Survey of Operating Systems:
§ 3: Basic UNIX Commands

Instructor: M.S. Schmalz

This section reviews UNIX file, directory, and system commands, and is organized as follows:

3.1. The UNIX Command Line Interface
3.2. Overview of UNIX File System
3.3. UNIX Documentation via man command
3.4. File Commands
3.5. Directory Commands
3.6. Basic System Commands

Information in this section was compiled from a variety of text- and Web-based sources, and is not to be used for any commercial purpose.

Reading Assignments and Exercises

UNIX implementations employ a shell that accepts user commands and invokes the appropriate OS processes. These OS routines can act on files, some of which can be executable, others of which are data or documentation files. Additionally, UNIX provides the man utility to help users view documentation about a given command or group of commands. UNIX file, directory, and basic system commands enable one to (a) navigate through a UNIX-based computer system and (b) locate, retrieve, modify, or store information organized in files or hierarchical structures called directories.

3.1. The UNIX Command Line Interface

Reading Assignments and Exercises

UNIX users run one of a collection of programs, the most frequently used of which is called a "shell". Numerous shells have been developed over the years (e.g., Korn shell, C shell, etc.) The shell you will use is the C-shell, whose syntax is similar to the "C" language. In point of information, the name of the C-shell program is "csh", which is the command you will invoke to run it.

When you run the C-shell, you will see a prompt on the screen. The prompt can be the name of your computer followed by a special character, like "%" or ">". The whole prompt might look like this:

Example of UNIX Prompt: hausdorff% ,

which would mean that (a) you are using a computer called "hausdorff", and (b) the shell is waiting for your keyboard input (signified by "%"). Since prompt formats vary widely, your shell's prompt might be different from the example shown above.

Convention. Throughout these course notes, the symbol "%" indicates the end of the prompt, the symbol after which you type a command or statement into the C-shell.

At the prompt, if you type the name of a program to be run followed by the Enter key, then the shell will run the program. When the program is done, you are sent back to the shell. Many programs that the shell runs are similar to traditional operating system commands (e.g., the directory listing command DIR in MS-DOS). To get a directory listing in UNIX, type "ls" and press to list the contents of your directory.

Example. If you want to invoke a UNIX directory listing, the system prompt and the string you type (emboldened) would appear as follows:

% ls

Many UNIX programs or commands have options that change the command's functionality. In the UNIX command line, options are usually preceded by a dash.

Example. Detailed information about files in the current directory can be obtained by typing ls -l, i.e.,

Detailed File Information: % ls -l

Here, the "-" precedes an option ("l", which stands for long).

Screen output from the ls -l command should look similar to the following. In an actual UNIX session, a user's login-name would be substituted for "bsimpson":

    total 6
    -rw-r--r--  1 bsimpson      123 Jun 23  1998 -i
    drwxr-S---  2 bsimpson      512 Apr  7 12:31 Articles/
    drwxr-s---  1 bsimpson      512 May  2 14:01 News/
    drwxr-s---  4 bsimpson     1024 Apr 11 14:17 programs/
    -rw-------  1 bsimpson     1891 May  4 11:56 dead.letter
    -rw-------  1 bsimpson    25247 May  4 11:33 mbox

In Section 3.3, we will describe in detail the meaning of the preceding display, as well as the function of UNIX file commands. Now that you know what the command line is and how to use it, we will digress briefly to discuss the UNIX file system.

3.2. Overview of UNIX File System

Reading Assignments and Exercises

When a disk is formatted, the physical disk is divided into a number of partitions, which are abstractions, each of which has an associated file system. The file system consists of hierarchically-arranged subdirectories and files that can be conveniently represented by an abstraction called a directory tree. A directory tree that might be found in a UNIX file system is exemplified in Figure 3.2.1.

Figure 3.2.1. Tree representation of an example UNIX file system.

