Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
count=XX creates a file of well−scattered pseudorandom numbers. However, assigning these numbers
to a variable in a script requires a workaround, such as filtering through od (as in above example) or
using dd (see Example 12−42).
There are also other means of generating pseudorandom numbers in a script. Awk provides a convenient
means of doing this.
Example 9−27. Pseudorandom numbers, using awk
#!/bin/bash
# random2.sh: Returns a pseudorandom number in the range 0 − 1.
# Uses the awk rand() function.
AWKSCRIPT=' { srand(); print rand() } '
# Command(s) / parameters passed to awk
# Note that srand() reseeds awk's random number generator.
echo −n "Random number between 0 and 1 = "
echo | awk "$AWKSCRIPT"
exit 0
# Exercises:
# −−−−−−−−−
# 1) Using a loop construct, print out 10 different random numbers.
# (Hint: you must reseed the "srand()" function with a different seed
# in each pass through the loop. What happens if you fail to do this?)
# 2) Using an integer multiplier as a scaling factor, generate random numbers
# in the range between 10 and 100.
# 3) Same as exercise #2, above, but generate random integers this time.
9.7. The Double Parentheses Construct
Similar to the let command, the ((...)) construct permits arithmetic expansion and evaluation. In its simplest
form, a=$(( 5 + 3 )) would set "a" to "5 + 3", or 8. However, this double parentheses construct is also a
mechanism for allowing C−type manipulation of variables in Bash.
Example 9−28. C−type manipulation of variables
#!/bin/bash
# Manipulating a variable, C−style, using the ((...)) construct.
echo
(( a = 23 )) # Setting a value, C−style, with spaces on both sides of the "=".
echo "a (initial value) = $a"
(( a++ )) # Post−increment 'a', C−style.
Chapter 9. Variables Revisited 101