Tutorial: Basic Unix continued

1.1 Input/Output redirection: Redirection to a file

• Type the following commands:

      cal
      cal 3 1991
      cal > file1
      more file1
      cal 3 1991 >> file1
      more file1

• Concatenate 2 files into one file using "cat" and file redirection.

A command can also read input from a file, using "<". The following two lines are equivalent:

      more file1
      more < file1

• Use file redirection to create a file named 'yearfile' in your directory. Put a whole year's calendar into it. Then append the next year's calendar.

1.2 Input/Output redirection: Pipes

The following two lines

      cat file1 file2 > sample_file
      wc < sample_file

can be combined into one line by using a pipe.

      cat file1 file2 | wc

This eliminates the need for temporary files.

• The command "grep" is used for searching through file content (eg. grep 'wc' file). The command "history" lists the last commands that were executed. Can you figure out how to combine "history" and "grep" so that all commands containing "wc" that were recently executed are printed? If you cannot figure it out right away, start by redirecting the output from history to a temporary file. Then combine "grep" and "history" into one line.

2.1 Wildcards and other special characters

• * means "any character including no characters"
• ? means "any single character"
• ; end of a command
• \ escape character

Execute the following commands: (Note: "touch" creates empty files for given filenames.)

      touch hello.txt progr.out newfile
      ls *
      ls n*
      ls *.*
      ls h?ll?.txt
      ls *.txt; ls *.out
      touch Hello\!

Exercises:

• Touch a file called "Hello World".
• List all files which have a three character extension.
• Create a directory "temp". Copy all files which contain a "t" in their name from your home directory into "temp".
• Remove all files from your directory "temp". Use the option which asks for confirmation for each file before deleting it.

3.1 Unix file permission

Three levels of permissions:

• r - read permission
• w - write permission
• x - execute permission

Three levels of ownership:

• u - user
• g - group
• o - others

Example: "-rwxr-x---" means:

• owner: rwx
• group: r-x
• other: ---

Permissions are changed with "chmod WHO+PERMISSION FILENAME".

Examples:

      chmod o+r newfile
      chmod g-x newfile

Permissions can also be changed using a numerical representation, which is calculated by interpreting the permissions string as three binary numbers. For example, "-rwxr-x---" corresponds to 111 101 000, which corresponds to 1*4 + 1*2 + 1*1, 1*4 + 0*2 + 1*1, 0*4 + 0*2 + 0*1, which corresponds to 750. Thus "chmod 750 FILENAME" changes the permissions to "-rwxr-x---".

Exercises

• List one of your directories. Then remove your own read and execute (search) permissions from the directory. Try to list the directory again. Restore the read access and try again. Finally restore the execute access.

• Remove the write permission from a file in your home directory, then try to add the current month to the end of the file. Restore the write permission and try again.

• Ask your neighbour for his/her username. List his/her home directory and find out which files (if any) you have read access to. Change one of your files, so that your neighbour can write to it. Test whether that works. Can your neighbour delete any of your files?

• Check the permissions for all your files and directories according to the following criteria: If you plan on storing any coursework (or back-ups of your coursework) on your Unix drive, set the permissions for those directories, so that no one else can read them. Keep in mind that if you have a public_html directory, this needs to be readable and executable by others. Your top-level directory needs to be executable by group and others if you want to store web pages in your public_html directory. But the top-level directory does not need to be readable by group or others.

4.1 Symbolic links

There are two different types of links: hard links and soft (or symbolic) links. Hard links point directly to a file and may not span different file systems or link to directories.

Soft links contain only an object's path name (a 'pointer') to the place where the actual data is stored. They can span file systems, and may refer to directories.

The following example for a hard link assumes that you have a directory called "temp":

      cal > hardfile
      ls -l
      ln hardfile temp/hardfile2
      ls -l temp/hardfile2

• Change the content of "hardfile" and check what this does to hardfile2.

      cal > softfile
      ls -l
      ln -s softfile temp/softfile2
      ls -l temp/softfile2

• Notice the difference in the listing between hard and soft links.
• Remove both hardfile and softfile and see what that does to the linked files.