3.2.1. UNIX System Directories

Several standard directories typically appear in a UNIX filesystem, as follows:

dev directory contains special device files that are used to drive hardware objects such as CD-ROM, floppy disk, etc. These files will be discussed in a later portion of the course, if time permits.

etc directory files are required for operation of the specific machine or system that you are using (also called machine-dependent or system-dependent files

home directory houses a collection of user directories on a given system. This is where your personal files would be kept if you had an account on a UNIX system such as grove.

opt directory typically contains application-specific subdirectories and files (e.g., programs such as FrameMaker or a public domain package such as the image editor xv or graphing package xvgr).

usr directory contains files that can be shared by all users (recall data and program sharing introduced in the MULTICS operating system)

var directory is comprised of files whose size varies with time, such as incoming mail and spooler files (e.g., print spool files for printout).

3.2.2. UNIX File Description

A Unix file is specified by a parameter block called an i-node. An i-node exists on disk for every file that is on that disk, and there exists a copy of the i-node in kernel memory for every open file. All of the information that UNIX knows about a file, except the file name, is stored in the i-node, which includes:

File access and file type, known as the mode.

File ownership information, which is important for security

Time stamps that record date and time of last modification, last access, and last modification of mode

Link count and File size in bytes, which pertain to the amount of information in a file

Addresses of physical blocks - these tell UNIX where to find the data for the file whose i-node is currently being examined

There exist 13 physical block addresses in an i-node. Each of these addresses is 3 bytes long, and the addresses point to the following information:

Addresses 1-10: The first ten block addresses refer to data blocks

Address 11 refers to a first-level index block that holds the addresses of further data blocks

Address 12 points to a second-level index block that holds the addresses of further index blocks

Address 13 refers to a third level index block (which holds the addresses of further second level index blocks, to allow for file expansion beyond the second level).

All physical addresses associated with a file are implicitly assumed to reside on the same disk, since standard UNIX provides no facility for a file to span more than one disk. There is no requirement that the physical addresses of a file should be contiguous (i.e., in adjacent sectors on the disk). Since UNIX is able to handle multiple files on one disk, it is unlikely that contiguity would offer any performance advanatages. UNIX also has no requirement that all logical blocks (file address specifications) should map to physical blocks (pieces of data on the disk). Instead, files can have "holes", which often happens with large, sparsely populated, direct-access files.

Now, let us discuss UNIX file size. For purposes of discussion, assume 512 byte blocks and 3 bytes per address, which is equivalent to a disk capacity of about 8 GByte. An index block of 512 bytes is capable of holding 170 3 byte addresses. Given this information, the largest file size in UNIX can be calculated as follows:

1. Directly addressed blocks 10 × 512 byte = 5,120 bytes

2. Blocks addressed via first level index block 170 × 512 byte = 87,040 bytes

3. There will be 170 index blocks addressed via the second level index block. This will address 170 × 170 × 512 bytes = 14,796,800 bytes

4. Via the third level index block there will be 170 × 170 × 170 × 512 bytes of addressable data, which yields 2,515,456,000 bytes.

Therefore, the total addressable space per file is 2,530,344,960 bytes (approximately 2.5 Gbytes). Note that BSD and other more recent versions of Unix use a larger disk i-node format that consists of 32 4 byte words. The block addresses now occupy 4 bytes rather than 3 and various other fields are larger. This trend is expected to be carried forth into future versions of UNIX implemented on progressively larger disk storage systems.

In particular, the BSD extensions include space for 32-bit user and group ids and an i-node generation number. This number, incremented when a free i-node is used for a different file, is employed by the network file system for file handle calculation. Additionally, in the BSD extension, all time fields have expanded to 64 bits so that the year 2038 (some say Y2.038K) problem (when the Unix standard &time variable maxes out, then wraps) can be avoided.

Once a file has been opened, the version of the i-node resident in memory contains the following information:

In-memory i-node status that indicates whether or not

a) the i-node is locked

b) a process is waiting for the i-node to be unlocked

c) the memory-resident i-node is dirty, i.e. differs from the version stored on disk

d) file modifications have been made that were not written to disk

Device number of the disk on which the file is resident

I-node number, occasionally called the i-number - On disk the i-nodes form an array and the i-number is inferred from the i-node's position in this array.

Pointers to other memory-resident i-nodes

Reference count that indicates the number of instances the file is (or, in some UNIX implementations, has been) active or open.

