Shell (八)

测试

测试结构

一个if/then结构测试一列命令的退出状态是否为0（因为依照惯例，0意味着命令执行成功），如果是0则会执行一个或多个命令。
有一个命令** [** ([左方括]是特殊字符). 它和test是同义词,因为效率的原因，它被内建在shell里。这个命令的参数是比较表达式或者文件测试，它会返回一个退出状态指示比较的结果(0表示真，1表示假)。
[[]] 同样是测试命令，是一个比较关键字。
if 命令不仅能测试由方括号括起来的条件，也能测试任何命令。

root@ubuntu:/home/newer# if cmp 1.txt keepme &>/dev/null; then echo "same"; else echo "diffrent";fi
diffrent

root@ubuntu:/home/newer# if grep -r dog 1.txt &>/dev/null; then echo "dog"; fi
dog

root@ubuntu:/home/newer# if [$abc] &>/dev/null; then echo "uninitialized variable is true"; else echo "uninitailized variable is fause"; fi
uninitailized variable is fause

#!/bin/bash
 
 #  小技巧:
 #  如果你不确定某一条件怎么被求值,
 #+ 可以用一个if-test结构来测试.
 
 echo
 
 echo "Testing \"0\""
 if [ 0 ]      # 0
 then
   echo "0 is true."
 else
   echo "0 is false."
 fi            # 0为真.
 
 echo
 
 echo "Testing \"1\""
 if [ 1 ]      # 1
 then
   echo "1 is true."
 else
   echo "1 is false."
 fi            # 1为真.
 
 echo
 
 echo "Testing \"-1\""
 if [ -1 ]     # -1
 then
   echo "-1 is true."
 else
   echo "-1 is false."
 fi            # -1为真.
 
 echo
 
 echo "Testing \"NULL\""
 if [ ]        # NULL (空条件)
 then
   echo "NULL is true."
 else
   echo "NULL is false."
 fi            # NULL为假.
 
 echo
 
 echo "Testing \"xyz\""
 if [ xyz ]    # 字符串
 then
   echo "Random string is true."
 else
   echo "Random string is false."
 fi            # 任意字符串为true.
 
 echo
 
 echo "Testing \"\$xyz\""
 if [ $xyz ]   # 变量$xyz为null值, 但...
               # 它只是一个未初始化的变量.
 then
   echo "Uninitialized variable is true."
 else
   echo "Uninitialized variable is false."
 fi            # 未初始化的变量为false.
 
 echo
 
 echo "Testing \"-n \$xyz\""
 if [ -n "$xyz" ]            # 进一步实验核实.
 then
   echo "Uninitialized variable is true."
 else
   echo "Uninitialized variable is false."
 fi            # 未始初化的变量为false.
 
 echo
 
 
 xyz=          # 已初始化, 但设置成null值.
 
 echo "Testing \"-n \$xyz\""
 if [ -n "$xyz" ]
 then
   echo "Null variable is true."
 else
   echo "Null variable is false."
 fi            # Null值变量为假.
 
 
 echo
 
 
 # 什么时候"false"为真?
 
 echo "Testing \"false\""
 if [ "false" ]              #  "false"是一个字符串.
 then
   echo "\"false\" is true." #+ 它被测试为真.
 else
   echo "\"false\" is false."
 fi            # "false"为真.
 
 echo
 
 echo "Testing \"\$false\""  # 再来，未初始化的变量.
 if [ "$false" ]
 then
   echo "\"\$false\" is true."
 else
   echo "\"\$false\" is false."
 fi            # "$false"变量为假.
               # 现在, 我们取得了预期的效果.
 
 #  如果我们测试未初始化的变量"$true"会发生什么?
 
 echo
 
 exit 0

结构

if [ condition1 ]
then
   command1
   command2
   command3
elif [ condition2 ]
# 和else if相同
then
   command4
   command5
else
   default-command
fi

等价的测试

#!/bin/bash

echo

if test -z "$1"
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

if /usr/bin/test -z "$1"      # 和内建的"test"命令一样.
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

if [ -z "$1" ]                # 和上面代码块的功能一样
#   if [ -z "$1"                应该来说会运行, 但是...
#+  Bash给出错误说少了一个封闭的右方括.
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo


if /usr/bin/[ -z "$1" ]       # 同样和上面的代码块一样.
# if /usr/bin/[ -z "$1"       # 工作, 但还是给出一个错误信息.
#                             # 注意:
#                               这个已经在bash 3.x版本被修补好了。
then
  echo "No command-line arguments."
else
  echo "First command-line argument is $1."
fi

echo

exit 0

Shell (八)

测试

测试结构

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