- •Table of Contents
- •Chapter 1. Why Shell Programming?
- •2.1. Invoking the script
- •2.2. Preliminary Exercises
- •Part 2. Basics
- •Chapter 3. Exit and Exit Status
- •Chapter 4. Special Characters
- •Chapter 5. Introduction to Variables and Parameters
- •5.1. Variable Substitution
- •5.2. Variable Assignment
- •5.3. Bash Variables Are Untyped
- •5.4. Special Variable Types
- •Chapter 6. Quoting
- •Chapter 7. Tests
- •7.1. Test Constructs
- •7.2. File test operators
- •7.3. Comparison operators (binary)
- •7.4. Nested if/then Condition Tests
- •7.5. Testing Your Knowledge of Tests
- •Chapter 8. Operations and Related Topics
- •8.1. Operators
- •8.2. Numerical Constants
- •Part 3. Beyond the Basics
- •Chapter 9. Variables Revisited
- •9.1. Internal Variables
- •9.2. Manipulating Strings
- •9.2.1. Manipulating strings using awk
- •9.2.2. Further Discussion
- •9.3. Parameter Substitution
- •9.4. Typing variables: declare or typeset
- •9.5. Indirect References to Variables
- •9.6. $RANDOM: generate random integer
- •9.7. The Double Parentheses Construct
- •Chapter 10. Loops and Branches
- •10.1. Loops
- •10.2. Nested Loops
- •10.3. Loop Control
- •10.4. Testing and Branching
- •Chapter 11. Internal Commands and Builtins
- •11.1. Job Control Commands
- •Chapter 12. External Filters, Programs and Commands
- •12.1. Basic Commands
- •12.2. Complex Commands
- •12.3. Time / Date Commands
- •12.4. Text Processing Commands
- •12.5. File and Archiving Commands
- •12.6. Communications Commands
- •12.7. Terminal Control Commands
- •12.8. Math Commands
- •12.9. Miscellaneous Commands
- •Chapter 13. System and Administrative Commands
- •Chapter 14. Command Substitution
- •Chapter 15. Arithmetic Expansion
- •Chapter 16. I/O Redirection
- •16.1. Using exec
- •16.2. Redirecting Code Blocks
- •16.3. Applications
- •Chapter 17. Here Documents
- •Chapter 18. Recess Time
- •Part 4. Advanced Topics
- •Chapter 19. Regular Expressions
- •19.1. A Brief Introduction to Regular Expressions
- •19.2. Globbing
- •Chapter 20. Subshells
- •Chapter 21. Restricted Shells
- •Chapter 22. Process Substitution
- •Chapter 23. Functions
- •23.1. Complex Functions and Function Complexities
- •23.2. Local Variables
- •23.2.1. Local variables make recursion possible.
- •Chapter 24. Aliases
- •Chapter 25. List Constructs
- •Chapter 26. Arrays
- •Chapter 27. Files
- •Chapter 28. /dev and /proc
- •28.2. /proc
- •Chapter 29. Of Zeros and Nulls
- •Chapter 30. Debugging
- •Chapter 31. Options
- •Chapter 32. Gotchas
- •Chapter 33. Scripting With Style
- •33.1. Unofficial Shell Scripting Stylesheet
- •Chapter 34. Miscellany
- •34.2. Shell Wrappers
- •34.3. Tests and Comparisons: Alternatives
- •34.4. Optimizations
- •34.5. Assorted Tips
- •34.6. Oddities
- •34.7. Portability Issues
- •34.8. Shell Scripting Under Windows
- •Chapter 35. Bash, version 2
- •Chapter 36. Endnotes
- •36.1. Author's Note
- •36.2. About the Author
- •36.3. Tools Used to Produce This Book
- •36.3.1. Hardware
- •36.3.2. Software and Printware
- •36.4. Credits
- •Bibliography
- •Appendix A. Contributed Scripts
- •Appendix C. Exit Codes With Special Meanings
- •Appendix D. A Detailed Introduction to I/O and I/O Redirection
- •Appendix E. Localization
- •Appendix F. History Commands
- •Appendix G. A Sample .bashrc File
- •Appendix H. Converting DOS Batch Files to Shell Scripts
- •Appendix I. Exercises
- •Appendix J. Copyright
Advanced Bash−Scripting Guide
indirect referencing more intuitive.
9.6. $RANDOM: generate random integer
$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 − 32767. $RANDOM should not be used to generate an encryption key.
Example 9−20. Generating random numbers
#!/bin/bash
#$RANDOM returns a different random integer at each invocation.
#Nominal range: 0 − 32767 (signed 16−bit integer).
MAXCOUNT=10
count=1
echo |
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echo "$MAXCOUNT random numbers:" |
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echo "−−−−−−−−−−−−−−−−−" |
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while [ "$count" −le $MAXCOUNT ] |
# Generate 10 ($MAXCOUNT) random integers. |
do |
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number=$RANDOM |
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echo $number |
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let "count += 1" # Increment count. |
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done |
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echo "−−−−−−−−−−−−−−−−−" |
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#If you need a random int within a certain range, use the 'modulo' operator.
#This returns the remainder of a division operation.
RANGE=500
echo
number=$RANDOM
let "number %= $RANGE"
echo "Random number less than $RANGE −−− $number"
echo
#If you need a random int greater than a lower bound,
#then set up a test to discard all numbers below that.