3.2.3. UNIX Directories

Note that the file name and the i-number which uniquely identifies the i-node is stored in a directory entry, which in UNIX is implemented as a file. When discriminating between directories and files, the UNIX kernel will not allow a directory to be opened for writing using normal file system calls, to preserve file system integrity. Otherwise, a user could change i-node information at will, with disastrous results to his or her file system.

Previous versions of Unix (System 5 and earlier) used a directory format comprised of a sequence of 16 byte records. In particular, the first 2 bytes held the i-number and the remaining 14 bytes held the file name. Hence, there was a limit of 14 characters on the maximum size of a file name. When a file was deleted, the i-number of the associated directory record was set to zero to mark the record as having been freed up for future use. However, the filename was not set to null.

More recent versions of Unix have adopted the more flexible structure for directory entries shown in the following figure.

Figure 3.2.2. High-level diagram of i-node structure.

When the user invokes the file deletion command, the space occupied by the file's directory entry is combined with the free space at the end of the previous directory entry (this is denoted as amalgamated space in Figure 3.2.2). The information block size field is then incremented to reflect the amount of space allocated. This allows reasonably long file names without requiring large directory records. A disadvantage is more complex manipulation of directories, which are read via a special system call (getdent() - get a directory entry). In programming practice, manipulation of directories from within programs is handled via library routines, to safeguard filesystems from user-inflicted damage.

3.3. UNIX Documentation via man command

Reading Assignments and Exercises

The majority of operating systems have an on-line help facility. UNIX expresses its help information in terms of man pages, which are supposed to resemble pages from a software manual.

In UNIX, the man command displays information from specially-formatted reference manuals. These are useful, reasonably complete manual pages that the user can select by (a) command name, or (b) one-line summaries selected either by keyword (-k option), or by the name of an associated file (-f option). If no manual page can be found for a given command, option, or keyword, then the man program prints an error message.

The man command is invoked using the following syntax:

machine% man {keyword}

For example, on the SunOS system, entering man exit after the command line prompt yields the following information for the exit command:

    Fortran Library Routines                                 EXIT(3F)

    NAME
         exit - terminate process with status

    SYNOPSIS
         subroutine exit (status)
         integer*4 status
    
    DESCRIPTION
         exit  flushes  and  closes all the files of the process, and
         notifies the parent process if it is executing a wait.   The
         low-order  8 bits of status are available to the parent pro-
         cess.  These 8 bits are shifted left 8 bits, and  all  other
         bits  are zero.  Therefore, status should be in the range of
         256 - 65280.
    
         This call never returns.
    
         The C function exit may cause  cleanup  actions  before  the
         final `sys exit'.
    
         If you call exit without an argument, you get a warning mes-
         sage, and a zero is automatically provided as an argument.
    
    FILES
         libF77.a
    
    SEE ALSO
         exit(2), fork(2), fork(3f), wait(2), wait(3f)
    
    SunOS 5.6             Last change: 98/09/16                     1

This documentation has several categories, as follows:

NAME - Gives the name of the command or programming language keyword, together with a short description of functionality

SYNOPSIS - Expanded description of functionality with respect to the string that follows NAME

DESCRIPTION - Fully describes the command or programming language keyword, including options and functionality in detail, often with examples

FILES - Shows what system files are associated with the command or programming language keyword

SEE ALSO - Provides other commands or programming language keywords that can be used as arguments to related man calls.

Additionally, at the bottom of the man page is a date of most recent modification and a version number (in the above example, SunOS 5.6) that tells you how recent the help or documentation is. The man command further supports the bugs category, which tells the user or system analyst what known problems are associated with the command or programming language keyword. In a simple command like exit listed above, there are few if any known bugs. However, other commands might have bugs associated with them, as do public-domain programs that can be run from UNIX.

Additional Exercise: Find four UNIX commands that have bugs associated with them, where the bug(s) is(are) listed on the man page for the given command.(Hint: Be prepared to answer a question similar to this on a midterm exam.)

