1. Suppose we have a file directories.txt where each line contains the name of a directory that we want to create. Our friend proposes that we run cat directories.txt | mkdir into order to do this? Will this command work? If not, how can we change this command to get it to work/could we improve it in any way?

One correct way to solve this is the command:

xargs mkdir < dir.txt

Many people might write this as the following. It’s fine, but it is a bit unnecessary to invoke the cat command and using a | when we already have input redirection (<) available.

cat dir.txt | xargs mkdir
  1. Come up with a general statement about when we need to use the xargs command.

We use xargs whenever we want to take something that would be coming as input to use them as command-line arguments. So for the last problem, the list of files is coming from a file that we can only redirect as input, but mkdir doesn’t read from stdin, it only takes command-line arguments.

On top of this, people often think of using xargs when you want to run one command over many possible input values.

  1. Suppose we want to run Mystery.java and save standard output to a file named out1.txt and have it print standard output to the console. How would we achieve this?
java Mystery.java | tee out1.txt
  1. Again, suppose we are running Mystery.java. This time, however, we want standard output and standard error to print to the console, in addition to having all output printed to out1.txt. How would we achieve this?
java Mystery.java 2>&1 | tee out1.txt
  1. Suppose we have the intro_survey.csv from the previous homework assignment. Using cut, how many unique answers were there to the question “what is your favorite candy”?
tail -n+2 intro_survey.csv | cut-d, -f1 | sort -f | uniq -i | wc -l
  1. Write a command that would delete all directories, recursively, starting from the current directory. Be careful about where you run this command!! Don’t run it from your home directory!
find -type d | xargs rm