These are my notes from the MIT's The Missing Semester of Your CS Education.

Lecture 2: Shell Tools and Scripting

Echo and interpolation 🤖

• echo will take as many arguments as you want and will treat each space as a beginning of new argument; for that reason, you can pass arguments as:
  • echo Hello World (which will introduce new lines)
  • echo Hello\World (one line)
  • echo "Hello World" (one line)
  • echo 'Hello World' (one line)
• declaring variables happens with no spaces: name=Sylwia; you invoke the variable with a dollar sign: $name
• you can interpolate the value of the variable with double quotes:

  name=Sylwia
  echo $name #Sylwia
  echo "My name is $name" #My name is Sylwia
  echo 'My name is $name' #My name is $name

• you can save an output of a command to a variable, e.g.:

  date=$(date)
  echo "today is $date" # today is Fri Nov 20 13:34:55 EST 2020

Variables 📝

• $0 stands for the name of the script
• $# - number of arguments passed in
• $$ - pid (process id) of this command
• $1 (up to $9) - the first (ninth) element in a collection
• $@ - all the arguments
• $? — error code from the previous command, e.g.

echo "Hello" #Hello
$? #0 because everything went well
mkdir test
!! #mkdir test; mkdir: test: File exists
echo $? #1
true 
echo $? #0 because true always evaluates to no errors
false 
echo $? #1

• $_ — last argument of the previous command
• !! — repeats the last command (e.g. sudo !! will pass the last command as an argument to sudo)

Logical operators 🤓

• || — it prints the left-hand side if it's true, otherwise, the right-hand side will print;
• && — it prints if both sides are true;
• ; - will print no matter what
• -ne - comparison operator (non-equal)

Searching and finding 👀

• ls *.sh - an example of globbing; lists any file that has this combination of letters
• ls ?.sh - looks for any file that has just one character before the .sh
• ls -R will list all the files recursively
• find . -name src -type d - recursively find in the current folder a file with a name of "src" and the type of "directory"
• find . -path '**/test/*.py' -type f - find in the current folder all the files that are in any /test subfolder located in any subfolder
• find . -mtime -1 finds all the files that have been modified in the last day
• find . -name "*.tmp" -exec rm {} \; - find all files with the .tmp ending execute removal
• fd . -name "*.tmp" - uses regex for looking for files and ignores git files
• 🚨 grep -R foobar . - looks for examples of foobar in all the files in this directory
• ripgrep - same but nicely-formatted and you can get a few lines of context

History 📙

• ctrl + r - reverse search
• history — shows all command history
• history 1 - shows last command
• history 100 | grep ls - shows all commands in the last 100 command history entries that contained "ls"

Misc 🧀

• convert image.{png,jpg} - same as convert image.png image.jpg
• touch foo{,1,2,3} will create 4 files (foo, foo1, etc.)
• touch project{1,2}/main/foo{,1,2} will create 6 files (3 in each path)
• touch {foo,bar}/{a..j} will create a to j files in both dirs
• #!/usr/local/python gives the shell path to execute a command in another language; you could achieve the same with #!/usr/bin/env python (if you don't know where it lives)
• shellcheck is a debugging tool for scripting; it takes an argument of a file

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