5.1 Processes

ps	ps (all processes of terminal) ps has lots of options but they depend on the flavour of Unix
top	shows top processes in the system
jobs	shows background jobs
bg	bg [[job]] puts specified job into background
fg	fg [[job]] puts specified job into foreground
kill	kill [-signo] pid1 ... sends signal to process default for signo: -15 (usually terminates process) -9 (always terminates process)
sleep	example: (sleep 5; echo "Hello")&
at	at time (scheduling processes at a certain time) example: at 4:30am tomorrow RETURN bigjob RETURN ^D
nice	nice command & (executes command at a lower priority, useful for batch jobs)
wait	wait [pid] (waits for termination of child process)
nohup	nohup bigjob & job keeps running after logout

Exercises:

• Try different ps options. Type "more &" then "nano &" then "ps -l". Which of the processes is the parent? What is the id of the parent process? In what states are the processes?

• Type "jobs". Then bring the "more" process into the foreground. Find the process id of the "nano" process and kill it.

• Type "emacs". Then put the process into the background using a second terminal window. Then kill the emacs process.

• Find out what the current time is using "date". Using "at" print the current month to a new file exactly 2 minutes later.

6.1 Shells

      printenv SHELL

Overview of shells:

shell	name	user start-up files	system-wide files
sh	Bourne shell or Posix shell	.profile	/etc/profile
ksh	Korn shell	.profile	/etc/profile
csh	C shell	.login, .cshrc	/etc/csh.login
tcsh	T C shell	.login, .tcshrc, .cshrc	/etc/csh.login, /etc/tcsh.login
bash	bash: GNU Bourne Again SHell	.bash_profile, .bash_login, .profile	/etc/profile

A shell can be started by typing its name (eg "csh"). The command "printenv" shows all environment variables which are currently set. Shells can be customised according to user preferences and system-wide preferences. The system preferences are in the /etc directory and usually executed before the user preferences. Some shells (csh and tcsh) have separate files for when a user logs in (.login) and when a user starts a shell (.cshrc). The alias command can be used to create a name for a command or command sequence. For example, "alias rm 'rm -i'" makes sure that rm is always used with the -i option. Such an alias can be stored in the start-up file.

• Find out what shell you are currently using. Edit the start-up file for your shell (or create a start-up file if you don't already have one). Edit the file so that it prints the current date and time when you start the shell. IMPORTANT: when editing a start-up file, you need to make sure that it doesn't contain any errors before you logout. After editing the file, start a new shell (by typing the name of the shell) and test whether it works. If everything is ok, you can exit the shell with "exit". It is a good idea to copy the original file to a backup version (eg .cshrc_backup) before you make changes. If you mess up your start-up files you MAY NOT BE ABLE TO LOGIN TO YOUR ACCOUNT again. If that ever happens, there is still something you can do. Find out what that is. (No, asking C&IT is not the right answer in this case.)

• Check whether there is a command called "e". If there isn't, add an alias to your start-up file which abbreviates your favourite editor as "e".

7.1 Some more advanced exercises

• Write an alias which stores your current working directory in a file "~/.curdir" every time you use the cd command. Make sure only one directory is ever present in the .curdir file.

• Have a message "Welcome, USER!" printed to the terminal exactly 1 minute after a new shell is started (replace USER with the username).

• Write a command that deletes all letters from a file and then sorts it numerically.

• Copy some web-page to a file. Using your favourite editor (or any Unix commands you know), change the file into a text file, ie. replace all <p> tags by blank lines; delete all HTML tags from the text; insert line breaks so that lines don't wrap across a line; if the file contains PC line breaks, delete these; delete unnecessary blank space. Try to achieve this as efficiently as possible, using only a few commands.