1. LING 408/508: Computational Techniques for Linguists
Lecture 6

2. Shell Arithmetic at the shell prompt:
expr (Need spaces cf. expr 1+3)
expr 2 '*' 2 (cf. expr 2 * 2)
echo `expr 1 + 3`
i=2 (NO SPACES! cf. i = 2)
expr $i + 1
let x= (cf. let x=1 + 3)
echo $x
let i=$i+1 (also ok let i=i+1)
echo $i
((x = 1+ 3)) (spaces not significant)
echo $((1+3))
((i=i+1)) (also ok let i=$i+1)

3. Comparison operators Examples: Format: 2 5 1 echo $x $i test $x -le $i
echo $? (exit status)
test $x -le $i -a $i -lt $x
echo $? 1
Format:
if [ $x OP $y ]; then …
(else/elif…) fi
[ …. ] is known as test
OP:
-eq equals
-ne not equals
-gt greater than
-ge greater than or equals
-lt less than
-le less than or equals

4. Input At a terminal: read –p "Name: " name
read –p "Enter X and Y: " x y
echo $x
echo $y

5. Positional Parameters
In a shell script:
$1: first parameter
$#: number of parameters
Program:
#!/bin/bash
echo "Number of parameters: $#"
if [ $# -eq 1 ]; then
echo "1st parameter: $1" fi
Output:
sh test.sh
Number of parameters: 0
sh test.sh 45
Number of parameters: 1
1st parameter: 45
sh test.sh 45 56
Number of parameters: 2

6. Running shell scripts
Run the program in the current directory:
(./ needed if current directory is not in PATH)
./test.sh
-bash: ./test.sh: Permission denied
ls -l test.sh
-rw-r--r-- 1 sandiway staff 98 Sep 4 09:14 test.sh
chmod u+x test.sh
-rwxr--r-- 1 sandiway staff 98 Sep 4 09:14 test.sh
Number of parameters: 0
Supply program filename as a parameter to sh/bash:
sh is dash, not bash anymore
sh test.sh
bash test.sh
source test.sh
. test.sh
(. = source)

7. First Non-trivial Program
Let’s write a simple shell-script BMI calculator
it can solicit input from the terminal or take command line arguments
try the metric one first

8. First Non-trivial Program
First pass:
Several things wrong about this program…

9. BMI calculator Did you notice bash can only do integers?
can use bc (an arbitrary precision calculator)
scale = # of decimal places
echo "scale=2;2/3" | bc -q
.66
but test comparisons (-gt etc.) would then be a pain in the butt
can re-scale the formula:
Example:
weight in kg * 1,000,000 / (height in cm)**2
echo $((68* / (165 * 165)))
2497 (24.97)
Explanation:
echo "…" | means send "..." to next command
bc –q means quiet mode
Explanation:
`…` is command substitution
bc -l, use the mathlib
In bc, a(1) computes tan-1(1), which is π/4
pi=`echo "scale=50; 4*a(1)" | bc -l` (-l math library)
echo $pi

10. First Non-trivial Program
Some possibilities:

11. Homework 3 Modify the BMI calculator to:
accept either command line arguments or read from the terminal if they’re missing
print the weight status message according to the following table:
modify the calculator to accept input in both metric and traditional units
make sure you supply examples of your program working!

12. Homework 3 Instructions: email me submit everything in one PDF file!
subject of course number, your name, homework 3
cite any discussion or source
due date: Monday 10th by midnight