Directories, covered in Chapter 4, “Working with Directories,” store both special and ordinary files. For users familiar with Windows or Mac OS, UNIX directories are equivalent to folders.
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In UNIX there are three basic types of files:
An ordinary file is a file on the system that contains data, text, or program instructions. In this chapter, you look at working with ordinary files.
Special files are covered in Chapter 5, “Manipulating File Attributes.” Some special files provide access to hardware such as hard drives, CD-ROM drives, modems, and Ethernet adapters. Other special files are similar to aliases or shortcuts and enable you to access a single file using different names.
First, list the files and directories stored in the current directory. Use the following command:
$ ls
Here’s a sample directory listing:
bin hosts lib res.03 ch07 hw1 pub test_results ch07.bak hw2 res.01 users docs hw3 res.02 work
This output indicates that several items are in the current directory, but this output does not tell us whether these items are files or directories. To find out which of the items are files and which are directories, specify the -F option to ls:
$ ls -F
Now the output for the directory is slightly different:
bin/ hosts lib/ res.03 ch07 hw1 pub/ test_results ch07.bak hw2 res.01 users docs/ hw3 res.02 work/
As you can see, some of the items now have a / at the end: each of these items is a directory. The other items, such as hw1, have no character appended to them. This indicates that they are ordinary files.
When the -F option is specified to ls, it appends a character indicating the file type of each of the items it lists. The exact character depends on your version of ls. For ordinary files, no character is appended. For special files, a character such as !, @, or # is appended to the filename.
For more information on the exact characters your version of ls appends to the end of a filename when the -F option is specified, please check the UNIX manual page for the ls command. You can do this as follows:
$ man ls
So far, you have seen ls list more than one file on a line. Although this is fine for humans reading the output, it is hard to manipulate in a shell script. Shell scripts are geared toward dealing with lines of text, not the individual words on a line. Without using external tools, such as the awk language covered in Chapter 17, “Filtering Text Using awk,” it is hard to deal with the words on a line.
In a shell script it is much easier to manipulate the output when each file is listed on a separate line. Fortunately ls supports the -1 option to do this. For example,
$ ls -1
produces the following listing:
bin ch07 ch07.bak docs hosts hw1 hw2 hw3 lib pub res.01 res.02 res.03 test_results users work
So far you have used ls to list visible files and directories, but ls can also list invisible or hidden files and directories. An invisible file is one whose first character is the dot or period character (.). UNIX programs (including the shell) use most of these files to store configuration information. Some common examples of hidden files include the files
All files that do not start with the . character are considered visible.
To list invisible files, specify the -a option to ls:
$ ls -a
The directory listing now looks like this:
. .profile docs lib test_results .. .rhosts hosts pub users .emacs bin hw1 res.01 work .exrc ch07 hw2 res.02 .kshrc ch07.bak hw3 res.03
As you can see, this directory contains many invisible files.
Notice that in this output, the file type information is missing. To get the file type information, specify the -F and the -a options as follows:
$ ls -a -F
The output changes to the following:
./ .profile docs/ lib/ test_results ../ .rhosts hosts pub/ users .emacs bin/ hw1 res.01 work/ .exrc ch07 hw2 res.02 .kshrc ch07.bak hw3 res.03
With the file type information you see that there are two hidden directories (. and ..). These two directories are special entries that are present in all directories. The first one, ., represents the current directory. The second one, .., represents the parent directory. We discuss these concepts in greater detail in section “The Directory Tree” of Chapter 4.
In the previous example, the command that you used specified the options to ls separately. These options can also be grouped together. For example, the commands
$ ls -aF $ ls -Fa
are the same as the command
$ ls -a -F
As you can see, the order of the options does not matter to ls. As an example of option grouping, consider the equivalent following commands:
ls -1 -a -F ls -1aF ls -a1F ls -Fa1
Any combination of the options -1, -a, and -F produces identical output:
./ ../ .emacs .exrc .kshrc .profile .rhosts bin/ ch07 ch07.bak docs/ hosts hw1 hw2 hw3 lib/ pub/ res.01 res.02 res.03 test_results users work/
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