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M.2. Writing Scripts

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


Self-reproducing Script

Write a script that backs itself up, that is, copies itself to a file named backup.sh.

Hint: Use the cat command and the appropriate positional parameter.

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 USB flash drive, then press ENTER. Finally, save the file to the flash drive after making certain the flash drive has properly mounted by parsing the output of df.

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.

Changing the line spacing of a text file

Write a script that reads each line of a target file, then writes the line back to stdout, but with an extra blank line following. This has the effect of double-spacing the file.

Include all necessary code to check whether the script gets the necessary command-line argument (a filename), and whether the specified file exists.

When the script runs correctly, modify it to triple-space the target file.

Finally, write a script to remove all blank lines from the target file, single-spacing it.

Backwards Listing

Write a script that echoes itself to stdout, but backwards.

Automatically Decompressing Files

Given a list of filenames as input, this script queries each target file (parsing the output of the file command) for the type of compression used on it. Then the script automatically invokes the appropriate decompression command (gunzip, bunzip2, unzip, uncompress, or whatever). If a target file is not compressed, the script emits a warning message, but takes no other action on that particular file.

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.

Checking whether a process is still running

Given a process ID (PID) as an argument, this script will check, at user-specified intervals, whether the given process is still running. You may use the ps and sleep commands.


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

Lottery Numbers

One type of lottery involves picking five different numbers, in the range of 1 - 50. Write a script that generates five pseudorandom numbers in this range, with no duplicates. The script will give the option of echoing the numbers to stdout or saving them to a file, along with the date and time the particular number set was generated. (If your script consistently generates winning lottery numbers, then you can retire on the proceeds and leave shell scripting to those of us who have to work for a living.)


Integer or String

Write a script function that determines if an argument passed to it is an integer or a string. The function will return TRUE (0) if passed an integer, and FALSE (1) if passed a string.

Hint: What does the following expression return when $1 is not an integer?

expr $1 + 0

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.


Simulate the functionality of the deprecated banner command in a script.

Removing Inactive Accounts

Inactive accounts on a network waste disk space and may become a security risk. Write an administrative script (to be invoked by root or the cron daemon) that checks for and deletes user accounts that have not been accessed within the last 90 days.

Enforcing Disk Quotas

Write a script for a multi-user system that checks users' disk usage. If a user surpasses a preset limit (100 MB, for example) in her /home/username directory, then the script automatically sends her a warning e-mail.

The script will use the du and mail commands. As an option, it will allow setting and enforcing quotas using the quota and setquota commands.

Logged in User Information

For all logged in users, show their real names and the time and date of their last login.

Hint: use who, lastlog, and parse /etc/passwd.

Safe Delete

Implement, as a script, a "safe" delete command, sdel.sh. Filenames passed as command-line arguments to this script are not deleted, but instead gzipped if not already compressed (use file to check), then moved to a ~/TRASH directory. Upon invocation, the script checks the ~/TRASH directory for files older than 48 hours and permanently deletes them. (An better alternative might be to have a second script handle this, periodically invoked by the cron daemon.)

Extra credit: Write the script so it can handle files and directories recursively. This would give it the capability of "safely deleting" entire directory structures.

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 23-8.

Quadratic Equations

Solve a quadratic equation of the form Ax^2 + Bx + C = 0. Have a script take as arguments the coefficients, A, B, and C, and return the solutions to four decimal places.

Hint: pipe the coefficients to bc, using the well-known formula, x = ( -B +/- sqrt( B^2 - 4AC ) ) / 2A.

Table of Logarithms

Using the bc and printf commands, print out a nicely-formatted table of eight-place natural logarithms in the interval between 0.00 and 100.00, in steps of .01.

Hint: bc requires the -l option to load the math library.

Sum of Matching Numbers

Find the sum of all five-digit numbers (in the range 10000 - 99999) containing exactly two out of the following set of digits: { 4, 5, 6 }. These may repeat within the same number, and if so, they count once for each occurrence.

Some examples of matching numbers are 42057, 74638, and 89515.

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.


Borrowing the ASCII graphics from Example A-42, write a script that plays the well-known gambling game of craps. The script will accept bets from one or more players, roll the dice, and keep track of wins and losses, as well as of each player's bankroll.


Write a script that plays the child's game of tic-tac-toe against a human player. The script will let the human choose whether to take the first move. The script will follow an optimal strategy, and therefore never lose. To simplify matters, you may use ASCII graphics:

   o | x |
     | x |
     | o |
   Your move, human (row, column)?
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.

Logging Logins

Parse /var/log/messages to produce a nicely formatted file of user logins and login times. The script may need to run as root. (Hint: Search for the string "LOGIN.")

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.

Given ASCII text input either from stdin or a file, adjust the word spacing to right-justify each line to a user-specified line-width, then send the output to stdout.

Mailing List

Using the mail command, write a script that manages a simple mailing list. The script automatically e-mails the monthly company newsletter, read from a specified text file, and sends it to all the addresses on the mailing list, which the script reads from another specified file.

