Lecture 9 Discussion Questions

Suppose I have a bash script named script1.sh with the following contents

#!/bin/bash

book="Educated"
echo "$book is a fantastic book"
echo 'you should totally read $book'
echo "hunter highly recommends $BOOK"

What would be the result of running ./script1.sh (You may assume that it has execute permissions)?

Educated is a fantastic book
you should totally read $book
hunter highly recommends

Write a shell script named if_args.sh which takes two command line arguments and prints “these are sum lucky numbers!” if their sum is equal to 7, and prints a sad face otherwise.

#!/bin/bash

arg1=$1
arg2=$2
let sum="$arg1+$arg2"

# Good to debug with echoing
# echo "$arg1, $arg2, $sum"

if [ $sum -eq 7 ]; then
    echo "these are sum lucky numbers!"
else
    echo ":("
fi

Write a shell script named loop.sh that loops over all files in the current directory and prints out the contents of the file, with newlines between each output. (Note, we could probably do this without a loop, but for the sake of learning let’s use a loop here). For the sake of this problem you can assume there are no sub-directories in the current directory. If you finish early, try make this work when there are sub-directories.

#!/bin/bash

# First attempt, doesn't handle directories
for file in $(ls); do
    cat $file
done

# Second attempt to ignore directories
# One option: Change what you loop over
#   Example: find . -type f -maxd
#   Example: ls -p | grep -v "/"
# Another option: Change loop logic
for file in $(ls); do
    if [ -f $file ]; then
        cat $file
    fi
done

Write a shell script named filter.sh which takes any amount of numbers as arguments and prints out all the numbers which are even (Note that bash has a % operator just like Java). If no numbers are given as inputs, print out a help message describing how to use the program.

#!/bin/bash

# Check if the number of arguments is non-zero
if [ $# -eq 0 ]; then
    echo "USAGE: ./filter.sh NUMBERS..."
    exit 1
else

# Loop over all arugments, printing out if they are even
for n in $@; do
    let i="$n % 2"
    if [ $i -eq 0 ]; then
        echo $n
    fi
done

Write a shell script named traveler.sh which goes into each directory (only one deep) from the current directory and prints out it’s contents.

#!/bin/bash

for dir in $(ls -p | grep "/"); do
   cd $dir
   ls
   cd ..
done

# Doesn't change the directory in the parent shell!
cd dir1