Description
At the beginning of your lab period, watch your lab instructor perform a brief demonstration of the
LINUX/UNIX command printf, which you will need to use for one of the questions in this lab exercise.
Alternatively, read about this command in the Sobell text, pages 917 – 920.
Complete the tasks or problems below. For each task, copy-and-paste the contents of your terminal window
(including the commands that you typed, and any output produced by the commands you gave) into a text
file called lab4.txt. However, do not include extraneous or superfluous commands or output; only include
content relevant and essential to the specified task. Unless otherwise specified, all commands should be run
on tuxworld using the bash shell. Then, with a text editor, add to lab4.txt your solution to questions 4
and 5. Also with the text editor add text and identifying information to clearly distinguish which commands/output/code correspond to each task. Submit lab4.txt through moodle when done. This lab is out
of a total of 16 marks, with each question (2a, 3b, 4, etc.) being worth one mark, with the exception of parts
1a and 1b, which are worth 2 marks each. Marks may be docked for extraneous, irrelevant, or superfluous
content. The submission is due at 11:55 p.m. on Thursday, October 6. Note that the lab specification is
three pages in length.
1. (a) The <<< operator in bash allows you to provide a string as standard input to a command. For
example, this command will print the content of shell variable VARIABLE if it contains the letter
“a”.
grep a <<< “$VARIABLE”
That is, grep(1) is invoked with (search) pattern “a”. Since no filename argument is provided
to grep(1), the command reads its input from the standard input. Because of the leading dollar
sign (‘$’) before the variable name, the shell expands the variable VARIABLE to its value (a string).
Finally, cause of “<<<”, the shell arranges for this string to appear on the standard input to
grep(1) using an invisible pipe.
Write a command following the model above that will print the contents of the variable POSTAL_CODE
only if it is a valid Canadian postal code (with uppercase letters and with a space in the middle). Demonstrate the operation of your command with cases where (the value of) POSTAL_CODE
is a properly-formatted postal code, and where it is not. In each case, set the shell variable
POSTAL_CODE to your test value prior to issuing the grep command. Remember to check cases
like “junkS7N 5C9garbage”.
(b) Download the file mailinfo.txt that is available as an ancillary file for this lab. You do not need
to show a log of downloading the file in lab4.txt.
Devise a grep command that will output the lines from mailinfo.txt that start with an uppercase
“I” or “O”, end in “box”, and contain at least one lowercase letter in between. There may be other
1
characters between the “I” or “O”, and “box”. For example, the following lines, among others,
will be matches:
Inbox
Outbox
The following lines, among others, will not:
the Inbox
INBOX
Outbox1
InbOX
Ibox
I box
Demonstrate that your grep command works as specified. You will need two grep commands, one
where it outputs the lines from mailinfo.txt that match your pattern, and a second grep(1)
that outputs the lines that do not.
(c) Repeat question 1b, except output the number of lines matching the above pattern, rather than
the lines themselves. Do this with a single, simple grep command; do not employ a pipeline.
2. Copy the file /etc/passwd to your current working directory (it could be a subdirectory of your home
directory for completing lab 4). Call the destination file passwd. This file consists of several rows
and columns, with rows separated by newlines and columns separated by colons. The first column
represents a username. Use UNIX commands to perform the following tasks. For each task, your
command should read as input the content of your local passwd file and output only the information
requested, with no additional information. Each question must be answered using a single command,
except as noted. The command may be a pipeline (a single command, involving a series of one or
more pipes, “|”). More information on the format of a passwd file is available via the command “man
5 passwd”.
Note that for the purposes of working with file passwd (and to simplify the question) do not be
concerned about the settings of environment variables LC_ALL or LC_COLLATE. Also use the default
ordering criterion of sort(1) in completing these tasks.
(a) Output the list of usernames in passwd. (You do not need to use pipes for this one). The output
is to contain just the username field from the file.
(b) Output the list of usernames in reverse sorted order.
(c) Output the username that is “greatest” or “largest” (comes last when the usernames are placed
in sorted order, or comes first when the usernames are place in reverse sorted order).
(d) Output the first character of the “greatest” username.
(e) The same as in 2d, except the character must be converted to uppercase.
(f) Save the “greatest” username (c.f. question 2c) in a file called max_username.txt. Then use
the more command (i.e. a second command) to output the content of max_username.txt to the
standard output.
3. (a) Set the shell variable COLUMN1_HEADING to “Name” and COLUMN2_HEADING to “Student number”.
Then use the printf command along with the above variables to output the following to the
terminal.
Name Student number
John 123456789
You can use a single printf command to print both lines, or complex command consisting of two
consecutive printf commands (each producing one line of output) separated by ‘;’.
2
(b) Use the printf command to output exactly the following to the terminal. In doing so, do not use
more escape characters than are absolutely necessary.
$COLUMN1_HEADING\t$COLUMN2_HEADING\nJohn\t123456789\n
4. Explain why it might be a good idea to always put braces around the body of an if statement in
C/C++, even if the body contains just one line of code. Consider, for example, the following code
fragment:
if (i==3)
putchar( c );
Suppose a statement is added to the code as follows:
if (i==3)
putchar( c );
flag=false;
5. For each of the following pairs of code fragments, state which one you think has better coding style.
In part (a) assume that there is significance to the choice of function name, as poor as it is.
(a) i. cs(); /* clear screen to restart cycle */
ii. cs(); /* restart cycle */
(b) i. char *strncpy(char *s1, const char *s2, size_t n) {
char *p;
for (p=s1; *p && 0<n; –n) /* Copy n chars */
*p++ = *s2++; /* from s2 */
for (; 0<n; –n) /* Write zero or */
*p++ = 0; /* more nulls */
return s1;
}
ii. char *strncpy(char *s1, const char *s2, size_t n) {
char *p;
for (p=s1; *p && 0<n; –n) /* Copy up to n */
*p++ = *s2++; /* chars from s2 */
for (; 0<n; –n) /* Write zero or */
*p++ = 0; /* more nulls */
return s1;
}
In this question, do not rewrite the code. Simply indicate which fragment of the pair demonstrates
better programming style. Hence your answer in each case will be either “i” or “ii”.
