Linux : Working with Files

Listing Files

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.
"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
l .profile, the Bourne shell ( sh) initialization script
l .kshrc, the Korn shell ( ksh) initialization script
l .cshrc, the C shell ( csh) initialization script
l .rhosts, the remote shell configuration file
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.


Option Grouping

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/
Sams