Linux Fundamentals, Part 3 - Revision history

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‎Before You Startdfhdhgdfghdfghdfhgdghdgfhdhgfdfgh

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‎Before You Start

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‎Resources

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← Older revision		Revision as of 08:17, 30 June 2012
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−	== Before You ~~Startdfhdhgdfghdfghdfhgdghdgfhdhgfdfgh~~ ==	+	== Before You Start ==

	=== About this tutorial ===		=== About this tutorial ===

← Older revision		Revision as of 08:16, 30 June 2012
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−	== Before You ~~Start~~ ==	+	== Before You Startdfhdhgdfghdfghdfhgdghdgfhdhgfdfgh ==

	=== About this tutorial ===		=== About this tutorial ===

@@ Line 816: / Line 816: @@
 Congratulations on finishing Part 3 of this tutorial series! At this point, you should know how to locate information in system and Internet documentation, and you should have a good grasp of the Linux permissions model, user account management, and login environment tuning.
-== Resources ==
+=== Resources ===
 Be sure to check out the various Linux documentation resources covered in this tutorial -- particularly the [http://www.tldp.org/ Linux Documentation Project]. You'll find its collection of guides, HOWTOs, FAQs, and man pages to be invaluable. Be sure to check out [http://www.lwn.net/ Linux Weekly News] as well.

