Week 04 Tutorial Questions

Objectives

A COMP2041 student wrote this script, named start_lab04.sh, to run before the Week 4 lab.

cat start_lab04.sh
#! /bin/dash

cd ~/labs/04

ex1=jpg2png
ex2=email_image
ex3=date_image
ex4=tag_music

But when he ran his script, it didn't seem to work:

pwd
/home/z1234567
./start_lab04.sh
pwd
/home/z1234567
echo $ex1 $ex2 $ex3 $ex4

Why not? How can the script be fixed?

Write a shell script, is_business_hours which exits with a status of 0 if the current time is between 9am - 5pm, and otherwise exits with a status of 1.
The date(1) command prints the current time in a format like this:
```
date
Sun 29 Jun 2025 04:35:19 AEST
```
COMP2041 student Shruti has a 'friends' subdirectory in her home directory that contains images of her many friends.
Shruti likes to view these images often and would like to have them appear in other directories within her CSE account so she has written a shell script to symlink them to the current directory:
```
for image_file in $(ls ~/friends); do
    ln -s "~/friends/$image_file" .
done
```
The links created by Shruti's script are broken.

Why? How can she fix her script?
The course code for COMP2041 has been changed to COMP2042 and the course code for COMP9044 has been changed to COMP9042.

Write a shell script, update_course_code.sh which appropriately changes the course code in all the files it is given as arguments.
Modify update_course_code.sh so if given a directory as an argument it updates the course codes in files found in that directory and its sub-directories.

CSE systems have a command, mlalias, which prints information about a specified mail alias.
For example:

mlalias cs2041.25T2.tutors
          Alias: cs2041.25T2.tutors
    Description: Maling List For COMP(2041|9044) 2025 T2 Tutors
          Flags: personal, public, unprivileged, members_can_post, closed
      Addresses:
                 andrewt
                 z9300162
                 dylanb
                 z5115658
                 annab
                 z5218654
                 z3121449
                 z5075632
                 z5160350
                 z5208931
                 z5256103
                 z5310685
                 z5311762
                 z5316004
                 z5358496
                 z5358896
                 z5360341
                 z5360529
                 z5361210
                 z5361534
                 z5362371
                 z5367318
                 z5416950
                 z5419252
                 z5420326
                 z5420849
                 z5421214
                 z5425577
                 z5428854
                 z5431362
                 z5437869
                 z5477887
                 z5477935
                 z5478419
                 z5479211
                 z5481971
                 z5483411
                 z5487536
                 z5487891
                 z5488112
                 z5488548
                 z5492103
                 z5513422
                 z5591556
                 z5592103
                 z5612340
         Owners: cs2041
        Senders: @COMP2041_Lecturer, @COMP2041_Supervisor

Convert the output of the mlalias command into a new line separated list of UNSW zIDs,
like this:

CSE systems have a command, acc, which prints information about a specified user.
For example:

acc z5218654
           User Name : z5218654            Aliases : annab
                 Uid : 41545                   Gid : 41545
             Expires : 15mar2026

              Groups : cs2041[29sep2025], cs1521[29sep2025]
       Group classes :
        User classes : 3785_Student, COMP1521_Supervisor[29sep2025]
                     : COMP2041_Supervisor[29sep2025]
                     : COMP3311t2_Student[29sep2025]
        Misc classes :

                Name : Ms Brew, Anna (Anna Brew)
            Position : Casual Academic (Sch: Computer Science & Eng)
         UNSW Number : 5218654
           UNSW Mail : a.brew@unsw.edu.au
           UNSW Home : //INFPWFS219.ad.unsw.edu.au/Student039$/z5218654
            CSE Home : /import/reed/4/z5218654

Write a pipeline which converts the output of acc into a new line separated list of courses the person is enrolled as a student in,
like this:

COMP3311

Make sure you don't include COMP1521 or COMP2041, as these are a supervisor role (course admin).

Use the pipelines from the above 2 questions to write a shell script which prints a list of courses taken by COMP2041 tutors with counts of how many COMP2041 tutors take each,
like this:
```
    7 COMP6771
    6 COMP3900
    6 COMP3331
    5 COMP3311
    5 COMP3121
...
```
Write a shell script named is_prime.sh which given an integer as an argument, tests whether it is prime and prints a suitable message:
```
is_prime.sh 42
42 is not prime
is_prime.sh 113
113 is prime
```
Your script should exit with a non-zero exit status if its argument is not prime.

Write a second script named primes.sh which uses the first script to print all primes less than a specified value, e.g:
```
primes.sh 100
2
3
5
7
11
13
17
...
79
83
89
97
```
We are working on a C program in an editor, and we'd like to run a script in another window which recompiles the program every time we save the file in the editor.

Write a shell script named recompile.sh which given a C file as argument, recompiles the program if the file changes and if the compile succeeds runs the program.

Hint: you can use the program stat to print the time when a file was last modified (as seconds since 1970-01-01) , like this:
```
$ stat -c '%Y' main.c
1615346166
```
Repeatedly checking if a file has changed is not ideal. It consumes CPU/power.

The program inotifywait is available on many Linux systems.

inotifywait -e main.c will exit when main.c is modified

inotifywait is efficient because waits for the operating system interface to notify it the file has changed..

Modify your script to use inotifywait
The shell variable $PATH contains a colon-separated list of directories. which will be searched when executing a command.

Write a shell script named which.sh which given a program name as argument, uses ls to print details of matching files in directories in $PATH.

For example:
```
./which.sh cat
-rwxr-xr-x 1 root root 43936 Sep 24 18:36 /bin/cat
./which.sh clang
llrwxrwxrwx 1 root root 24 Jan 12 01:49 /usr/bin/clang -> ../lib/llvm-11/bin/clang
./which.sh lost
lost not found
```
The shell builtin which does something similar:
```
which cat
/bin/cat
which clang
/usr/bin/clang
which lost
```
but don't try using which. Use the usual programs we've been using to write scripts such as tr and test.

Think about if any characters in directory names migh break your answer, e.g spaces.