3.3.1. Use of keyword (-k) Option

Occasionally, a user might know the documentation that he or she is looking for in a general sense (e.g., by topic), but not by exact command or keyword. UNIX man utilities provide a semantic keyword cross-reference capability that facilitates a limited type of content-directed search. For example, to find man page information about a loader program, we first need to know what commands or keywords are associated with the keyword loader. Thus, we type:

machine% man -k loader

which usually produces the following display on a Sun workstation running SunOS or Solaris:

    AutoLoader AutoLoader (3) - load subroutines only on demand
    DynaLoader DynaLoader (3) - Dynamically load C libraries into Perl
    code .PP dl_error(), dl_findfile(), dl_expandspec(),
    dl_load_file(), dl_find_symbol(), dl_find_symbol_anywhere(),
    dl_undef_symbols(), dl_install_xsub(), dl_load_flags(),
    bootstrap() \- routines used by DynaLoader modules ExtUtils
    ExtUtils (3) - make a bootstrap file for use by DynaLoader
    SelfLoader SelfLoader (3) - load functions only on demand download
    download (1) - host resident PostScript font downloader download
    download (1) - host resident PostScript font downloader

What the previous command did was to search the man database for commands whose descriptive string or SEE ALSO category contained the string loader. The terms that were returned, which could be used as arguments to the man command, include DynaLoader, ExtUtils, SelfLoader, and download. Note that man pages are sometimes repeated in the output of the man -k command, and occasionally there is much superfluous information.

3.3.2. The man filename (-f) Option

When one specifies man -f {file...}, where {file...} denotes a list of one or more filenames, man attempts to locate manual pages related to any of the given files. It strips the leading path name components from each file, and then prints one-line summaries containing the resulting base- name or names. This option also uses the windex database.

Additional Exercise: Read about the windex database routines (in the C language) by typing man windex. Use the man -k windex command to find out more about windex.

3.4. File Commands

Reading Assignments and Exercises

The UNIX file system supports viewing of directories and file contents, moving and copying of files, renaming files and setting permissions, and other similar tasks. These operations are standard on modern computer file systems.

3.4.1. Viewing Directory File Information

Recall the example output of the ls -l command given in Section 3.1:

total 6
-rw-r--r--  1 bsimpson      123 Jun 23  1991 -i
drwxr-S---  2 bsimpson      512 Apr  7 12:31 Articles/
drwxr-s---  1 bsimpson      512 May  2 14:01 News/
drwxr-s---  4 bsimpson     1024 Apr 11 14:17 programs/
-rw-------  1 bsimpson     1891 May  4 11:56 dead.letter
-rw-------  1 bsimpson    25247 May  4 11:33 mbox

The first column (e.g. -rw-r--r--), pertains to file permissions, namely, who has access to your files and what they can do with them (r = "read", w = "write", etc.).

The name (bsimpson) is the login-i.d. of the person who owns the files (should be you). The number after that is how many bytes the file uses, followed by the creation date/time and finally the file name. (Some systems have different formats for ls output, so check with your consultant or system administrator if you have questions.)

Example. In the preceding directory listing, file mbox is readable and writeable by bsimpson, occupies 25,247 bytes on disk, and was created May 4 of the current year at 11:33am.

Another option for the ls command is -a, where a denotes all. Certain files in your directory are usally invisible to the ls or ls -l command. (A file is invisible if its name begins with ".") These hidden files are usually system-oriented files that are transparent to a user's day-to-day operations. If you had to look at these every day, they would probably clutter up your directory listing. By typing

% ls -a

you can obtain a listing of all your regular files, plus several files beginning with ".", for example, ., .., .cshrc, .login, .newsrc and several others.

One can usually combine options on the ls command. For example, to detailed information on all of your files can be produced by typing ls -a -l or ls -l -a.

Efficient Usage Tip. With some commands (like ls) you can put several options after just one dash, for example,

ls -al or ls -la.

