Missing Semester 02 - Shell Tools and Scripting

More on Shell, scripting, variables, functions, arguments and, most importantly, the tools!

Written by Eva Dee on (about a 4 minute read).

Assigning Variables

  • foo=bar

Declaring a variable.

  • echo $foo

Accessing a variable.

❗ Spaces are reserved for separating the arguments (foo = bar is an invalid declaration).

Quotes

For literal strings, double or single quotes are equivalent.

But, variables won't expand inside single quotes (but yes in double-quotes)

  • 'Value is $foo' ⛔
  • "Value is $foo" ✅

Functions

Function example: (mcd - create and cd into that folder)

mcd () {
  mkdir -p "$1"
  cd "$1"
}
  • source [function name]

Execute commands from a file in the current shell.

  • source path/to/file

Evaluate the contents of a given file.

Special Variables

  • $0

The name of the script we are running.

  • $1

Access the 1st argument.

  • $#

The number of arguments we are giving to the command (length).

  • %_

The value of the last argument.

  • !!

Run the last command again (often used with sudo).

  • $?

Read the value of standard error. (false produces error code 1).

  • $$

Process id.

  • $@

Expands to all the arguments (spread).

  • ;

To concacenate different commands in the same line.

Short-circuit Operators

  • false || echo "Oops, fail" (prints Oops, fail)

  • true || echo "Will not be printed" (prints nothing)

  • true && echo "Things went well" (prints Things went well)

  • false && echo "Will not be printed" (prints nothing)

  • true ; echo "This will always run" (prints This will always run)

  • false ; echo "This will always run" (prints This will always run)

Process Substitution

Process substitution feeds the output of a process (or processes) into the stdin of another process.

cat <(ls) <(ls ..)

<( CMD ) will execute CMD and place the output in a temporary file and substitute the <() with that file’s name.

Test

Evaluate condition.

If it is true, returns 0 exit status, otherwise returns 1.

  • test -z \$GIT_BRANCH

Test if given variable is empty.

  • test -e filename

Test if file exists.

Globbing

Globbing or filename expansion: Filename expansion means expanding filename patterns or templates containing special characters.

For example, example.??? might expand to example.001 and/or example.txt.

  • touch foo{1,2,10}

  • touch {foo,bar}/{a..j}

Shell Functions vs Scripts

  • Functions have to be in the same language as the shell, while scripts can be written in any language. This is why including a shebang for scripts is important.

  • Functions are loaded once when their definition is read. Scripts are loaded every time they are executed.

  • Functions are executed in the current shell environment whereas scripts execute in their own process

Misc

  • shebang

The first line in the script that will inform the shell how to run this program.

  • foo=$(pwd)

Saves the output of the pwd command in the foo variable: echo "We are in $(pwd)

  • /dev/null

If you redirect output there, it will be discarded (this is so for tidying up your output)

  • ctrl + r

Backward search (press ctrl + r again to go through the results)

Shell Tools

  • brew info [tool]

👍 To look it up on Homebrew.

  • shellcheck (For bash script debugging)

  • imageMagick (Tools and libraries to manipulate images in many formats)

  • ffmpeg (Play, record, convert, and stream audio and video)

  • find

    • find . -name src -type d

    • find . -mtime -1 (modification time in the last day )

    • find . -naame "\*.tmp" -exec rm {} \;

  • fd (Simple, fast and user-friendly alternative to find)

  • ripgrep (For more colorful grep.)

    • rg -C 5 emoji src (Get 5 lines of context.)

    • history | grep convert

  • fzf (Type fzf to search over all your files.)

  • tree (Display directories as trees (with optional color/HTML output))

  • broot (New way to see and navigate directory trees)