Variable assignment and manipulation

CHECK IF VARIABLE IS SET:

${variable:-word} - If variable is not set, returns word, otherwise returns variable.

• $ variable=bob
• $ echo ${variable:-not set}
• bob
• $ unset variable
• $ echo ${variable:-not set}
• not set

${variable:=word} - If variable set, returns variable. If not set, assigns word to variable, then returns variable.

• $ variable=bob
• $ echo ${variable:=not set}
• bob
• $ unset variable
• $ echo ${variable:=not set}
• not set
• $ echo ${variable}
• not set

${variable:+word} - Returns word if variable isn't set. If variable is set, returns nothing

• $ variable=bob
• $ echo ${variable:+Variable already set}
• Variable already set
• $ unset variable
• $ echo ${variable:+Variable already set}
•  <this is the command returning nothing>

${variable:?word} - Return word if variable isn't set, and then terminate the shell with error.

• $ variable=bob
• $ echo ${variable:?word}
• bob
• $ unset variable
• $ echo ${variable:?word}
• bash: variable: word

${variable:?} - Return default error message if variable isn't set, and then terminate the shell with error.

• $ variable=bob
• $ echo ${variable:?word}
• bob
• $ unset variable
• $ echo ${variable:?word}
• bash: variable: parameter null or not set

 VARIABLE MANIPULATION

${variable:startnum} - Display variable starting at character startnum

• $ variable=1234567890
• $ echo ${variable:4}
• 567890

${variable:startnum:length} - Display variable starting at character startnum, for total of length characters.

• $ variable=1234567890
• $ echo ${variable:4:2}
• 56

${variable#pattern} - Finds the first and shortest instance of pattern, and removes it.

• $ variable="landing ground on island"
• $ echo ${variable#land*}
• ing ground on island

${variable##pattern} - Finds the first and longest instance of pattern, and removes it.

• $ variable="landing ground on island"
• $ echo ${variable##land*}
•  <this is the command returning nothing>

${variable%pattern} - Finds the last and shortest instance of pattern, and removes it.

• $ variable="landing ground on island"
• $ echo ${variable%*land}
• landing ground on is

${variable%%pattern} - Finds the last and longest instance of pattern, and removes it.

• $ variable="landing ground on island"
• $ echo ${variable%%*land}
•  <this is the command returning nothing>

${variable/pattern1/pattern2} - Replaces first instance of pattern1 with pattern2.

• $ variable=1234554321
• $ echo ${variable/1/00}
• 00234554321

${variable//pattern1/pattern2} - Replaces all instances of pattern1 with pattern2.

• $ variable=1234554321
• $ echo ${variable//1/00}
• 002345543200

${variable#pattern1/pattern2} - Replace pattern1 with pattern2 if variable begins with pattern1

${variable%pattern1/pattern2} - Replace pattern1 with pattern2 if variable ends with pattern1