3.4.2. Moving Files

The command mv, which means move, allows a user to move or rename files. For safety, mv should be used with the -i option, which asks you if you really want to overwrite a file. You are also prompted if you try to rename a file to a name that exists in the current directory. New users have the -i option set by default. The following discussion explains two different forms of mv that you can use.

mv file1 file2

This form of mv changes the name of a file (from file1 to file2). This command also applies to directories. For example, to rename a directory, use the directory name instead of one or more filenames.

mv file1 ... fileN directory

This form of the mv command will move one or more files (separated by spaces) into the designated directory, which is the last argument on the command line. Note that file1 ... fileN can be either file or directory names.

Example. Given the following UNIX session:

% ls

a.out*   cop3610/   emg3312/   private/   typescript 

% mv a.out typescript

remove typescript? n

% ls

a.out*   cop3610/   emg3312/   private/   typescript 

% mv a.out bogus

% ls

bogus*   cop3610/   emg3312/   private/   typescript 

% mv bogus typescript cop3610

% ls

cop3610/   emg3312/   private/

% ls cop3610

bogus*   typescript

The first mv attempted to rename a.out to `typescript (a file which already exists). Since the -i option is used by default, the question remove typescript? was returned because the file `typescript already exists. By answering y, the target file typescript would have been overwritten by the file a.out. Answering n simply cancels the move process.

In the second use of mv, a.out was renamed to bogus.

The third mv command moved the bogus and typescript files into the cop3610 directory.

3.4.3. Copying Files

To copy a file, use cp, which means copy. The cp command has two formats that are analogous to the mv command formats. The only difference between the these commands is that cp doesn't remove the original file. Thus we say that renaming a file is a destructive form of copying, because the source file is removed.

Similar to mv, cp also offers a -i option. New users have this option set by default. Again, note that cp and mv are almost identical in usage.

3.4.4. Removing Files

The command rm means remove and is used to erase files. For safety, this command should almost always be used with the interactive option -i. Thus, whenever you remove a file you will be asked to verify the removal of the file. New users have rm initialized to use the -i option by default.

Example.

% ls

a.out*   cop3610/   emg3312/   private/   tempfile   typescript 

% rm tempfile

rm: remove tempfile? y

% ls

a.out*   cop3610/   emg3312/   private/   typescript 

In this example, the file tempfile was removed. Note that rm asked if tempfile should really be removed (the corresponding prompt is emboldened).

Note: When you remove a file with rm, the file is no longer available from the user's perspective - you will not be able to get it back unless you request a tape backup dump from the systems staff. Most UNIX systems do not have an undelete command such as that found in some versions of MS-DOS or utility programs such as Norton Utilities^TM.

3.4.5. Changing File Permissions

On a multi-user system, keeping selected files out of the reach of prying eyes is important. As a (relatively weak) security measure, the Unix operating system has built-in file permissions feature.

By typing ls -l, we have seen that a long listing of the files in the current directory can be displayed. An example is displayed below. As noted previously, the letters and dashes on the left side of the listing represent the permissions set on each file or directory.

Example.

% ls -l

drwxr-xr-x 11 bsimpson        572 Nov 16 05:11 drafts/

-rw------- 1  bsimpson          1 Dec 10 20:04 termpaper

The letter in the first column describes the type of the file, while the other nine letters describe the file's permissions, which indicate who can access the file and how it can be accessed.

In order to understand the concept of file security at a basic level, the following permission codes are listed:

- : permission is not set

r : read permission is set

w : write permission is set

x : execute permission is set

d : the file represents a directory

The nine permission characters are partitioned into three sets of three characters each, where each set of three characters contains r, w and x codes. The three partitions comprise an access control list, and are described as follows:

user - Permissions in the first left-hand group of three characters control the user's access to the file. If the current user is not the owner of the file, then the user might not be able to access that file, depending on how the permissions are set. To find the owner of a file, use ls -l, and look at the username (e.g., bsimpson in the preceding example).

group - A collection of users can be formally aggregated in a group, which is a list of permissible usernames. For example, a group cop3610 could contain the usernames of all students enrolled for this course in a given semester. User permissions thus control the access that people in the group(s) assigned to the file have. You can use the command ls -lg to list all groups associated with each file.

other - Users in the other partition comprise all users not in the user or group partitions. The other permissions control access that users in the rest of the world (who can login to your system) have to each file.

Examples.