@@ Line 752: / Line 752: @@
 === Export and set -x ===
-Because of this behavior, variables can be set in ~/.bash_profile or /etc/profile and marked for export, and then never need to be set again. There are some options that cannot be exported, however, and so they must be put in put in your ~/.bashrc and your profile in order to be set consistently. These options are adjusted with the set built-in:
+Because of this behavior, variables can be set in '''~/.bash_profile''' or '''/etc/profile''' and marked for export, and then never need to be set again. There are some options that cannot be exported, however, and so they must be put in your '''~/.bashrc''' and your ''profile'' in order to be set consistently. These options are adjusted with the set built-in:
+<pre>
 $ set -x
+</pre>
-The -x option causes bash to print out each command it is about to run:
+The <span style="color:green">-x</span> option causes bash to print out each command it is about to run:
+<pre>
 $ echo $FOO
 $ echo foo
 foo
-This can be very useful for understanding unexpected quoting behavior or similar strangeness. To turn off the -x option, do set +x. See the bash man page for all of the options to the set built-in.
+=== Setting variables with "set" ===
-Setting variables with "set"
+The <span style="color:green">set</span> built-in can also be used for setting variables, but when used that way, it is optional. The bash command <span style="color:green">set FOO=foo</span> means exactly the same as <span style="color:green">FOO=foo</span>. Un-setting a variable is done with the <span style="color:green">unset</span> built-in:
+<pre>
 The set built-in can also be used for setting variables, but when used that way, it is optional. The bash command set FOO=foo means exactly the same as FOO=foo. Un-setting a variable is done with the unset built-in:
-−
 $ FOO=bar
 $ echo $FOO
@@ Line 772: / Line 772: @@
 $ unset FOO
 $ echo $FOO
-Unset vs. FOO=
+=== Unset vs. FOO= ===
+This is ''not'' the same as setting a variable to nothing, although it is sometimes hard to tell the difference. One way to tell is to use the <span style="color:green">set</span> built-in with no parameters to list all current variables:
-This is not the same as setting a variable to nothing, although it is sometimes hard to tell the difference. One way to tell is to use the set built-in with no parameters to list all current variables:
+<pre>
-−
 $ FOO=bar
 $ set | grep ^FOO
@@ Line 785: / Line 785: @@
 $ unset FOO
 $ set | grep ^FOO
-Using set with no parameters like this is similar to using the export built-in, except that set lists all variables instead of just those marked for export.
+=== Exporting to change command behavior ===
-Exporting to change command behavior
+Often, the behavior of commands can be altered by setting environment variables. Just as with new bash sessions, other programs that are started from your bash prompt will only be able to see variables that are marked for export. For example, the command <span style="color:green">man</span> checks the variable PAGER to see what program to use to step through the text one page at a time.
+<pre>
 Often, the behavior of commands can be altered by setting environment variables. Just as with new bash sessions, other programs that are started from your bash prompt will only be able to see variables that are marked for export. For example, the command man checks the variable PAGER to see what program to use to step through the text one page at a time.
-−
 $ PAGER=less
 $ export PAGER
 $ man man
+</pre>
-With PAGER set to less, you will see one page at a time, and pressing the space bar moves on to the next page. If you change PAGER to cat, the text will be displayed all at once, without stopping.
+With PAGER set to <span style="color:green">less</span>, you will see one page at a time, and pressing the space bar moves on to the next page. If you change PAGER to <span style="color:green">cat</span>, the text will be displayed all at once, without stopping.
+<pre>
 $ PAGER=cat
 $ man man
-Using "env"
+=== Using "env" ===
+Unfortunately, if you forget to set PAGER back to <span style="color:green">less</span>, <span style="color:green">man</span> (as well as some other commands) will continue to display all their text without stopping. If you wanted to have PAGER set to <span style="color:green">cat</span> just once, you could use the <span style="color:green">env</span> command:
-Unfortunately, if you forget to set PAGER back to less, man (as well as some other commands) will continue to display all their text without stopping. If you wanted to have PAGER set to cat just once, you could use the env command:
+<pre>
-−
 $ PAGER=less
 $ env PAGER=cat man man
 $ echo $PAGER
 less
-This time, PAGER was exported to man with a value of cat, but the PAGER variable itself remained unchanged in the bash session.
+== Summary and resources ==
 Summary and resources
-Summary
 Congratulations on finishing Part 3 of this tutorial series! At this point, you should know how to locate information in system and Internet documentation, and you should have a good grasp of the Linux permissions model, user account management, and login environment tuning.
-Be sure to check out the various Linux documentation resources covered in this tutorial -- particularly the Linux Documentation Project. You'll find its collection of guides, HOWTOs, FAQs, and man pages to be invaluable. Be sure to check out Linux Weekly News as well.
+== Resources ==
-The Linux System Administrators guide (available from the "Guides" section at www.tldp.org) is a good complement to this series of tutorials -- give it a read! You may also find Eric S. Raymond's Unix and Internet Fundamentals HOWTO to be helpful.
+Check out the other articles in this series:
-In the Bash by example article series, Daniel Robbins shows you how to use bash programming constructs to write your own bash scripts. This bash series (particularly Parts 1 and 2) is good preparation for the LPIC Level 1 exam and reinforces the concepts covered in this tutorial's "Tuning the user environment" section:
+In the "Bash by Example" article series, Daniel shows you how to use bash programming constructs to write your own bash scripts. This series (particularly Parts 1 and 2) will be good preparation for the LPIC Level 1 exam:
-    * Bash by example, Part 1: Fundamental programming in the Bourne-again shell
+You can learn more about sed in the Sed by Example article series. If you're planning to take the LPI exam, be sure to read the first two articles of this series.
-    * Bash by example, Part 2: More bash programming fundamentals
+*[[Sed by Example, Part 1]]
-    * Bash by example, Part 3: Exploring the ebuild system.
+*[[Sed by Example, Part 2]]
-If you're not familiar with the vi editor, check out Daniel's Vi intro -- the cheat sheet method tutorial. This tutorial will give you a gentle yet fast-paced introduction to this powerful text editor. Consider this must-read material if you don't know how to use vi.
+To learn more about awk, read the Awk by Example article series.
-For an intro to the Emacs editor, see the developerWorks tutorial, Living in Emacs.
+If you're not too familiar with the vi editor, I strongly recommend that you check out my [http://www-106.ibm.com/developerworks/edu/l-dw-linuxvi-i.html Vi -- the cheat sheet method] tutorial. This tutorial will give you a gentle yet fast-paced introduction to this powerful text editor. Consider this must-read material if you don't know how to use vi.
 __NOTOC__
 [[Category:Linux Core Concepts]]
 [[Category:Articles]]

