lesson03.md

Lesson 3 - Command Line Part II

Overview of today's lesson

Intermediate commands and skills

• Bash variables and commands
• Executing bash scripts and dot-files
• Parsing files
• Working with input and output
• Wildcards
• Brace expansion
• Control keys

Variables, bash scripts, and dot-files

Bash variables and commands

• A=0 — assign a variable
• echo VARIABLE — output the value of a variable or expression
• $PATH — your path variable (try: echo -e ${PATH//:/'\n'})
• $SHELL — your current shell
• $PPID — process ID
• $HOME — your home directory (another name for ~ or /Users/you)

Loops and logic

• for/do/done — for loop
• if/then/else/fi — if/then statement

for i in {1..3}
do
    echo $i
done

if [ -d "temp" ]; then
    echo "Directory exists."
else 
    mkdir temp
fi

Executing bash scripts and dot-files

• bash SCRIPT.sh — run a bash shell script
• source .DOTFILE — run a dot-file like your .bash_profile

Any of the commands we have used from the command line (the bash prompt $) can also be typed into a text file and executed by typing bash SCRIPT.sh from the command line.

More commands

We will test some of these commands with the example file survey_scores_2015_T.csv.

Parsing and sorting files

• cut -d "," -f 5- FILE — output 5th field through end using comma field delimiter
• sed 's/FIND/REPLACE/g' FILE — replace text in a file
• perl -e 's/FIND/REPLACE/g' FILE — run perl commands in the command line (advanced)
• sort FILE - sort a file alphabetically
• uniq FILE - get the unique lines in a sorted file

Working with input and output

• | — pipe output from one command to another (e.g., sort | uniq | wc)
• > — redirect (write) to file
• < — get output of file (other type of redirect)
• `COMMAND` — pass output of a command (e.g., in a for loop)

Wildcards and brace expansion

• ? — match any single character
• * — match any string of characters
• [set] — match any character in set
• [!set] — match any character not in set
• {start..end} — expand a range; e.g., b{ed,olt,ar}s, {2..5}, {d..h}

Control keys

Various keyboard shortcuts help you quickly and powerfully control the terminal. This section is adapted from The Best Keyboard Shortcuts for Bash (aka the Linux and macOS Terminal) by Lowell Heddings.

Make sure you know which is the Alt or Meta key on your computer. On a Mac, you can do this by setting your Terminal preferences to "Use Option as Meta key".

Enabling command-line editing

The instructions below assume you're using the default keyboard shortcut configuration in bash. By default, bash uses emacs-style keys. If you’re more used to the vi text editor, you can switch to vi-style keyboard shortcuts.

The following command will put bash into vi mode:

set -o vi

The following command will put bash back into the default emacs mode:

set -o emacs

Working with processes

• Ctrl+C — Interrupt (kill) the current foreground process running in in the terminal. This sends the SIGINT signal to the process, which is technically just a request—most processes will honor it, but some may ignore it.
• Ctrl+Z — Suspend the current foreground process running in bash. This sends the SIGTSTP signal to the process. Type bg to send the process to the background (similar to typing & at the end of the command). To return the process to the foreground later, use the fg process_name command.
• Ctrl+D — Close the bash shell. This sends an EOF (End-of-file) marker to bash, and bash exits when it receives this marker. This is similar to running the exit command.

Controlling the screen

• Ctrl+L — Clear the screen. This is similar to running the clear command.
• Ctrl+S — Stop all output to the screen. This is particularly useful when running commands with a lot of long, verbose output, but you don’t want to stop the command itself with Ctrl+C.
• Ctrl+Q — Resume output to the screen after stopping it with Ctrl+S.

Moving the cursor

• Ctrl+A or Home — Go to the beginning of the line.
• Ctrl+E or End — Go to the end of the line.
• Alt+B — Go left (back) one word.
• Ctrl+B — Go left (back) one character.
• Alt+F — Go right (forward) one word.
• Ctrl+F — Go right (forward) one character.
• Ctrl+XX — Move between the beginning of the line and the current position of the cursor. This allows you to press Ctrl+XX to return to the start of the line, change something, and then press Ctrl+XX to go back to your original cursor position. To use this shortcut, hold the Ctrl key and tap the X key twice.

Deleting Text

• Ctrl+D or Delete — Delete the character under the cursor.
• Alt+D — Delete all characters after the cursor on the current line.
• Ctrl+H or Backspace — Delete the character before the cursor.

Fixing Typos

• Alt+T — Swap the current word with the previous word.
• Ctrl+T — Swap the last two characters before the cursor with each other. You can use this to quickly fix typos when you type two characters in the wrong order.
• Ctrl+_ — Undo your last key press. You can repeat this to undo multiple times.

Cutting and pasting

• Ctrl+W — Cut the word before the cursor, adding it to the clipboard.
• Ctrl+K — Cut the part of the line after the cursor, adding it to the clipboard.
• Ctrl+U — Cut the part of the line before the cursor, adding it to the clipboard.
• Ctrl+Y — Paste the last thing you cut from the clipboard. The y here stands for “yank”.

Capitalizing characters

• Alt+U — Capitalize every character from the cursor to the end of the current word, converting the characters to upper case.
• Alt+L — Uncapitalize every character from the cursor to the end of the current word, converting the characters to lower case.
• Alt+C — Capitalize the character under the cursor. Your cursor will move to the end of the current word.

Tab completion

Tab completion is a very useful bash feature. While typing a file, directory, or command name, press Tab and bash will automatically complete what you’re typing, if possible. If not, bash will show you various possible matches and you can continue typing and pressing Tab to finish typing.

• Tab — Automatically complete the file, directory, or command you’re typing.

Command history

• Ctrl+P or Up-Arrow — Go to the previous command in the command history.
• Ctrl+N or Dn-Arrow — Go to the next command in the command history.
• Alt+R — Revert any changes to a command pulled from your history.