-rw------- 1 instr cop3610 7830 Nov 19 15:06 hw2

Only the owner (instr) of the file hw2 has permission to read and write to the file.

-rw-r----- 1 instr cop3610 17820 Nov 19 15:06 hw1

This is a file that is readable by both the owner (instr) and by users that are in the cop3610 group. However, only the owner has write permission to the file (rw code in the left-hand group of three characters).

drwxr-xr-x 6 instr staff 512 Apr 19 22:27 /home/instr/410/

This is a directory (as shown by the left-hand d) that is readable and executable by everyone, but can be written to only by the owner (instr). The groups field contains staff, which is usually a group of privileged users.

-rw-rw-rw- 1 instr cop3610 783 Sep 18 15:06 temp

This file (temp) is readable and writable by all users.

When a file or directory is created, UNIX sets default file permissions according to the umask descriptor in your .cshrc file. To change the permissions on a file, the chmod command is used, which has the following form:

chmod mode file(s) 

where mode specifies the change of permissions on the specified file(s). The mode is specified as follows:

Step 1. Choose one or more permission partitions by specifying u (user), g (group), o (other), or a (all).

Step 2. Type + (add permission) or - (delete permission).

Step 3.Specify the permissions to be changed using r (read), w (write), or x (execute).

Example 1.

% ls -l

drwxr-xr-x 11 bsimpson         572 Nov 16 05:11 drafts/
-rw------- 1  bsimpson        5666 Dec 10 20:04 termpaper

% chmod go+r termpaper

This use of chmod gave read access for group and others to the file termpaper, as shown below:

% ls -l

drwxr-xr-x 11 bsimpson         572 Nov 16 05:11 drafts/
-rw-r--r-- 1  bsimpson        5666 Dec 10 20:04 termpaper

Example 2. Assume the file status shown at the end of the previous example.

% chmod o-rx drafts
% ls -l

drwxr-x--- 11 bsimpson         572 Nov 16 05:11 drafts/
-rw-r--r-- 1  bsimpson        5666 Dec 10 20:04 termpaper

This example removed read and execute access for others from the drafts directory.

Example 3. Assume the file status shown at the end of the previous example.

% chmod go-rwx termpaper

% ls -l

drwxr-x--- 11 bsimpson         572 Nov 16 05:11 drafts/
-rw------- 1  bsimpson        5666 Dec 10 20:04 termpaper

In this example, the chmod command is directed to remove all permissions for group and others from termpaper.

Note: To make a directory accessible to everyone, one must specify group and others read and execute permissions for the entire directory. For example:

chmod go+rx directory

3.4.6. Viewing File Contents

We begin with the cat command, then progress to the more command.

The command cat means concatenate and is often used to view short files. Supplying cat with multiple file names, as follows:

cat file1 file2 ... fileN

will display each file sequentially in a continuous stream of text. This is why the command is called concatenate. If a file is large and you want to use cat to view it, you will have to have quick reflexes, and use {Ctrl}-s and {Ctrl}-q to stop and restart scrolling of text so you can get a chance to view it. On modern computers, scrolling is usually so fast that you will likely lose the text you are trying to see. Thus, we recommend the use of commands such as more or less to view your files.

The commands more and less are commonly used to view files one screen at a time. When you use either of these commands, you will have an information bar at the bottom of the screen. For example, you can press the {spacebar} to go to the next screen, b to go back a page, or the {Return} to scroll the file forward a line at a time. When you finally get to the end of the file, more will return a Unix prompt, while less will wait for you to press q to quit. The following list of options will work for both more and lesscommands, unless otherwise indicated.

{spacebar} - Takes you to the next page.

b - Takes you to the previous page.

{Enter} key - Scrolls forward one line.

k - Scrolls backward one line (less only).

g - Takes you to the beginning of the file (less only).

G - Takes you to the end of the file (less only).

h - Shows you a help screen.

/pattern - Goes to the next occurrence of pattern in the file. When you finish typing in the pattern, you must press the {Enter} key. Here, pattern is a regular expression, which we will define later in this course.

n - Search forward for another occurrence of the pattern previously searched for with /.

r - Search backwards for a previous occurrence of the pattern previously searched for with /.

q - Quits the more or less program.

