Compiled list of questions

This list is compiled mostly from the answers below and some of my own, and is intended to be a quick reference for these questions.

shells

• where to set environmental variables
• How to permanently set environmental variables
• script runs from command line but not cron
• why you must quote - Gilles
• why you must quote - Stephane
• Why the “-” in the “#! /bin/sh -” shebang?
• #!/usr/bin/env bash vs #!/bin/bash
• `...` vs $(...)
• Difference between Login Shell and Non-Login Shell?
• Get exit status of process that's piped to another
• Difference Between: Terminal, Shell, TTY, and Console

sh

• shell arithmetic
• redirecting output
• handling filenames with unusual characters
• How to get the pid of the last executed command in shell script?
• How can I test if a variable is empty or contains only spaces?
• What are the shell's control and redirection operators?
• What is the “eval” command in bash?

bash

• how to show bash history
• How can I test if a variable is empty or contains only spaces?
• How to correctly add a path to PATH?
• In Bash, when to alias, when to script, and when to write a function?
• Why doesn't my Bash script recognize aliases?
• How can I get the size of a file in a bash script?

command line

• du is different than df
• finding the path of an executable
• batch renaming files
• getting system hardware info
• copy and paste on command line
• Delete a file starting with -
• Difference Between: Terminal, Shell, TTY, and Console
• How do I set an environment variable on the command line and have it appear in commands?
• How do I get the size of a directory on the command line?
• How to switch between users on one terminal?
• grep for multiple patterns
• How do I zip/unzip on the unix command line?
• Meaning of --
• How to set default file permissions for all folders/files in a directory?
• find - exec rm vs -delete
• Why is looping over find's output bad practice?

searching filesystems

• locate vs find: usage, pros and cons of each other
• Locating files and displaying their owner
• How to find inode number using “find” command?
• Getting all files that have been modified on a specific date
• List all files containing a specific string
• Script to find files with a specific extension but NOT containing specific text
• How to search for video files on Ubuntu?
• Checking if a file exists in several directories
• Find and move directories based on file type and date
• Shell script for moving oldest files?

text processing

• find and replace
• working with CSV files
• Append lines to file
• How to escape sed
• How can I use sed to replace a multi-line string?
• Why is using a shell loop to process text considered bad practice?
• How to add a newline to the end of a file?

regex

• different types of regex

history

• Is Linux a Unix?
• Where did the “wheel” group get its name?
• What exactly is POSIX?

other

• installing new software on old OS
• fixing/reinstalling grub
• monitoring your server
• understanding op-mode
• passwordless machine access
• installing a custom kernel
• running 32 bit programs on 64 bit systems
• Why was '~' chosen to represent the home directory?