Making and Listing Regular Files

The operating system is responsible for maintaining the file system. The file system maintained by a UNIX system consists of a hierarchy of files. Two types of files will be of interest to us: directories (folders) and regular files, i.e. files that are not directories. Regular files may hold data or programs. They may consist of text or be binary files that are not human-readable.

There are also hidden files. These files have a name beginning with a dot (.). To see them use the command ls -a or ls –all (ls -all on a Mac). Both files and directories can be hidden. Usually these items contain data on the configuration of your system and its applications.

You are used to working with regular files and directories in Windoze or MacOSX. Things in UNIX work the same way, but we will use commands to manage files instead of mouse clicking or dragging.

As we have already seen the UNIX command touch, which creates new files. This command creates an empty file for each argument given it. At your UNIX prompt, enter

unix> touch stuff

This creates the empty file named stuff in your current working directory.

Now let us analyze the anatomy of this command. The name of the command is touch; its purpose is to create an empty file. Since you do not see a - sign, there are no options being used. The argument is stuff. This is the name of the file you created. Create a few more empty files. Enter these commands

unix> touch foo
unix> touch bar

You may create several files at once by making a space-separated list as we show here.

unix> touch aardvark buffalo cougar dingo elephant

Now you have eight new files in your cwd. Next we will see how to list the files. Enter this command at the UNIX prompt

unix> ls

The command ls lists your files. Notice we had neither options nor arguments. If you created the files using touch as instructed, they should appear on your screen like this

unix> ls
unix> aardvark bar buffalo cougar dingo elephant foo stuff

The command ls has several options. One option is the l option; it list the files in long format. To invoke it, type

unix> ls -l

You will see a listing like this

unix> ls -l
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:50 aardvark
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:50 bar
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:50 buffalo
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:50 cougar
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:50 dingo
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:50 elephant
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:49 foo
-rw-rw-r--    1 morrison morrison        0 Jun  9 10:49 stuff

The first column reflects the permissions for the files. The sequence

-rw-rw-r--

indicates that you and your group have read/write permission and that others (“the world”) have read permission. We will discuss permissions in more detail when we discuss the management of directories.

You can see the name here is listed in two columns; on this machine morrison is in his own group. On another system, you may live in group with several other people; if so you will see the name of that group in one of these columns. The zero indicates the size of the file; each file we created is empty. Then there is a date, a time and the file name. This is the long format for file listing; it is seen by using the -l option in the ls command.

Another option is the -a option. This lists all files, including “hidden” files. Hidden files have a dot (.) preceding their name. To see them, enter

unix> ls -a

at the command line. One thing you are guaranteed to see, no matter where you are are are the directories .. (parent) and . (current). If you are in you home directory, You will see the files you saw using ls and several hidden files with mysterious names like bash_profile. Do not delete these; they provide the configuration for your account and do things like record preferences for applications you have used or installed. You can also list all files including hidden files by entering

unix> ls --all

You can use more than one option at once. For example, entering

unix> ls -al

unix> ls -a -l

unix> ls --all -l

shows all of your files and hidden files in long format. Try this now on your machine.

Note to Mac Users

Mac users should precede verbose commands with a single -. So on a Mac, you type

unix> ls -all -l

and not

unix> ls --all -l

Otherwise, your Mac will respond with a cryptic error message.

Next we will show how to display a file to the screen. UNIX commands that process files are called filters. Filters accept input from a file, and place output in another file. The default action of any UNIX filter is to get input from stdin and send its output to stdout. Any errors resulting from using the filter go to stderr, which by default is just stdout. If an argument is offered to a UNIX filter, that argument is generally a regular file, and it gets used in lieu of stdin.

Let us peek inside your .bash_profile file. Enter the command

unix> cat .bash_profile

The command name is cat, short for catalog (the file to the screen). The cat command is a filter that does not filtering at all; it simply dumps the entire file to the screen all at once. We are using no options, but the file name is an argument to cat. If a file is long and you want to see it one screenful at a time, use the filter more. The command more takes a file name as an argument and shows it on the screen a screenful at a time. You can hit the space bar to see the next screenful or use the down-arrow or enter key to scroll down one line at a time. To exit more at any time, type a q and more quits. You can use several arguments in cat or more and the indicated files will be displayed in seriatum.

Programming Exercises

What does the -r option cause ls to do?
What does the -t option cause ls to do? Try combining it with thte -l option and it becomes clear.
What does the -n option do to cat?

6.1 Globbing

Make a directory called tossme (you will toss this when done) and enter it by doing the following

unix> mkdir tossme
unix> cd tossme

Now create some empty files with touch like so.

MAC:Tue Jan 04:10:24:tossme> touch cat.html dog.html snake.css snake.py cow.c
MAC:Tue Jan 04:10:26:tossme> touch Turtle.java Egret.java tapir.py goat.cpp
MAC:Tue Jan 04:10:27:tossme> ls
Egret.java  cat.html    dog.html    snake.css   tapir.py
Turtle.java cow.c       goat.cpp    snake.py

The special character * is a wildcard standing for any sequence of zero or more characters. Using * to create wildcards is called “globbing” because it allows you to refer to a glob of files. Let us use it to list all files with the extension .py.

MAC:Tue Jan 04:10:27:tossme> ls *.py
snake.py tapir.py

Now let’s list all files that begin with the letter s.

unix> ls s*
snake.css snake.py

How do we list all files whose extension begins with a c?

unix> ls *.c*
cow.c     goat.cpp  snake.css

You can cat a glob of files; for example.

cat *.py

will put all files to stdout with a .py extension.