Either program works, but less is much more flexible. Backward scrolling is just just one of many features that less has, which more does not. We suggest using less if you want full-featured file display.

3.4.7. Wildcard Usage

In certain cases, UNIX supports application of a command to multiple files. The command length and complexity can be reduced via wildcard characters for efficient matching of filenames. Wildcard characters are:

1. ? -- matches any one character.

2. * -- matches any contiguous group (string) of zero or more characters.

To better understand the use of wildcards, let us consider the following examples.

Example. Let a directory contain the files file, file2, file3, fun, fun2, mbox, and readme. Here follows a terminal interactive session, where brackets ([ ]) contain explanations that do not appear on the computer monitor:

% ls *         [ * matches all filenames]
file   file2   file3   fun   fun2   mbox   readme

% ls f*        [ f* matches all filenames beginning with f]
file   file2   file3   fun   fun2

% ls file?     [ file? matches all filenames of length 5 beginning with file]
file2  file3

% ls ????      [ ???? matches all filenames of length 4]
file   mbox

The first example shows ls with *, which matches all files in the directory (since all filenames have 0 or more characters).

In the second example, f* matches all the files beginning with an f, which are listed.

The third example uses file? to match all filenames that begin with the word file and have one character following that word.

In the fourth example, four ? characters in a row match all filenames that are four and only four characters long.

Wildcards should be used with caution. For example, when used with a destructive command like rm, the wildcard "*" could help you remove all the files in your directory!

3.5. Directory Commands

Reading Assignments and Exercises

Having covered file commands, we now turn to commands related to directories, which are collections of files and other directories.

3.5.1. Discovering your Location

It is often difficult to remember where you are within a given file system, due to the tree structure that usually has many levels. To make the location of the current directory clear to you, UNIX provides the pwd command, which means present working directory.

Example. Using the previous login i.d. of bsimpson, typing the command pwd in B. Simpson's home directory would yield the following interactive session:

% pwd

/home/bsimpson

This also holds for any location that you are at in the file system.

3.5.2. Changing the Directory

To reset the current directory, which is like moving from one directory to another directory, UNIX provides the cd command, which means change directory. Whenever you need to move to your home directory, just type cd with no arguments. If you specify a directory name as an argument, cd will attempt to located that directory, then set it as the current directory if it is a valid directory in the UNIX filesystem.

Example. Suppose you want to view files in bsimpson's `cop3610' directory. Then, you would type:

% cd ~bsimpson/cop3610
% pwd

/home/bsimpson/cop3610

%ls -l

and the filenames would be displayed.

Important Note: the tilde preceding bsimpson is expanded by the Unix C-shell into the full pathname of that person's home directory, so you do not have to enter a potentially long pathname.

Suppose B. Simpson has an assign1 directory under cop3610. To move there, type

% cd assign1
% pwd

/home/bsimpson/cop3610/assign1

If you use `..' as the argument, cd will bring you up a level in the directory tree.

Example. Assume the filesystem from the previous example.

% cd ..

% pwd

/home/bsimpson/cop3610

3.5.3. Viewing Directory Contents

We've already overviewed the ls command for listing directory contents, together with the "l" and "a" options. Several commonly-used options follow:

-a : Lists all the files in the directory, including hidden files.

-F : Appends a single character to filenames on the display only that aren't text files, to denote file type. For example, directories have a trailing / and executable files have a trailing *. New users have this option set by default. Hence, the examples of ls in these course notes assume that this option is being used.

-l : Lists in long format, telling (from left to right) the file's permissions, number of links to the file, the file's owner, the file's size in bytes, and the time the file was last modified. Examples were provided previously.

-g : In conjunction with the -l option, the -g option of ls includes the file's group following the owner field for each file.

-R : Used to generate a recursive listing of all directories encountered below the level of the current directory.

Note that ls lists the current directory by default, if you do not specify the name of a file or directory you wish to list. Some interesting results can occur, as shown below.

