Appendix I. Exercises

Write a script to carry out each of the following tasks.


Home Directory Listing

Perform a recursive directory listing on the user's home directory and save the information to a file. Compress the file, have the script prompt the user to insert a floppy, then press ENTER. Finally, save the file to the floppy.

Converting for loops to while and until loops

Convert the for loops in Example 10-1 to while loops. Hint: store the data in an array and step through the array elements.

Having already done the "heavy lifting", now convert the loops in the example to until loops.


Print (to stdout) all prime numbers between 60000 and 63000. The output should be nicely formatted in columns (hint: use printf).

Unique System ID

Generate a "unique" 6-digit hexadecimal identifier for your computer. Do not use the flawed hostid command. Hint: md5sum /etc/passwd, then select the first 6 digits of output.


Archive as a "tarball" (*.tar.gz file) all the files in your home directory tree (/home/your-name) that have been modified in the last 24 hours. Hint: use find.

Safe Delete

Write, as a script, a "safe" delete command, Filenames passed as command-line arguments to this script are not deleted, but instead gzipped and moved to a /home/username/trash directory. At invocation, the script checks the "trash" directory for files older than 48 hours and deletes them.


Managing Disk Space

List, one at a time, all files larger than 100K in the /home/username directory tree. Give the user the option to delete or compress the file, then proceed to show the next one. Write to a logfile the names of all deleted files and the deletion times.

Making Change

What is the most efficient way to make change for $1.68, using only coins in common circulations (up to 25c)? It's 6 quarters, 1 dime, a nickel, and three cents.

Given any arbitrary command line input in dollars and cents ($*.??), calculate the change, using the minimum number of coins. If your home country is not the United States, you may use your local currency units instead. The script will need to parse the command line input, then change it to multiples of the smallest monetary unit (cents or whatever). Hint: look at Example 23-4.

Lucky Numbers

A "lucky number" is one whose individual digits add up to 7, in successive additions. For example, 62431 is a "lucky number" (6 + 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the "lucky numbers" between 1000 and 10000.

Alphabetizing a String

Alphabetize (in ASCII order) an arbitrary string read from the command line.


Parse /etc/passwd, and output its contents in nice, easy-to-read tabular form.

Pretty-Printing a Data File

Certain database and spreadsheet packages use save-files with comma-separated values (CSVs). Other applications often need to parse these files.

Given a data file with comma-separated fields, of the form:
Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989
Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612
Reformat the data and print it out to stdout in labeled, evenly-spaced columns.


Logging File Accesses

Log all accesses to the files in /etc during the course of a single day. This information should include the filename, user name, and access time. If any alterations to the files take place, that should be flagged. Write this data as neatly formatted records in a logfile.

Strip Comments

Strip all comments from a shell script whose name is specified on the command line. Note that the "#! line" must not be stripped out.

HTML Conversion

Convert a given text file to HTML. This non-interactive script automatically inserts all appropriate HTML tags into a file specified as an argument.

Strip HTML Tags

Strip all HTML tags from a specified HTML file, then reformat it into lines between 60 and 75 characters in length. Reset paragraph and block spacing, as appropriate, and convert HTML tables to their approximate text equivalent.

Hex Dump

Do a hex(adecimal) dump on a binary file specified as an argument. The output should be in neat tabular fields, with the first field showing the address, each of the next 8 fields a 4-byte hex number, and the final field the ASCII equivalent of the previous 8 fields.


Solve a 4 x 4 determinant.

Hidden Words

Write a "word-find" puzzle generator, a script that hides 10 input words in a 10 x 10 matrix of random letters. The words may be hidden across, down, or diagonally.


Anagram 4-letter input. For example, the anagrams of word are: do or rod row word. You may use /usr/share/dict/linux.words as the reference list.

Please do not send the author your solutions to these exercises. There are better ways to impress him with your cleverness, such as submitting bugfixes and suggestions for improving this book.