SHELL学习笔记三
for var in list do commands done
$ cat test1 #!/bin/bash # basic for command for test in Alabama Alaska Arizona Arkansas California Colorado do echo The next state is $test done $ ./test1 The next state is Alabama The next state is Alaska The next state is Arizona The next state is Arkansas The next state is California The next state is Colorado $
- 使用转义字符（反斜线）来将单引号转义；
- 使用双引号来定义用到单引号的值。
$ cat test2 #!/bin/bash # another example of how not to use the for command for test in I don\'t know if "this'll" work do echo "word:$test" done $ ./test2 word:I word:don't word:know word:if word:this'll word:work $
$ cat test4 #!/bin/bash # using a variable to hold the list list="Alabama Alaska Arizona Arkansas Colorado" list=$list" Connecticut" for state in $list do echo "Have you ever visited $state?" done $ ./test4 Have you ever visited Alabama? Have you ever visited Alaska? Have you ever visited Arizona? Have you ever visited Arkansas? Have you ever visited Colorado? Have you ever visited Connecticut? $
$ cat test5 #!/bin/bash # reading values from a file file="states" for state in $(cat $file) do echo "Visit beautiful $state" done $ cat states Alabama Alaska Arizona Arkansas Colorado Connecticut Delaware Florida Georgia $ ./test5 Visit beautiful Alabama Visit beautiful Alaska Visit beautiful Arizona Visit beautiful Arkansas Visit beautiful Colorado Visit beautiful Connecticut Visit beautiful Delaware Visit beautiful Florida Visit beautiful Georgia $
造成这个问题的原因是特殊的环境变量 IFS ，叫作内部字段分隔符（internal field separator）。IFS 环境变量定义了bash shell用作字段分隔符的一系列字符。默认情况下，bash shell会将下列字符当作字段分隔符：
- 空格
- 制表符
- 换行符
将这个语句加入到脚本中，告诉bash shell在数据值中忽略空格和制表符。对前一个脚本使用 这种方法，将获得如下输出。 $ cat test5b #!/bin/bash # reading values from a file file="states" IFS=$'\n' for state in $(cat $file) do echo "Visit beautiful $state" done $ ./test5b Visit beautiful Alabama Visit beautiful Alaska Visit beautiful Arizona Visit beautiful Arkansas Visit beautiful Colorado Visit beautiful Connecticut Visit beautiful Delaware Visit beautiful Florida Visit beautiful Georgia Visit beautiful New York Visit beautiful New Hampshire Visit beautiful North Carolina $
警告 在处理代码量较大的脚本时，可能在一个地方需要修改 IFS 的值，然后忽略这次修改，在脚本的其他地方继续沿用 IFS 的默认值。一个可参考的安全实践是在改变 IFS 之前保存原来的 IFS 值，之后再恢复它。 这种技术可以这样实现：
IFS.OLD=$IFS
IFS=$'\n'
<在代码中使用新的IFS值>
IFS=$IFS.OLD
这就保证了在脚本的后续操作中使用的是 IFS 的默认值。
$ cat test6 #!/bin/bash # iterate through all the files in a directory for file in /home/rich/test/* do if [ -d "$file" ] then echo "$file is a directory" elif [ -f "$file" ] then echo "$file is a file" fi done $ ./test6 /home/rich/test/dir1 is a directory /home/rich/test/myprog.c is a file /home/rich/test/myprog is a file /home/rich/test/myscript is a file /home/rich/test/newdir is a directory /home/rich/test/newfile is a file /home/rich/test/newfile2 is a file /home/rich/test/testdir is a directory /home/rich/test/testing is a file /home/rich/test/testprog is a file /home/rich/test/testprog.c is a file $
