linux/unix interview questions

 1. difference single quote, double quote, back tick

• single quote: preserve literal value
• double quote: variable expansion and command substitution
• back tick: command substitution

2. difference between >, >>, 2>

• >: redirect stdout to file and overwrite
• >>: redirect stdout to file and append
• 2>: redirect stderr to file and overwrite

3. command1 & command2 & wait

• wait for concurrent and background running commands to finish

4. trap 'rm -f /tmp/tempfile' EXIT

• do cleanup when script exit normal/abnormally
• put this line at beginning of script

5. difference source vs exec a script

• source execute in current shell, afterwards variable and function remains
• exec create subshell to execute, afterwards nothing left

6. difference test, [], [[]]

• test and [] are the same 
• [[]] is better