Generating Passwords

Generate pseudorandom 8-character passwords, using characters in the ranges [0-9], [A-Z], [a-z]. Each password must contain at least two digits.

Monitoring a User

You suspect that one particular user on the network has been abusing his privileges and possibly attempting to hack the system. Write a script to automatically monitor and log his activities when he's signed on. The log file will save entries for the previous week, and delete those entries more than seven days old.

You may use last, lastlog, and lastcomm to aid your surveillance of the suspected malefactor.

Checking for Broken Links

Using lynx with the -traversal option, write a script that checks a Web site for broken links.


Testing Passwords

Write a script to check and validate passwords. The object is to flag "weak" or easily guessed password candidates.

A trial password will be input to the script as a command-line parameter. To be considered acceptable, a password must meet the following minimum qualifications:

  • Minimum length of 8 characters

  • Must contain at least one numeric character

  • Must contain at least one of the following non-alphabetic characters: @, #, $, %, &, *, +, -, =


  • Do a dictionary check on every sequence of at least four consecutive alphabetic characters in the password under test. This will eliminate passwords containing embedded "words" found in a standard dictionary.

  • Enable the script to check all the passwords on your system. These probably do not reside in /etc/passwd.

This exercise tests mastery of Regular Expressions.

Cross Reference

Write a script that generates a cross-reference (concordance) on a target file. The output will be a listing of all word occurrences in the target file, along with the line numbers in which each word occurs. Traditionally, linked list constructs would be used in such applications. Therefore, you should investigate arrays in the course of this exercise. Example 15-12 is probably not a good place to start.

Square Root

Write a script to calculate square roots of numbers using Newton's Method.

The algorithm for this, expressed as a snippet of Bash pseudo-code is:

#  (Isaac) Newton's Method for speedy extraction
#+ of square roots.

guess = $argument
#  $argument is the number to find the square root of.
#  $guess is each successive calculated "guess" -- or trial solution --
#+ of the square root.
#  Our first "guess" at a square root is the argument itself.

oldguess = 0
# $oldguess is the previous $guess.

tolerance = .000001
# To how close a tolerance we wish to calculate.

loopcnt = 0
# Let's keep track of how many times through the loop.
# Some arguments will require more loop iterations than others.

while [ ABS( $guess $oldguess ) -gt $tolerance ]
#       ^^^^^^^^^^^^^^^^^^^^^^^ Fix up syntax, of course.

#      "ABS" is a (floating point) function to find the absolute value
#+      of the difference between the two terms.
#             So, as long as difference between current and previous
#+            trial solution (guess) exceeds the tolerance, keep looping.

   oldguess = $guess  # Update $oldguess to previous $guess.

#  =======================================================
   guess = ( $oldguess + ( $argument / $oldguess ) ) / 2.0
#        = 1/2 ( ($oldguess **2 + $argument) / $oldguess )
#  equivalent to:
#        = 1/2 ( $oldguess + $argument / $oldguess )
#  that is, "averaging out" the trial solution and
#+ the proportion of argument deviation
#+ (in effect, splitting the error in half).
#  This converges on an accurate solution
#+ with surprisingly few loop iterations . . .
#+ for arguments > $tolerance, of course.
#  =======================================================

   (( loopcnt++ ))     # Update loop counter.

It's a simple enough recipe, and seems at first glance easy enough to convert into a working Bash script. The problem, though, is that Bash has no native support for floating point numbers. So, the script writer needs to use bc or possibly awk to convert the numbers and do the calculations. It may get rather messy . . .

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.

Monitoring Processes

Write a script to continually monitor all running processes and to keep track of how many child processes each parent spawns. If a process spawns more than five children, then the script sends an e-mail to the system administrator (or root) with all relevant information, including the time, PID of the parent, PIDs of the children, etc. The script appends a report to a log file every ten minutes.

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.

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.

XML Conversion

Convert an XML file to both HTML and text format.

Chasing Spammers

Write a script that analyzes a spam e-mail by doing DNS lookups on the IP addresses in the headers to identify the relay hosts as well as the originating ISP. The script will forward the unaltered spam message to the responsible ISPs. Of course, it will be necessary to filter out your own ISP's IP address, so you don't end up complaining about yourself.

As necessary, use the appropriate network analysis commands.

For some ideas, see Example 15-41 and Example A-30.

Optional: Write a script that searches through a list of e-mail messages and deletes the spam according to specified filters.

Creating man pages

Write a script that automates the process of creating man pages.

Given a text file which contains information to be formatted into a man page, the script will read the file, then invoke the appropriate groff commands to output the corresponding man page to stdout. The text file contains blocks of information under the standard man page headings, i.e., NAME, SYNOPSIS, DESCRIPTION, etc.

Example A-41 is an instructive first step.

Morse Code
Convert a text file to Morse code. Each character of the text file will be represented as a corresponding Morse code group of dots and dashes (underscores), separated by whitespace from the next. For example:
Invoke the "morse.sh" script with "script"
as an argument to convert to Morse.

$ sh morse.sh script

... _._. ._. .. .__. _
s   c    r   i   p   t
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.

