Lecture 2 - Shell Tools and Scripting

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Exercises

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1. Read man ls and write an ls command that lists files in the following manner

   • Includes all files, including hidden files
   • Sizes are listed in human readable format (e.g. 454M instead of 454279954)
   • Files are ordered by recency
   • Output is colorized

   A sample output would look like this

    -rw-r--r--   1 user group 1.1M Jan 14 09:53 baz
    drwxr-xr-x   5 user group  160 Jan 14 09:53 .
    -rw-r--r--   1 user group  514 Jan 14 06:42 bar
    -rw-r--r--   1 user group 106M Jan 13 12:12 foo
    drwx------+ 47 user group 1.5K Jan 12 18:08 ..
   

    ls -l -a -h -t --color
   

   ls lists all directories and files in the current folder. -l flag lists everything in a ‘long’ format. -a flag ensures that entries starting with . which are hidden are also shown. -h prints sizes in human readable format eg. 10K, 100M, 1.5G etc. -t enables the result to be sorted by modification time with newest first and finally --color colorizes the output.

2. Write bash functions marco and polo that do the following. Whenever you execute marco the current working directory should be saved in some manner, then when you execute polo, no matter what directory you are in, polo should cd you back to the directory where you executed marco. For ease of debugging you can write the code in a file marco.sh and (re)load the definitions to your shell by executing source marco.sh.

   # marco.sh
   marco() {
       export CURR_DIR=$(pwd)
   }
   
   polo() {
       cd "$CURR_DIR"
   }
   

   marco saves the current directory in a variable called CURR_DIR. Then using the export command the CURR_DIR variable is set as an environment variable. Therefore, it can be accessed later on when required.

   Now whenpolo is executed it uses this environment variable to cd into the saved directory.

3. Say you have a command that fails rarely. In order to debug it you need to capture its output but it can be time consuming to get a failure run. Write a bash script that runs the following script until it fails and captures its standard output and error streams to files and prints everything at the end. Bonus points if you can also report how many runs it took for the script to fail.

    # script.sh
    #!/usr/bin/env bash
   
    n=$(( RANDOM % 100 ))
   
    if [[ n -eq 42 ]]; then
       echo "Something went wrong"
       >&2 echo "The error was using magic numbers"
       exit 1
    fi
   
    echo "Everything went according to plan"
   

   Remember to make script.sh executable first by using chmod +x ./script.sh. Otherwise solution.sh won’t work.

    # solution.sh
    #!/usr/bin/env bash
   
    RUN=0
   
    until [[ "$?" -ne 0 ]]; do
        RUN=$((RUN+1))
        ./script.sh &> outputstream.txt
    done
   
    echo "The script failed after $RUN runs"
    cat outputstream.txt
   

4. As we covered in the lecture find’s -exec can be very powerful for performing operations over the files we are searching for. However, what if we want to do something with all the files, like creating a zip file? As you have seen so far commands will take input from both arguments and STDIN. When piping commands, we are connecting STDOUT to STDIN, but some commands like tar take inputs from arguments. To bridge this disconnect there’s the xargs command which will execute a command using STDIN as arguments. For example ls | xargs rm will delete the files in the current directory.

   Your task is to write a command that recursively finds all HTML files in the folder and makes a zip with them. Note that your command should work even if the files have spaces (hint: check -d flag for xargs).

    find . -type f -name "*.html" | xargs -d '\n' tar cf compressed.tar.gz
   

   find will recursively find all the .html files in the current directory. We pipe the STDOUT of this command into the xargs program. The -d flag discards all the \n’s we get as part of find’s output. This cleaned output is then supplied as STDINPUT to tar which compresses all the required files into a zip called compressed.tar.gz.

5. (Advanced) Write a command or script to recursively find the most recently modified file in a directory. More generally, can you list all files by recency?

   Open to suggestions. I think we can use find . -type d to get a list of all sub-directories. Then we can use ls -l for each of those sub-directories and append all outputs together in a file and sort the time column to get a final list of files sorted by modification time. Can’t seem to put to put it together though. Happy if one of you could chip in! 😄

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