Lecture 6 – Sep 16th, 2019

Setup

1. Log in to clyde.
2. Create a directory and cd into it.
3. Copy ~steve/ex/guess to the current directory. (Remember, bash will expand ~steve to be Prof. Checkoway’s home directory.)

Task

1. Modify guess to make it interactive by letting the user make multiple guesses until the answer is correct using either a while or until loop.

   To make it “fun,” you probably shouldn’t print out the correct answer on a wrong guess. Mine looks like this

   steve@clyde:~/blah$ ./guess
   Guess a number between 1 and 10: 1
   Incorrect. Guess again: 3
   Incorrect. Guess again: 8
   Good guess!
   

2. Write a new script high-low that picks a random number between 1 and 100 and then asks the user to guess the value. After each guess, if the answer is incorrect, tell the user if the correct value is higher or lower. After the user guesses correctly, tell them how many guesses it took.

   Remember, < and > are for comparing strings. You need to use -lt and -gt to compare integers. If you wish, you can use the break command to break out of loops.

   Mine looks like this.

   steve@clyde:~/blah$ ./high-low
   Guess a number between 1 and 100: 50
   50 is too low. Guess again: 75
   75 is too high. Guess again: 63
   63 is too high. Guess again: 57
   57 is too high. Guess again: 53
   53 is too low. Guess again: 55
   55 is too low. Guess again: 56
   You got it in 7 guesses.
   

3. Write a new script scripts that outputs the name of all files in /usr/bin that are shell scripts. You can identify if a file is a shell script by running the file command on the file. This will print out a one line description about the file. For example,

   steve@clyde:~/blah$ file /usr/bin/zipgrep
   /usr/bin/zipgrep: POSIX shell script, ASCII text executable
   steve@clyde:~/blah$ file /usr/bin/awk
   /usr/bin/awk: symbolic link to /etc/alternatives/awk
   steve@clyde:~/blah$ file /usr/bin/host
   /usr/bin/host: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, for GNU/Linux 3.2.0, BuildID[sha1]=c3ab218fd658e9fd02c6636bfc5a5d9b4a068737, stripped
   

   Check out the -q option in the man page for grep and remember that the “then” arm of an if statement runs when the cmd in if cmd; then returns 0.

4. Run shellcheck on guess, high-low, and scripts. Fix any errors or warnings. E.g., “SC2086: Double quote to prevent globbing and word splitting.” means you should add some double quotes around your use of variables.