACKUPS:

     $ cp [a-z]*/*.[ch]* BACKUPS

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• Using Your Unix Account - Contents
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