This chapter is from the book
This chapter is from the book
This chapter is from the book
Basic File Management
Course Objectives Covered
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Executing Commands at the Command Line (3036)
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Common Command Line Tasks (3036)
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Changing Directories and Listing Directory Contents (3036)
In this section, you'll learn about a number of utilities used for managing files and directories. Some of these utilities—such as cd and ls—are of such importance that it was impossible to get this far in the book without using them in some of the examples. Here, we will expand upon them and explore their functions and uses in greater detail.
Working with cd and pwd
The cd command is used to change the directory you are working in. If you enter the command with no parameters, it will move you to whatever directory is defined by the HOME variable. If you specify any parameter with it, it is seen as denoting the directory you want to change to.
Some characters that can be of great use with cd are the single period (.) and double period (..). The former represents whatever directory you are currently in, whereas the latter represents the parent directory of the current one.
The pwd utility shows the present working directory—the one you are currently in. The same value that it returns is contained in the environmental variable PWD.
NOTE
Always remember that more than one item may have the same name in Linux. As a general rule, all uppercase entries represent variables, and all lowercase entries are files and utilities.
The following are some examples of how these two utilities can be used:
$ pwd /usr/bin $ echo $HOME /usr/home/edulaney $ cd $ pwd /usr/home/edulaney $
This sequence showed the present working directory to be /usr/bin and the HOME variable to be equal to /usr/home/edulaney. Entering cd without any parameters changed to that directory.
$ pwd /usr/home/edulaney $ cd / $ pwd / $ cd /usr/home/edulaney $ pwd /usr/home/edulaney $ cd .. $ pwd /usr/home $
In this sequence, the first change is to the root directory (/), then to /usr/home/edulaney. Using the shortcut for the parent of this directory, it was then possible to move back one directory.
Absolute and Relative Addressing
There are two methods of specifying paths to anything—files, directories, and so on: absolute and relative. When you give an absolute path, you take nothing into consideration, and you give a value that is always true. When you give a relative path, you take into account where you currently are, and you give a path relative to that.
To use an analogy, suppose two people live in the same city and state: Muncie, Indiana. The first person lives at 1909 Rosewood, and the second lives at 4104 Peachtree. If the first person wants to find/visit the second person, they can find out where they are via an absolute path known as their mailing address:
4104 Peachtree Lane
Muncie, IN 47304
This address says to
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Find Indiana.
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Within Indiana, find Muncie.
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Within Muncie, find the section of the city falling within the 47304 zip code.
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Within all earlier confines, find Peachtree Lane.
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On Peachtree Lane, go to house number 4104.
NOTE
Absolute addresses will never change and will point to the entity regardless of where you are coming from.
The absolute address is the same whether the person coming to visit lives in Muncie or in Alaska. However, because the first person does live in Muncie as well, we can also tell them how to reach their destination using relative addressing:
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Take Rosewood to Bethel and turn left.
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Take Bethel to Jackson and turn right.
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Take Jackson across the railroad tracks to the stop sign at Hawthorne.
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Turn left on Hawthorne and go to the next stop, which is Peachtree.
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Turn right on Peachtree Lane, and go to the first house on the right.
NOTE
Relative addresses will always change and are relative to where you are coming from.
Table 3.2 illustrates a few examples that can be used with the cd command.
Examples of Using the cd Command
)
Working with ls
The ability to list files and directories is one of the most essential to any operating system, and the ls utility performs this function for Linux. When given by itself it lists the names of files and directories beneath the current directory in a column-style format. Entries are always—by default—given in alphabetical order and there is nothing to differentiate names of directories from names of files. An example would be
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NOTE
The listings are always in alphabetical order by default, with all uppercase entries coming before lowercase entries.
There are a slew of options that can be used with this command, and one of the most useful is -F, which will indicate what type of entry is being displayed:
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Entries without any trailing characters added are standard files. Entries with a "/" on the end—such as Desktop—are directories. Those entries with a trailing asterisk (*) are executable. Symbolic links are signified by an at (@) symbol.
Another useful option is -a, which will show all files. By default, hidden files are not displayed by ls. A hidden file is any file that has a period (.) as its first character:
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NOTE
Periods can appear anywhere within a filename and appear as many times as you want. The only time they have any special significance is when they are the very first character of the name.
If you use -A in place of -a, it will leave off the first two entries ("." and ".."). Undoubtedly, however, the most useful option of all is -l, which will display a long list of the files. Entries look like this
drwx------- 5 root root 1024 Aug 30 11:12
There are essentially seven columns here, and they can be broken out as follows:
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The permissions on the entry. Permissions are more fully discussed throughout subsequent chapters, but for now it is important to realize that the first character identifies what type of entry it is. A "-" indicates a file, whereas a "d" is a directory. Other possibilities for the first character are "c" for a character special file (such as a terminal), "b" for a block special device (such as a tape drive), "l" for a symbolic link, or "p" for a named pipe.
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The number of links. If this is a directory, it will be equal to the number of entries beneath it. If it is a file, it is equal to the number of ways to reference it.
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The name of the owner who created (or now owns) the entity.
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The name of the group owning the file.
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The size of the file.
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The date of creation, or of modification into the current format.
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The name.
Table 3.3 offers some of the other options for ls and their purpose.
Options for the ls Command
|
Option |
Purpose |
|
-c |
List in order of time of last change/modification instead of alphabetical order |
|
-d |
List directories |
|
-G |
When used with -l, don't show the group |
|
-i |
Show the inode number (pointer) to each entry |
|
-n |
Show the owner and group by their numeric values instead of by name |
|
-o |
Same as -lG |
|
-r |
Reverse the order of the display |
|
-R |
Recursively show entries in subdirectories as well |
|
-S |
Sort |
|
-u |
Sort by last access time |
|
-w |
Specify screen width |
|
-x |
Show lines instead of columns |
|
-X |
Alphabetize by extension |
Applying a touch
There are essentially three dates associated with a file or entry: creation, modification, and access. Using the touch utility, you can change the access and modification time associated with a file:
$ ls -l brio -rw-r--r-- 1 root root 28 Aug 22 10:26 brio $ touch brio $ ls -l brio -rw-r--r-- 1 root root 28 Aug 30 16:01 brio $
There are a few options that can be used with the touch utility as well, as summarized in the following table:
|
Option |
Purpose |
|
-a |
Only change the access time |
|
-m |
Only change the modification time |
|
-r |
Use the time/date associated with a reference file to make the change instead of the current time/date |
An example of the latter would be
$ ls -l tuesday wednesday -rw-r--r-- 1 root root 85 Aug 22 10:26
NOTE
If you use the touch utility with the name of a file that does not exist, it will create the file with the current date and time and a size of zero.