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`Copyright 1990, 1989, 1988, 1987, 1986, 1985, 1984, 1983 AT&T
`All Rights Reserved
`Printed in USA
`Published by Prentice-Hall, Inc.
`A Division of Simon & Schuster
`Englewood Cliffs, New Jersey 07632
`
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`
`IMPORTANT NOTE TO USERS
`While every effort has been made to ensure the accuracy of all information in this document, AT&T
`assumes no liability to any party for any loss or damage caused by errors or omissions or by state(cid:173)
`ments of any kind in this document, its updates, supplements, or special editions, whether such er(cid:173)
`rors are omissions or statements resulting from negligence, accident, or any other cause. AT&T furth(cid:173)
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`
`TRADEMARKS
`DOCUMENTER'S WORKBENCH is a registered trademark of UNIX System Laboratories, Inc.
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`
`10 9 8 7 6 5 4 3 2
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`ISBN
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`0-13-947052-2
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`UNIX
`PRESS
`A Prentice Hall Title
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`Accessing and Manipulating Files
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`If the -w or -c option had been specified instead, the command would have
`reported the number of words or characters, respectively, in the file.
`Figure 3-21 summarizes the syntax and capabilities of the we command.
`
`Rgure 3-21: Summary of the we Command
`
`Command Recap
`we - count lines, words, and characters in a file
`
`command
`
`WC
`
`options
`-1, -w, -c
`
`arguments
`
`file(s)
`
`Description: we counts lines, words, and characters in the
`specified file(s), keeping a total count of all tallies
`when more than one file is specified.
`
`Options
`
`I Remarks,
`
`-1 counts the number of lines in the specified
`file(s)
`-w counts the number of words in the
`specified file(s)
`-c counts the number of characters in the
`specified file(s)
`
`When a file name is specified in the command line,
`it is printed with the count(s) requested.
`
`Protecting Your Files: the chmod Command
`The command chmod (short for change mode) allows you to decide who can
`read, write, and use your files and who cannot. Because the UNIX operating
`system is a multi-user system, you usually do not work alone in the file system.
`System users can follow path names to various directories and read and use
`files belonging to one another, as long as they have permission to do so.
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`Acce ssing and Manipulating Files
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`If you own a file, you can decide who has the right to read it, write in it (make
`changes to it), or, if it is a program, to execute it. You can also restrict permis(cid:173)
`sions for directories with the chmod command. When you grant execute per(cid:173)
`mission for a directory, you allow the specified users to cd to it and list its con(cid:173)
`tents with the ls command.
`To assign these types of permissions, use the following three symbols:
`allows system users to read a file or to copy its contents
`r
`allows system users to write changes into a file (or a copy of it)
`w
`allows system users to run an executable file
`x
`
`To specify the users to whom you are granting (or denying) these types of per(cid:173)
`mission, use these three symbols:
`u
`you, the owner of your files and directories ( u is short for user)
`members of the group to which you belong (the group could
`g
`consist of team members working on a project, members of a
`department, or a group arbitrarily designated by the person
`who set up your UNIX system account)
`all other system users
`
`o
`
`When you create a file or a directory, the system automatically grants or denies
`permission to you, members of your group, and other system users. You can
`alter this automatic action by modifying your environment (see Chapter 9 for
`details). Moreover, regardless of how the permissions are granted when a file is
`created, as the owner of the file or directory you always have the option of
`changing them. For example, you may want to keep certain files private and
`reserve them for your exclusive use. You may want to grant permission to read
`and write changes into a file to members of your group and all other system
`users as well. Or you may share a program with members of your group by
`granting them permission to execute it.
`How to Determine Existing Permissions
`You can determine what permissions are currently in effect on a file or a direc(cid:173)
`tory by using the command that produces a long listing of a directory's con(cid:173)
`tents: ls -1. For example, typing ls -1 and pressing the RETURN key while
`in the directory named starship/bin in the sample file system produces the
`following output:
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`Accessing and Manipulating Files
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`Permissions for the display and list files and the tools directory are shown
`on the left of the screen under the line total 35, and appear in this format:
`-rwxr-xr-x (for the display file)
`-rw-r--r-- (for the list filer)
`drwx--x--x (for the tools directory)
`
`After the initial character, which describes the file type (for example,
`a - (dash) symbolizes a regular file and a d a directory), the other nine charac(cid:173)
`ters that set the permissions comprise three sets of three characters. The first set
`refers to permissions for the owner, the second set to permissions for group
`members, and the last set to permissions for all other system users. Within each
`set of characters, the r, w, and x show the permissions currently granted to each
`category. If a dash appears instead of an r, w, or x, permission to read, write,
`or execute is denied.