FLOOR=200
number=0 #initialize
while [ "$number" −le $FLOOR ] do
number=$RANDOM done
echo "Random number greater than $FLOOR −−− $number" echo
# May combine above two techniques to retrieve random number between two limits.
9.6. $RANDOM: generate random integer |
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Advanced Bash−Scripting Guide
number=0 #initialize
while [ "$number" −le $FLOOR ] do
number=$RANDOM
let "number %= $RANGE" # Scales $number down within $RANGE. done
echo "Random number between $FLOOR and $RANGE −−− $number" echo
# Generate binary choice, that is, "true" or "false" value. BINARY=2
number=$RANDOM T=1
let "number %= $BINARY"
# let "number >>= 14" gives a better random distribution
# (right shifts out everything except last binary digit). if [ "$number" −eq $T ]
then
echo "TRUE" else
echo "FALSE"
fi
echo
# May generate toss of the dice. SPOTS=7 # Modulo 7 gives range 0 − 6. DICE=2
ZERO=0
die1=0
die2=0
# Tosses each die separately, and so gives correct odds.
while [ "$die1" −eq $ZERO ] do
# Can't have a zero come up.
let "die1 = $RANDOM % $SPOTS" # Roll first one. done
while [ "$die2" −eq $ZERO ] do
let "die2 = $RANDOM % $SPOTS" # Roll second one. done
let "throw = $die1 + $die2"
echo "Throw of the dice = $throw" echo
exit 0
Just how random is RANDOM? The best way to test this is to write a script that tracks the distribution of
"random" numbers generated by RANDOM. Let's roll a RANDOM die a few times...
Example 9−21. Rolling the die with RANDOM
9.6. $RANDOM: generate random integer |
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Advanced Bash−Scripting Guide |
#!/bin/bash |
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# How random is RANDOM? |
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RANDOM=$$ |
# Reseed the random number generator using script process ID. |
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PIPS=6 |
# A die has 6 pips. |
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MAXTHROWS=600 |
# Increase this, if you have nothing better to do with your time. |
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throw=0 |
# Throw count. |
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zeroes=0 |
# Must initialize counts to zero. |
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ones=0 |
# since an uninitialized variable is null, not zero. |
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twos=0 |
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threes=0 |
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fours=0 |
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fives=0 |
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sixes=0 |
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print_result () |
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{ |
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echo |
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echo |
"ones = |
$ones" |
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echo |
"twos = |
$twos" |
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echo |
"threes = $threes" |
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echo |
"fours = |
$fours" |
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echo |
"fives = |
$fives" |
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echo |
"sixes = |
$sixes" |
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echo |
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} |
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update_count() |
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{ |
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case |
"$1" in |
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0) |
let "ones += 1";; |
# Since die has no "zero", this corresponds to 1. |
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1) |
let "twos += 1";; |
# And this to 2, etc. |
2)let "threes += 1";;
3)let "fours += 1";;
4)let "fives += 1";;
5)let "sixes += 1";;
esac
}
echo
while [ "$throw" −lt "$MAXTHROWS" ] do
let "die1 = RANDOM % $PIPS" update_count $die1
let "throw += 1" done
print_result
#The scores should distribute fairly evenly, assuming RANDOM is fairly random.
#With $MAXTHROWS at 600, all should cluster around 100, plus−or−minus 20 or so.
#Keep in mind that RANDOM is a pseudorandom generator,
#and not a spectacularly good one at that.
#Exercise for the reader (easy):
#Rewrite this script to flip a coin 1000 times.
#Choices are "HEADS" or "TAILS".
9.6. $RANDOM: generate random integer |
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Advanced Bash−Scripting Guide
exit 0
As we have seen in the last example, it is best to "reseed" the RANDOM generator each time it is invoked. Using the same seed for RANDOM repeats the same series of numbers. (This mirrors the behavior of the random() function in C.)
Example 9−22. Reseeding RANDOM
#!/bin/bash
# seeding−random.sh: Seeding the RANDOM variable.
MAXCOUNT=25 |
# How many numbers to generate. |
random_numbers ()
{
count=0
while [ "$count" −lt "$MAXCOUNT" ] do
number=$RANDOM echo −n "$number " let "count += 1"
done
}
echo; echo |
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RANDOM=1 |
# Setting RANDOM seeds the random number generator. |
random_numbers |
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echo; echo |
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RANDOM=1 |
# Same seed for RANDOM... |
random_numbers |
# ...reproduces the exact same number series. |
echo; echo |
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RANDOM=2 |
# Trying again, but with a different seen... |
random_numbers |
# gives a different number series. |
echo; echo |
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# RANDOM=$$ seeds RANDOM from process id of script.
#It is also possible to seed RANDOM from 'time' or 'date'.
#Getting fancy...
SEED=$(head −1 /dev/urandom | od −N 1 | awk '{ print $2 }')
#Pseudo−random output fetched from /dev/urandom (system pseudo−random "device"),
#then converted to line of printable (octal) numbers by "od",
#finally "awk" retrieves just one number for SEED.
RANDOM=$SEED random_numbers
echo; echo
exit 0
9.6. $RANDOM: generate random integer |
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