Expansions and escaping

Pathname expansion

• The mechanism by which wildcards work is called pathname expansion.
• Examples:
  • echo *
  • echo *.md
  • echo [[:upper:]]*
  • echo test/*/sample.txt

Tilde expansion

• Tilde (~) expands into the path of the users home directory.
• You can also expand a specific users home directory using ~username
• Examples:
  • echo ~
  • echo ~heisenberg

Arithmetic expansion

• This can be used to perform arithmetic operations through expansions.
• Usage: &((expression))
• Examples:
  • echo $((2 + 2))
  • echo $(((3 ** 3 + 3) / 10))
  • echo The remainder when 10 is divided by 3 is $((10 % 3))

Brace expansion

• This is used to text strings using a sequence or pattern.
• Usage: {range or pattern}
• Examples:
  • echo Front-{A,B,C}-Back
  • echo {1..5}
  • echo {01..09}
  • echo {A..Z}
  • echo {Z..A}
  • echo a{A{1,2},B{3,4}}b
  • mkdir logFile-{01..12}-{2010..2020}
  • touch logFile-{01..12}-{2010..2020}/log.txt

Parameter expansion

• It is used to expand system variables.
• Example:
  • echo $USER

Command substitution

• This allows us to use the output of a command as an expansion.
• Usage: $(command)
• Examples:
  • ls -l $(which cp)
  • file $(ls -d /usr/bin/* | grep zip)
• Command substitution has another syntax, it uses backquotes instead of dollar sign and parantheses.
• Usage: `command`
• Examples:
  • ls -l `which cp`

Double quotes

• Allows you to use a string as it is, without any special characters.
• Remember, parameter expansion, arithmetic expansion, and command substitution still take place within double quotes.
• Examples:
  • ls -l "two words.txt"
  • echo "$USER $((2+2)) $(cal)"
  • echo $(cal) will give shitty output. Use echo "$(cal)" instead.

Single quotes

• If we need to suppress all expansions, we use single quotes.
• Example:
  • echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER'

Escaping characters

• \ is used to escape any character that can have a special meaning.
• Examples:
  • echo "The balance for user $USER is: \$5.00"
  • mv bad\ filename.txt goodFilename.txt

Backslash escape sequences

• In addition to its role as the escape character, the backslash is used as part of a notation to represent certain special characters called control codes.
• Escape sequences:
  • \a - Bell
  • \b - Backspace
  • \n - Newline
  • \r - Carriage return
  • \t - Tab
• Examples:
  • sleep 10; echo -e "Time's up\a
  • sleep 10; echo "Time's up" $'\a'