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CS 340 Class Notes Summer 2021

Lecture 5: Shell Scripting/Version Control Systems

(05/12/2021)

Review Commands

  • tmux -ls - view of how many sessions are active
  • tmux a - attach to tmux session
  • ls - l <file> - long form output for ls. Permissions will appear in the form:
    • -{user permissions}-{group permissions}-{other permissions} {owner} {group} {file size in bytes} {modified date} {file name}
    • an '@' indicates extended permissions
  • tail -n +<number> - gets the lines of a file after the specified number

Shell Script

  • Recall how we need a 'shbang' (#!) at the top of a shell script file. This tells the shell what program to run this file.
  • We also need to change file modifier chmod +x to make it an executable.
  • To access the command line arguments, the shell conveniently creates an array of strings with the executed command and its args. e.g. ["./covid.sh", "New York"].
  • We can access these variables by their index, namely $0, $1, and so on.
  • Note that accessing an undefined variable gives an empty string, which is useful to check if a command-line arg was provided.
  • The following are equivalent ways of checking if a first command-line argument was provided:
    • if [ $1 ]
    • if [ -n $1 ] - -n checks for non empty string. -z would check if it is empty.
  • For variable assignment, we add no spaces around =.
    • state=$1
  • Integer math in a shell script needs to be surrounded by $(()) to indicate to the shell that these values are not strings
    • echo 2 + 2 - 2 + 2
    • echo $(( 2 + 2)) - 4
  • In arithmetic expansion, math mode, the shell recognizes variables without having to use '$'.
    • $((var + 42)) as opposed to $(($var + 42))
  • Reminder: cut works on input or on a file, so we can echo a variable before piping it it to cut
  • Reminder: grep -i is for case insensitive message
  • To check if a value is in a specific data set, say Paris as a possible state of the US, we can use [ -z grep $state ] since grep will return an empty string when it nothing is found.
  • Reminder: remove any temporary files with rm $temp_file before exiting with exit <exit code>

Covid Script

  • Last lecture we accessed a csv file of Covid-19 deaths and cases by day, we will be manipulating this data with a shell script to output more manageable data and allow a user to select a desired state
  • To convert cumulative to daily total covid cases we have to iterate across cases and deaths and subtract the current number from the previous number.

Tmux Navigation

  • Tmux does not let us do normal scrolling
    • We could pipe executable into less to see the result and scroll here
    • We can go into copy and paste mode in tmux with { in command mode. Get out with esc

How do we manage files so that we can support all these activities?

  1. Backups (USBs, Hard drives, etc)
    • Pros:
      • Recover old files
      • Share files
      • Custom versions
    • Cons:
      • Expensive
      • Scalability
      • Recursive Problem (our backups individually have the same problem we started with)
      • Hard to manage
      • Remember to bring
  2. Cloud Storage (Dropbox, OneDrive, etc)
    • Pros:
      • Off-site (always available)
      • Automatic
      • Some versioning
      • Sharing
    • Cons:
      • Collaboration
      • Security or privacy concerns (since handled by third party)
      • "Break the build" (say if adding something and saving it automatically breaks our code)
  3. Version Control System (git)
    • Modify files
    • Mark files that are ready to be added to the "repository" (global shared files)
    • Bundle up and share our changes with everyone
    • git is a version control system, GitHub is a website that hosts git repositories

Version Control Systems

  • Recall that maintenance is the dominant activity in software development.
    • Correcting faults
    • Improving design
    • Implementing Enhancements
    • Fix regressions (part of the code that used to work)
  • Software development does not happen in a vacuum
    • Team members
    • Multiple computers
    • Computer crashes

Bonus: Making Flags

  • Creating flags can get a little gnarly since they can show up in any order in the command line
  • GetOpt is a program that is useful for command-line options