7.3. 其它比较操作符

   1 #!/bin/bash
   2 
   3 a=4
   4 b=5
   5 
   6 #  这儿的"a"和"b"既能被看成是整数也能被看着是字符串。
   7 #  在字符串和数字之间不是严格界定的，
   8 #+ 因为Bash变量不是强类型的。
   9 
  10 #  Bash允许变量的整数操作和比较，
  11 #+ 但这些变量必须只包括数字字符.
  12 #  不管怎么样，你应该小心考虑.
  13 
  14 echo
  15 
  16 if [ "$a" -ne "$b" ]
  17 then
  18   echo "$a is not equal to $b"
  19   echo "(arithmetic comparison)"
  20 fi
  21 
  22 echo
  23 
  24 if [ "$a" != "$b" ]
  25 then
  26   echo "$a is not equal to $b."
  27   echo "(string comparison)"
  28   #     "4"  != "5"
  29   # ASCII 52 != ASCII 53
  30 fi
  31 
  32 # 在这个实际的例子中,"-ne"和"!="都能工作.
  33 
  34 echo
  35 
  36 exit 0

   1 #!/bin/bash
   2 #  str-test.sh: 测试null字符串和没有用引号引起的字符串,
   3 #+ but not strings and sealing wax, not to mention cabbages and kings . . .
   4 
   5 # 用if [ ... ]结构
   6 
   7 
   8 # 如果一个字符串变量没有被初始化，它里面的值未定义。
   9 # 这种情况称为"null"值(不同于零).
  10 
  11 if [ -n $string1 ]    # $string1没有被声明或初始化.
  12 then
  13   echo "String \"string1\" is not null."
  14 else  
  15   echo "String \"string1\" is null."
  16 fi  
  17 # 错误结果.
  18 # Shows $string1 as not null, although it was not initialized.
  19 
  20 
  21 echo
  22 
  23 
  24 # 让我们再试试.
  25 
  26 if [ -n "$string1" ]  # 这次, $string1变量被引号引起.
  27 then
  28   echo "String \"string1\" is not null."
  29 else  
  30   echo "String \"string1\" is null."
  31 fi                    # 在一个测试方括里用引号引起变量!
  32 
  33 
  34 echo
  35 
  36 
  37 if [ $string1 ]       # 这次,变量$string1什么也不加.
  38 then
  39   echo "String \"string1\" is not null."
  40 else  
  41   echo "String \"string1\" is null."
  42 fi  
  43 # This works fine.
  44 # The [ ] test operator alone detects whether the string is null.
  45 # However it is good practice to quote it ("$string1").
  46 #
  47 # As Stephane Chazelas points out,
  48 #    if [ $string1 ]    has one argument, "]"
  49 #    if [ "$string1" ]  has two arguments, the empty "$string1" and "]" 
  50 
  51 
  52 
  53 echo
  54 
  55 
  56 
  57 string1=initialized
  58 
  59 if [ $string1 ]       # Again, $string1 stands naked.
  60 then
  61   echo "String \"string1\" is not null."
  62 else  
  63   echo "String \"string1\" is null."
  64 fi  
  65 # Again, gives correct result.
  66 # Still, it is better to quote it ("$string1"), because . . .
  67 
  68 
  69 string1="a = b"
  70 
  71 if [ $string1 ]       # Again, $string1 stands naked.
  72 then
  73   echo "String \"string1\" is not null."
  74 else  
  75   echo "String \"string1\" is null."
  76 fi  
  77 # Not quoting "$string1" now gives wrong result!
  78 
  79 exit 0
  80 # Thank you, also, Florian Wisser, for the "heads-up".

   1 #!/bin/bash
   2 # zmore
   3 
   4 #用'more'查看被压缩过的文件
   5 
   6 NOARGS=65
   7 NOTFOUND=66
   8 NOTGZIP=67
   9 
  10 if [ $# -eq 0 ] # 等同于:  if [ -z "$1" ]
  11 # $1可以存在,但为空:  zmore "" arg2 arg3
  12 then
  13   echo "Usage: `basename $0` filename" >&2
  14   # 错误信息打印到标准错误.
  15   exit $NOARGS
  16   # 返回65作为脚本的退出状态(错误码).
  17 fi  
  18 
  19 filename=$1
  20 
  21 if [ ! -f "$filename" ]   # Quoting $filename allows for possible spaces.
  22 then
  23   echo "File $filename not found!" >&2
  24   # 错误信息打印到标准错误.
  25   exit $NOTFOUND
  26 fi  
  27 
  28 if [ ${filename##*.} != "gz" ]
  29 # 在方括号里使用变量替换.
  30 then
  31   echo "File $1 is not a gzipped file!"
  32   exit $NOTGZIP
  33 fi  
  34 
  35 zcat $1 | more
  36 
  37 # 用过滤程序'more.'
  38 # 如果你想，也可以用'less'代替。
  39 
  40 
  41 exit $?   # 脚本返回管道的退出状态
  42 # 实际上"exit $?"是多余的,因为在任何情况下，
  43 # 脚本会返回最后一个执行命令的退出码。