Example. Suppose you are in bsimpson's home directory, and you type ls -a. If the directory is not secured, you might see the following:

% ls -a
./      .cshrc     .login       .msgsrc     cop3610/     private/
../     .emacs     .logout      a.out*      emg3312/     typescript

Here, the two files called "." and ".." are directory links. In particular, "." is a link to the current directory, and ".." is a link to the parent of the current directory. You can backtrack upward through a directory structure by using "cd .." to pop up one directory level.

Additional Exercise. Use man ls to learn the options of the ls command.

3.5.4. Creating a New Directory

Users can create new directories using the mkdir command. Prior to this, one must determine where the directory is to be placed. For example, a directory can be located one level below one's home directory or subdirectory.

Example. Let bsimpson create a directory within his emg3312 directory:

% pwd                     [Locate the parent directory of the new directory]
/home/bsimpson/emg3312

% mkdir drafts            [Make the new directory]
% ls                      [Check to be sure that the new directory is there]

drafts/    termpaper

Recall that, when you create a directory, its permissions and the default permissions of all its children (files and directories) are set according to the umask setting. It is always wise to view permissions with the ls -l command, then change them with the chmod command.

3.5.5. Removing an Existing Directory

Example. Assuming that bsimpson has a drafts directory, let us try to remove that directory using rmdir:

% pwd                              [check the current directory]
/home/bsimpson/emg3312
          [this is the current directory]

% ls                               [list contents of current directory]
drafts/ termpaper

% rmdir drafts                     [try to remove the drafts directory]
rmdir: drafts: Directory not empty [this is an error message from rmdir]
 

% ls drafts                        [o.k., let's see what is in "drafts"]
monologue notes
                 [contents of directory "drafts"]

% rm drafts/*                      [try to remove contents of directory "drafts"]
rm: remove drafts/monologue? y     ["rm" asks do you really want to do this?
rm: remove drafts/notes? y           ...and you answer "y" for yes]

% ls drafts                        [check to see if files have been removed]
                                   [no listing => directory "drafts" is empty]

% rmdir drafts                     [now we can remove "drafts"]
% ls                               [so what is left under emg3312?]
termpaper                          [...only the "termpaper" directory remains]
 

Note that bsimpson first had to remove (or move) the files from the directory drafts before removing the directory itself. Note also that the files were removed with rm followed by a wildcard ("*").

3.5.6. Directory and File Command Summary

Here follows a summary of the UNIX file and directory commands covered in this section:

pwd

(present working directory) Display the full pathname of the current directory.

cd directory

(change directory) Changes the current directory. If you specify no arguments, then cd will bring you back to your home directory.

ls directory ...

(list directory) Lists the contents of a directory or directories. If the directory name is unspecified, then ls will list the contents of the current directory. You can also supply filename(s) instead of a directory name, to get more information about one or more files.

mkdir directory ...

(make directory) Create a directory or directories.

rmdir directory ...

(remove directory) Remove an empty directory or directories (the directories must not contain any files).

cat file1 ...

(concatenate) Used to view a file or files continuously on your terminal. If you use cat to view a long file, it is necessary to use {Ctrl}-s keys to pause the screen and {Ctrl}-q keys to unpause. Otherwise, file contents will be displayed too fast for you to read them. For files longer than one screen, it is recommended that you use either more or less to view the file.

more file

less file

These two programs display a file one screen at a time, and offer viewing options such as paging backwards through a file and pattern searching. In a typical UNIX idiosyncrasy, less is the more sophisticated of the two and has features that aren't found in more.

rm file1 ...

(remove) Removes the specified file(s).

mv file1 file2

mv file1 ... fileN directory

(move) Moves a file or directory. In the first form, file1 will be moved to (renamed as) file2. The second form will move a number of files into a directory which you specify as the final argument on the command line. Directory names can be used in place of the filenames in either of the forms, to move or rename directories.

cp file1 file2

cp file1 ... fileN directory

(copy) Copies files. In the first form, file1 will be copied to a file called file2. The second form will copy a number of files into a directory which you specify as the final argument on the command line.

chmod mode file1 ...

(change mode) Changes file permissions. The mode specifies how permissions are to be changed for the listed file(s).