`The following diagram summarizes this breakdown for the file named
`display.
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`~ rwxr-xr-x
`
`~ Permission to write to
`1 \
`
`\ " the file denied to
`group and other
`
`read write execute
`
`As you can see, the owner has r, w, and x permissions and members of the
`group and other system users have r and x permissions.
`There are two exceptions to this notation system. Occasionally the letter s or
`the letter 1 may appear in the permissions line, instead of an r, w or x. The
`letter s (short for set user ID or set group ID) represents a special type of per(cid:173)
`mission to execute a file. It appears where you normally see an x (or - ) for the
`user or group (the first and second sets of permissions). From a user's point of
`view it is equivalent to an x in the same position; it implies that execute permis(cid:173)
`sion exists. It is significant only for programmers and system administrators.
`(See the System Administrator's Guide for details about setting the user or group
`ID.) The letter 1 indicates that locking will occur when the file is accessed. It
`does not mean that the file has been locked.
`
`How to Change Existing Permissions
`After you have determined what permissions are in effect, you can change them
`by executing the chmod command in the following format:
`chmod uiho+permission file(s)<CR>
`
`or
`
`chmod who=permission file(s)<CR>
`The following list defines each component of this command line.
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`Accessing and Manipulating Files
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`chmod
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`who
`
`name of the program
`
`one of three user groups (u, g, or o)
`u = user
`g = group
`o = others
`
`+ or -
`
`instruction that grants(+) or denies(-) permission
`
`permission any combination of three authorizations (r, w, and x)
`r = read
`w = write
`x = execute
`
`file(s)
`
`file (or directory) name(s)
`listed; assumed to be branches from
`your current directory,
`unless you use full pathnames.
`
`The following examples show a few possible ways to use the chmod command.
`As the owner of display, you can read, write, and run this executable file.
`You can protect the file against being accidentally changed by denying yourself
`write (w) permission. To do this, type the command line:
`chmod u-w display<CR>
`After receiving the prompt, type ls -1 and press the RETURN key to verify that
`this permission has been changed, as shown in the following screen.
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`Accessing and Manipulating Files
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`As you can see, you no longer have permission to write changes into the file.
`You will not be able to change this file until you restore write permission for
`yourself.
`Now consider another example. Notice that permission to write into the file
`display has been denied to members of your group and other system users.
`However, they do have read permission. This means they can copy the file into
`their own directories and then make changes to it. To prevent all system users
`from copying this file, you can deny them read permission by typing:
`chmod go-r display<CR>
`The g and o stand for group members and all other system users, respectively,
`and the -r denies them permission to read or copy the file. Check the results
`with the ls -1 command.
`
`A Note on Permissions and Directories
`You can use the chmod command to grant or deny permission for directories as
`well as files. Simply specify a directory name instead of a file name on the com(cid:173)
`mand line.
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`However, consider the impact on various system users of changing permissions
`for directories. For example, suppose you grant read permission for a directory
`to yourself (u), members of your group (g), and other system users (o). Every
`user who has access to the system will be able to read the names of the files
`contained in that directory by running the ls -1 command. Similarly, granting
`write permission allows the designated users to create new files in the directory
`and remove existing ones. Granting permission to execute the directory allows
`designated users to move to that directory (and make it their current directory)
`by using the cd command.
`An Alternative Method
`There are two methods by which the chmod command can be executed. The
`method described above, in which symbols such as r, w, and x are used to
`specify permissions, is called the symbolic method.
`An alternative method is the octal method. Its format requires you to specify
`permissions using three octal numbers, ranging from Oto 7. (The octal number
`system is different from the decimal system that we typically use on a day-to(cid:173)
`day basis.) To learn how to use the octal method, see the chmod(1) entry in the
`User's Reference Manual.
`Figure 3-22 summarizes the syntax and capabilities of the chmod command.
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