Shell Scripting

Notes on shell scripting, as well as some zsh- and bash-specific stuff.

Notes

• zsh has two representations for array variables like PATH and CDPATH, of which the lower case variant is an actual array

• ~ is a special character. Therefore, if you use it in a string variable and then try to do something with it, ~ will be evaluated literally, and not expanded to the value of $HOME. So use $HOME in these cases instead

• Functions can be declared with/without function keyword and parens

  • my_function (), function my_function, etc

• To pass arguments to a function, just use spaces

  • my_function "hello" "world"

  • If passing variables, sometimes it's good practice to put them in quotes in case the variables have spaces in them

• The export keyword is only necessary when sub-processes need access to the variable

  • So if you don't use export, when you execute programs they won't have access to the variable. But the resulting shell will

• To do string transformation on strings in zsh upon expansion, use expansion flags

• To rename the current directory: mv ../current_dir ../new_dir_name or mv ../{test,test2} (uses brace expansion for brevity)

• Assigning test/[[]] results to a variable (tl;dr: there's not a great way to do this). This would be nice for code readability

• Can't assign variable to command exit status directly

  • But can get around it using $?:

    • (grep foo bar.txt); var1=$?

  • bash - Assigning exit code to a shell local variable - Unix & Linux Stack Exchange

• macOS has a weird quirk where filenames with a : show a / for them instead

  • Can get around this if you need to by using this guy instead: ꞉ -- it's a different character but looks pretty similar

• For conditionals, 0 is truthy and 1 is falsey

• For multline string usage, use HereDoc

• For stripping trailing characters, use parameter expansion %: ${var%/}, (stripping /)

Links

• Lowercasing and Uppercasing strings in bash/zsh

• Bash scripting cheatsheet

• Assigning default values to shell variables with a single command in bash - Stack Overflow

• zsh - string substitutions: all occurrences - Unix & Linux Stack Exchange

• A Quick Primer on Bash Booleans (StackOverflow)

• bash - How to change the output color of echo in Linux - Stack Overflow

• How to exit bash function if error? - Stack Overflow

• Shell Tools and Scripting · the missing semester of your cs education