Classic Thirteen Questions about Shell Scripts You Should Know

• #Classic Thirteen Questions about Shell

• 1. Why is it called shell?

• 2 . What is the relationship between Shell prompt (PS1) and Carriage Return (CR)?

• 3. When someone else echoes, you also echo. How much do you know about echo?

• 4. What is the difference between "" (double quotation mark) and (single quotation mark)?

• 5. var=value? What is the difference between export and export?

• 6. What is the difference between exec and source?

• 7. What is the difference between ( ) and { }?

• 8. () and ${}. What’s the difference?

• 9. and *?

• 10. What is the difference between && and ||?

• 11. What is the difference between > and 0c4af0acd46919dd0ff35ca00a788b6e 与 c01b8772d0f6397290d6d9199e3ce834 来改变送出的数据信道(stdout, stderr)，使之输出到指定的档案。

  ls my.file no.such.file 1> file.out 2>file.err
  # 2>&1 就是将stderr并进stdout做输出
  ls my.file no.such.file 1> file.out 2>&1
  # /dev/null 空
  ls my.file no.such.file >/dev/null 2>&1
  
  cat < file > file
  # 在 IO Redirection 中，stdout 与 stderr 的管道会先准备好，才会从 stdin 读进资料。 
  # 也就是说，在上例中，> file 会先将 file 清空，然后才读进 < file ， 
  # 但这时候档案已经被清空了，因此就变成读不进任何数据了

  12. 你要if还是case呢？

  # if
  echo -n "Do you want to continue?(Yes/No):"
  read YN
  if [ "$YN"=Y -o "$YN"=y -o "$YN"="Yes" -o "$YN"="yes" -o "$YN"="YES"];then
  echo "continue"
  else
  exit 0
  fi
  
  # case
  echo -n "Do you want to continue?(Yes/No):"
  read YN
  case "$YN" in
  [Yy]|[Yy][Ee][Ss])
  echo "continue"
  ;;
  *)
  exit 0
  esac

  13. for what? while与until差在哪？

  # for
  for ((i=1;i<=10;i++))
  do
   echo "num is $i"
  done
  
  # while
  num=1
  while [ "$num" -le 10 ]; do
   echo "num is $num"
   num=$(($num + 1))
  done
  
  # until
  num=1
  until [ "$num" -gt 10 ]; do
   echo "num is $num"
   num=$(($nu + 1))
  done

  • break 是结束 loop
  • return 是结束 function
  • exit 是结束 script/shell

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