@@ Line 439: / Line 439: @@
 === The elusive first digit ===
 And to conclude this section, we finally take a look at the elusive first digit of a numeric mode. As you can see, this first digit is used for setting the sticky, suid, and sgid bits:
-suid 	sgid 	sticky 	mode digit
+{| {{table}}
-on 	on 	on 	7
+!suid
-on 	on 	off 	6
+!sgid
-on 	off 	on 	5
+!sticky
-on 	off 	off 	4
+!mode digit
-off 	on 	on 	3
+|-
-off 	on 	off 	2
+|on
-off 	off 	on 	1
+|on
-off 	off 	off 	0
+|on
 Here's an example of how to use a 4-digit numeric mode to set permissions for a directory that will be used by a workgroup:
+<pre>
 # chmod 1775 /home/groupfiles
-As homework, figure out the meaning of the 1755 numeric permissions setting. :)
+== Linux account managment ==
 Linux account managment
-Introducing /etc/passwd
 In this section, we'll look at the Linux account management mechanism, starting with the /etc/passwd file, which defines all the users that exist on a Linux system. You can view your own /etc/passwd file by typing less /etc/passwd.

@@ Line 377: / Line 377: @@
 |}
-Using our table, the last three digits of 0077 expand to ---rwxrwx. Now, remember that the umask tells the system which permissions to disable. Putting two and two together, we can see that all "group" and "other" permissions will be turned off, while "user" permissions will remain untouched.
+Using our table, the last three digits of 0077 expand to ---rwxrwx. Now, remember that the <span style="color:green">umask</span> tells the system which permissions to disable. Putting two and two together, we can see that all "group" and "other" permissions will be turned off, while "user" permissions will remain untouched.
-Introducing suid and sgid
 When you initially log in, a new shell process is started. You already know that, but you may not know that this new shell process (typically bash) runs using your user id. As such, the bash program can access all files and directories that you own. In fact, we as users are totally dependent on other programs to perform operations on our behalf. Because the programs you start inherit your user id, they cannot access any filesystem objects for which you haven't been granted access.
 For example, the passwd file cannot be changed by normal users directly, because the "write" flag is off for every user except root:
+<pre>
 $ ls -l /etc/passwd
 -rw-r--r--    1 root     wheel        1355 Nov  1 21:16 /etc/passwd
+</pre>
 However, normal users do need to be able to modify /etc/passwd (at least indirectly) whenever they need to change their password. But, if the user is unable to modify this file, how exactly does this work?
-Thankfully, the Linux permissions model has two special bits called suid and sgid. When an executable program has the suid bit set, it will run on behalf of the owner of the executable, rather than on behalf of the person who started the program.
+=== suid ===
+Thankfully, the Linux permissions model has two special bits called <span style="color:green">suid</span> and <span style="color:green">sgid</span>. When an executable program has the <span style="color:green">suid</span> bit set, it will run on behalf of the owner of the executable, rather than on behalf of the person who started the program.
-Now, back to the /etc/passwd problem. If we take a look at the passwd executable, we can see that it's owned by root:
 $ ls -l /usr/bin/passwd
 -rwsr-xr-x    1 root     wheel       17588 Sep 24 00:53 /usr/bin/passwd
-You'll also note that in place of an x in the user's permission triplet, there's an s. This indicates that, for this particular program, the suid and executable bits are set. Because of this, when passwd runs, it will execute on behalf of the root user (with full superuser access) rather than that of the user who ran it. And because passwd runs with root access, it's able to modify the /etc/passwd file with no problem.
+=== suid/sgid caveats ===
-suid/sgid caveats
+We've seen how <span style="color:green">suid</span> works, and <span style="color:green">sgid</span> works in a similar way. It allows programs to inherit the group ownership of the program rather than that of the current user.
-We've seen how suid works, and sgid works in a similar way. It allows programs to inherit the group ownership of the program rather than that of the current user.
+{{fancyimportant|Here's some miscellaneous yet important information about suid and sgid. First, suid and sgid bits occupy the same space as the x bits in a ls -l listing. If the x bit is also set, the respective bits will show up as s (lowercase). However, if the x bit is not set, it will show up as a S (uppercase).}}
-Important!
 Here's some miscellaneous yet important information about suid and sgid. First, suid and sgid bits occupy the same space as the x bits in a ls -l listing. If the x bit is also set, the respective bits will show up as s (lowercase). However, if the x bit is not set, it will show up as a S (uppercase).