while test command do other commands done
$ cat test10 #!/bin/bash # while command test var1=10 while [ $var1 -gt 0 ] do echo $var1 var1=$[ $var1 - 1 ] done $ ./test10 10 9 8 7 6 5 4 3 2 1 $
使用多个测试命令
while 命令允许你在 while 语句行定义多个测试命令。只有最后一个测试命令的退出状态码会被用来决定什么时候结束循环。如果你不够小心，可能会导致一些有意思的结果。下面的例子将说明这一点。
$ cat test11 #!/bin/bash # testing a multicommand while loop var1=10 while echo $var1 [ $var1 -ge 0 ] do echo "This is inside the loop" var1=$[ $var1 - 1 ] done $ ./test11 10 This is inside the loop 9 This is inside the loop 8 This is inside the loop 7 This is inside the loop 6 This is inside the loop 5 This is inside the loop 4 This is inside the loop 3 This is inside the loop 2 This is inside the loop 1 This is inside the loop 0 This is inside the loop -1 $
until 命令和 while 命令工作的方式完全相反。 until 命令要求你指定一个通常返回非零退出状态码的测试命令。只有测试命令的退出状态码不为 0 ，bash shell才会执行循环中列出的命令。一旦测试命令返回了退出状态码 0 ，循环就结束了。
until test commands do other commands done
$ cat test12 #!/bin/bash # using the until command var1=100 until [ $var1 -eq 0 ] do echo $var1 var1=$[ $var1 - 25 ] done $ ./test12 100 75 50 25 $
通常必须遍历存储在文件中的数据。这要求结合已经讲过的两种技术：
- 使用嵌套循环
- 修改 IFS 环境变量
通过修改 IFS 环境变量，就能强制 for 命令将文件中的每行都当成单独的一个条目来处理，即便数据中有空格也是如此。一旦从文件中提取出了单独的行，可能需要再次利用循环来提取行中的数据。
典型的例子是处理/etc/passwd文件中的数据。这要求你逐行遍历/etc/passwd文件，并将 IFS变量的值改成冒号，这样就能分隔开每行中的各个数据段了。
#!/bin/bash # changing the IFS value IFS.OLD=$IFS IFS=$'\n' for entry in $(cat /etc/passwd) do echo "Values in $entry –" IFS=: for value in $entry do echo " $value" done done $ Values in rich:x:501:501:Rich Blum:/home/rich:/bin/bash - rich x 501 501 Rich Blum /home/rich /bin/bash Values in katie:x:502:502:Katie Blum:/home/katie:/bin/bash - katie x 506 509 Katie Blum /home/katie /bin/bash
- break 命令
- continue 命令
break 命令接受单个命令行参数值： break n 其中 n 指定了要跳出的循环层级。默认情况下， n 为 1 ，表明跳出的是当前的循环。如果你将n 设为 2 ， break 命令就会停止下一级的外部循环。 $ cat test20 #!/bin/bash # breaking out of an outer loop for (( a = 1; a < 4; a++ )) do echo "Outer loop: $a" for (( b = 1; b < 100; b++ )) do if [ $b -gt 4 ] then break 2 fi echo " Inner loop: $b" done done $ ./test20 Outer loop: 1 Inner loop: 1 Inner loop: 2 Inner loop: 3
continue 命令可以提前中止某次循环中的命令，但并不会完全终止整个循环。可以在循环内部设置shell不执行命令的条件。这里有个在 for 循环中使用 continue 命令的简单例子。
$ cat test21 #!/bin/bash # using the continue command for (( var1 = 1; var1 < 15; var1++ )) do if [ $var1 -gt 5 ] && [ $var1 -lt 10 ] then continue fi echo "Iteration number: $var1" done $ ./test21 Iteration number: 1 Iteration number: 2 Iteration number: 3 Iteration number: 4 Iteration number: 5 Iteration number: 10 Iteration number: 11 Iteration number: 12 Iteration number: 13 Iteration number: 14 $
#shell会将 for 命令的结果重定向到文件output.txt中，而不是显示在屏幕上。 for file in /home/rich/* do if [ -d "$file" ] then echo "$file is a directory" elif echo "$file is a file" fi done > output.txt
