CST8177 - Lab #4

Shell Features and File Globs

In-Lab Demo

1. Execute a command line using redirection

2. Demonstrate shell features, including at least variable expansion and command substitution

3. Demonstrate file globbing of several varieties

Working with the shell

In Unix and Linux, "shell" is the term used to describe the command-line interpreter. In this course, we will use the bash shell because of its wide popularity on Linux, although there are other shells available such as csh and sh.

Exercise #1 (review): Working with shell features

File name completion

Purpose	Command line example
Complete a file name (to avoid typing long file names or making a typo in a complex name)	cp .bashrc abc.bashcopy vi abc[TAB]

Aliasing

History Mechanism

Purpose	Command line example
Display the current history buffer	history | less
Re-execute the last command	!!
Re-execute any previous command using keyboard keys	Use the up and down arrow keys
Re-execute any previous command using the event number	!n (where n is the event number as listed in the history output)
Edit a command	Use the up and down arrow keys to select the command and edit it

Redirection & command grouping

A file descriptor is a numeric identifier (small unsigned integer) that a UNIX system uses to identify an open file that is attached to a process. Note: A file descriptor is created by a process through issuing an open() system call for the file name. A file descriptor ceases to exist when it is no longer held by any process. By default a process is set up with three file descriptors:

• 0 (stdin), usually associated with the current console (/dev/pts/n) - keyboard

• 1 (stdout), usually associated with the current console (/dev/pts/n) - screen

• 2 (stderr), usually associated with the current console (/dev/pts/n) - screen

Note that pts is used for a terminal window, a pseudo-terminal. An actual terminal will be tty.

Create an empty directory and touch x to create a file in it. Use the command ls x y to provide examples of the actions listed below. Use only one command for each.

Purpose	Command line solution
Redirect stdout to a file named out and view the file.
Redirect stderr to a file named err and view the file.
Redirect the output such that stdout is written to out and stderr is written to err from the single command.
Append stdout to a file named out and view the file.
Append stderr to a file named err and view the file.
Redirect stdout to a file named out and redirect stderr so it is written to the same file as stdout (but do not name the file for stderr). View the file out.
Redirect both stderr and stdout to a file named out and view the file.
Reverse the redirection sequence in the question above and repeat. What's the difference, if any?

There are also some useful command separators you should be aware of. Try each of these (to cause mkdir dir to fail, create it twice in the same directory):

• ; - the semi-colon executes one command at a time and is equivalent to the [Enter] key between commands.

  Example: mkdir dir; cd dir

• && - the double ampersand executes the second command only if the first command executed successfully.

  Example: mkdir dir && echo Directory creation successful

• || - the double pipe executes the second command only if the first command executed unsuccessfully.

  Example: mkdir dir || echo Could not make new directory

Exercise #2: Working with shell expansion/substitution

Execute the commands listed below, record the output here, and examine the output.

Brace expansion

• echo post{script,office,ure}

• mkdir -p /home/teacher/{CST8207,CST8177}/{F,W}0{0,1}

How many directories have been created (excluding teacher)? _______________________

Tip: Use find /home/teacher | wc -l and adjust as needed.

Tilde expansion

• ls -d ~ ____________________________________________________________________________

Variable expansion

• echo "My search path is: $PATH" ________________________________________________

  _______________________________________________________________________________________

• cd /

• ls -d "home" ________________________________________________________________

• ls -d "HOME" ________________________________________________________________

• ls -d "$HOME" ________________________________________________________________

• ls -d "${HOME}" ________________________________________________________________

Command substitution

• ls -ld $(find /home -maxdepth 1 -uid 500)

  ________________________________________________________________________________

• Compare with: find /home -maxdepth 1 -uid 500 -ls

  _______________________________________________________________________________________

Briefly describe the significant differences ____________________________________________

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Arithmetic Expressions

• echo Result $(( 3 + 6 - 2 )) ________________________________________________

• echo Result $(( 67 + 3 / 2 )) ________________________________________________

• echo Result $(( (8 - 4) * 2 )) ________________________________________________

• echo Result $(( 8 - 4 * 2 )) ________________________________________________

• echo Result $(( 5 + 4 )) ________________________________________________

File globbing

• Create a directory called dir.

• Create files in the current directory: touch x{1,2,3} x{1,2,3}0

• cp x?0 dir

  How many files have been copied (see above for counting tips)? ____________________

• Delete all files in the current directory (do not use -f or -r): rm x*

• Delete the directory dir

• touch hda hdb hdc hdd

• ls hd[abc] What is the output of this command?

  _______________________________________________________________________________________

• ls hd[a-c] What is the output of this command?

  _______________________________________________________________________________________

• rm hd[a-d]

• ls hd[a-d] What is the output of this command?

  _______________________________________________________________________________________

• touch hda1 hdb2 hdc3 hdd4

• ls hd[b-d][1-3] What is the output of this command?

  _______________________________________________________________________________________

• rm hd[a-d][1-4]

• rm hd[a-d][1-4] What is the output of this command?

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Exercise #3: Working with metacharacters

• The special character: question mark, escaped by the backslash

  • touch your # Create this empty file

  • echo How are you?

  • echo How are you\? What is the difference between these commands?

  _______________________________________________________________________________________

• The special character: single quote, escaped by the backslash

  • echo Don't you need $5.00?

    Note: Use Ctrl+D to terminate the input prompt >

  • echo Don\'t you need $5.00?

  • echo Don\'t you need '$5.00?'

  • Write your own echo command, using another method to allow the $ and ? to print correctly:

  _______________________________________________________________________________________

• The special character: double quote, escaped by the single quote

  • echo Mother yelled "Time to eat!"

  • echo 'Mother yelled "Time to eat!"'

  • Write your own echo command, using another method to allow the "s to print correctly:

  _______________________________________________________________________________________

Exercise #4: Working with quotes

Remove the special meaning that the space, the newline, and the # have for the shell.

• This command does not produce the desired result: useradd -c Alan Turing enigma

• Each of these will work fine (delete all but one with userdel)

  • useradd -c "Alan Turing" enigma

  • useradd -c 'Alan Turing' enigma

  • useradd -c Alan\ Turing enigma

• This command does not produce the desired result: grep Alan Turing /etc/passwd

  • Fix it using double quotes:

_______________________________________________________________________________________

• Fix it using single quotes:

_______________________________________________________________________________________

• Fix it using the escape character:

_______________________________________________________________________________________

• This command does not produce the desired result: alias la=ls -A

  • Fix the command line using double quotes:

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• To execute a statement that spans several lines, use the escape character (\) immediately before [Enter]. Show the result of the line-spanning echo:

  • echo hello sailor[Enter]

  • echo hello \[Enter]

sailor[Enter]

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• Test a line-spanning echo, but put a space after the \ and before the [ENTER]. What happens?

_______________________________________________________________________________________

• Show a correctly working multi-line grep or alias command:

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