-Important!
-Another important note: suid and sgid come in handy in many circumstances, but improper use of these bits can allow the security of a system to be breached. It's best to have as few suid programs as possible. The passwd command is one of the few that must be suid.
-Changing suid and sgid
-Setting and removing the suid and sgid bits is fairly straightforward. Here, we set the suid bit:
+{{fancyimportant|Another important note: suid and sgid come in handy in many circumstances, but improper use of these bits can allow the security of a system to be breached. It's best to have as few suid programs as possible. The passwd command is one of the few that must be suid.}}
 # chmod u+s /usr/bin/myapp
+</pre>
-And here, we remove the sgid bit from a directory. We'll see how the sgid bit affects directories in just a few panels:
+And here, we remove the <span style="color:green">sgid</span> bit from a directory. We'll see how the <span style="color:green">sgid</span> bit affects directories in just a few panels:
+<pre>
 # chmod g-s /home/drobbins
-Permissions and directories
+=== Permissions and directories ===
-−
 So far, we've been looking at permissions from the perspective of regular files. When it comes to directories, things are a bit different. Directories use the same permissions flags, but they are interpreted to mean slightly different things.
 For a directory, if the "read" flag is set, you may list the contents of the directory; "write" means you may create files in the directory; and "execute" means you may enter the directory and access any sub-directories inside. Without the "execute" flag, the filesystem objects inside a directory aren't accessible. Without a "read" flag, the filesystem objects inside a directory aren't viewable, but objects inside the directory can still be accessed as long as someone knows the full path to the object on disk.
 And, if a directory has the "sgid" flag enabled, any filesystem objects created inside it will inherit the group of the directory. This particular feature comes in handy when you need to create a directory tree to be used by a group of people that all belong to the same group. Simply do this:
+<pre>
 # mkdir /home/groupspace
 # chgrp mygroup /home/groupspace
 # chmod g+s /home/groupspace
-Now, any users in the group mygroup can create files or directories inside /home/groupspace, and they will be automatically assigned a group ownership of mygroup as well. Depending on the users' umask setting, new filesystem objects may or may not be readable, writable, or executable by other members of the mygroup group.
+=== Directories and deletion ===
 Directories and deletion
-−
 By default, Linux directories behave in a way that may not be ideal in all situations. Normally, anyone can rename or delete a file inside a directory, as long as they have write access to that directory. For directories used by individual users, this behavior is usually just fine.
-However, for directories that are used by many users, especially /tmp and /var/tmp, this behavior can be bad news. Since anyone can write to these directories, anyone can delete or rename anyone else's files -- even if they don't own them! Obviously, it's hard to use /tmp for anything meaningful when any other user can type rm -rf /tmp/* at any time and destroy everyone's files.
+However, for directories that are used by many users, especially '''/tmp''' and '''/var/tmp''', this behavior can be bad news. Since anyone can write to these directories, anyone can delete or rename anyone else's files -- even if they don't own them! Obviously, it's hard to use '''/tmp''' for anything meaningful when any other user can type <span style="color:green">rm -rf /tmp/*</span> at any time and destroy everyone's files.
-Thankfully, Linux has something called the sticky bit. When /tmp has the sticky bit set (with a chmod +t), the only people who are able to delete or rename files in /tmp are the directory's owner (typically root), the file's owner, or root. Virtually all Linux distributions enable /tmp's sticky bit by default, but you may find that the sticky bit comes in handy in other situations.
+Thankfully, Linux has something called the sticky bit. When '''/tmp''' has the sticky bit set (with a <span style="color:green">chmod +t</span>), the only people who are able to delete or rename files in '''/tmp''' are the directory's owner (typically root), the file's owner, or root. Virtually all Linux distributions enable '''/tmp''''s sticky bit by default, but you may find that the sticky bit comes in handy in other situations.
-The elusive first digit
 And to conclude this section, we finally take a look at the elusive first digit of a numeric mode. As you can see, this first digit is used for setting the sticky, suid, and sgid bits:
 suid 	sgid 	sticky 	mode digit