The obvious followup to this is to extend the hex dump script into a disassembler. Using a lookup table, or some other clever gimmick, convert the hex values into 80x86 op codes.

Emulating a Shift Register

Using Example 26-15 as an inspiration, write a script that emulates a 64-bit shift register as an array. Implement functions to load the register, shift left, shift right, and rotate it. Finally, write a function that interprets the register contents as eight 8-bit ASCII characters.

Calculating Determinants

Write a script that calculates determinants [1] by recursively expanding the minors. Use a 4 x 4 determinant as a test case.

Hidden Words

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

Optional: Write a script that solves word-find puzzles. To keep this from becoming too difficult, the solution script will find only horizontal and vertical words. (Hint: Treat each row and column as a string, and search for substrings.)


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.

Word Ladders

A "word ladder" is a sequence of words, with each successive word in the sequence differing from the previous one by a single letter.

For example, to "ladder" from mark to vase:

mark --> park --> part --> past --> vast --> vase
         ^           ^       ^      ^           ^

Write a script that solves word ladder puzzles. Given a starting and an ending word, the script will list all intermediate steps in the "ladder." Note that all words in the sequence must be legitimate dictionary words.

Fog Index

The "fog index" of a passage of text estimates its reading difficulty, as a number corresponding roughly to a school grade level. For example, a passage with a fog index of 12 should be comprehensible to anyone with 12 years of schooling.

The Gunning version of the fog index uses the following algorithm.

  1. Choose a section of the text at least 100 words in length.

  2. Count the number of sentences (a portion of a sentence truncated by the boundary of the text section counts as one).

  3. Find the average number of words per sentence.


  4. Count the number of "difficult" words in the segment -- those containing at least 3 syllables. Divide this quantity by total words to get the proportion of difficult words.


  5. The Gunning fog index is the sum of the above two quantities, multiplied by 0.4, then rounded to the nearest integer.

    G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )

Step 4 is by far the most difficult portion of the exercise. There exist various algorithms for estimating the syllable count of a word. A rule-of-thumb formula might consider the number of letters in a word and the vowel-consonant mix.

A strict interpretation of the Gunning fog index does not count compound words and proper nouns as "difficult" words, but this would enormously complicate the script.

Calculating PI using Buffon's Needle

The Eighteenth Century French mathematician de Buffon came up with a novel experiment. Repeatedly drop a needle of length n onto a wooden floor composed of long and narrow parallel boards. The cracks separating the equal-width floorboards are a fixed distance d apart. Keep track of the total drops and the number of times the needle intersects a crack on the floor. The ratio of these two quantities turns out to be a fractional multiple of PI.

In the spirit of Example 15-50, write a script that runs a Monte Carlo simulation of Buffon's Needle. To simplify matters, set the needle length equal to the distance between the cracks, n = d.

Hint: there are actually two critical variables: the distance from the center of the needle to the nearest crack, and the inclination angle of the needle to that crack. You may use bc to handle the calculations.

Playfair Cipher

Implement the Playfair (Wheatstone) Cipher in a script.

The Playfair Cipher encrypts text by substitution of digrams (2-letter groupings). It is traditional to use a 5 x 5 letter scrambled-alphabet key square for the encryption and decryption.

   C O D E S
   A B F G H
   I K L M N
   P Q R T U
   V W X Y Z

Each letter of the alphabet appears once, except "I" also represents
"J". The arbitrarily chosen key word, "CODES" comes first, then all
the rest of the alphabet, in order from left to right, skipping letters
already used.

To encrypt, separate the plaintext message into digrams (2-letter
groups). If a group has two identical letters, delete the second, and
form a new group. If there is a single letter left over at the end,
insert a "null" character, typically an "X."



For each digram, there are three possibilities.

1) Both letters will be on the same row of the key square:
   For each letter, substitute the one immediately to the right, in that
   row. If necessary, wrap around left to the beginning of the row.


2) Both letters will be in the same column of the key square:
   For each letter, substitute the one immediately below it, in that
   row. If necessary, wrap around to the top of the column.


3) Both letters will form the corners of a rectangle within the key square:
   For each letter, substitute the one on the other corner the rectangle
   which lies on the same row.

The "TH" digram falls under case #3.
T U           (Rectangle with "T" and "H" at corners)

T --> U
H --> G

The "SE" digram falls under case #1.
C O D E S     (Row containing "S" and "E")

S --> C  (wraps around left to beginning of row)
E --> S


To decrypt encrypted text, reverse the above procedure under cases #1
and #2 (move in opposite direction for substitution). Under case #3,
just take the remaining two corners of the rectangle.

Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a
fairly detailed description of the Playfair Cipher and its solution methods.

This script will have three main sections

  1. Generating the key square, based on a user-input keyword.

  2. Encrypting a plaintext message.

  3. Decrypting encrypted text.

The script will make extensive use of arrays and functions.


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


For all you fine people who failed second-year algebra, a determinant is a numerical quantity associated with a multidimensional matrix (array of numbers).
For the simple case of a 2 x 2 determinant:

  |a  b|
  |b  a|

The solution is a*a - b*b, where "a" and "b" represent numbers.