Shell-流程控制

一、if

首先看if是一个shell关键字：

[root@node1 shell]# type if
if is a shell keyword
[root@node1 shell]# help if
if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fiExecute commands based on conditional.The `if COMMANDS' list is executed.  If its exit status is zero, then the`then COMMANDS' list is executed.  Otherwise, each `elif COMMANDS' list isexecuted in turn, and if its exit status is zero, the corresponding`then COMMANDS' list is executed and the if command completes.  Otherwise,the `else COMMANDS' list is executed, if present.  The exit status of theentire construct is the exit status of the last command executed, or zeroif no condition tested true.Exit Status:Returns the status of the last command executed.

通过help命令，我们已经清楚了它的语法：if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]… [ else COMMANDS; ] fi，下面就以例子来说明一下：

[root@node1 shell]# if test 3 -gt 2;then echo "ok";fi
ok
[root@node1 shell]# if test 3 -gt 5;then echo "ok";else echo "error"; fi
error
[root@node1 shell]# if test 3 -eq 4;then echo ok;elif test 3 -eq 3;then echo 3;fi
3

二、while

[root@node1 shell]# type while
while is a shell keyword
[root@node1 shell]# help while
while: while COMMANDS; do COMMANDS; doneExecute commands as long as a test succeeds.Expand and execute COMMANDS as long as the final command in the`while' COMMANDS has an exit status of zero.Exit Status:Returns the status of the last command executed.

通过help命令，我们可以了解到while的语法：while COMMANDS; do COMMANDS; done

[root@node1 shell]# while ls /share;do echo ok;rm -rf /share;done
ls: 无法访问/share: 没有那个文件或目录
[root@node1 shell]#

输出1~5：

[root@node1 shell]# while test $a -le 5;do echo $a;((a++));done
1
2
3
4
5

三、for

[root@node1 shell]# type for
for is a shell keyword
[root@node1 shell]# help for
for: for NAME [in WORDS ... ] ; do COMMANDS; doneExecute commands for each member in a list.The `for' loop executes a sequence of commands for each member in alist of items.  If `in WORDS ...;' is not present, then `in "$@"' isassumed.  For each element in WORDS, NAME is set to that element, andthe COMMANDS are executed.Exit Status:Returns the status of the last command executed.
for ((: for (( exp1; exp2; exp3 )); do COMMANDS; doneArithmetic for loop.Equivalent to(( EXP1 ))while (( EXP2 )); doCOMMANDS(( EXP3 ))doneEXP1, EXP2, and EXP3 are arithmetic expressions.  If any expression isomitted, it behaves as if it evaluates to 1.Exit Status:Returns the status of the last command executed.

通过help，我们了解到for的语法为：1.增强for循环：for NAME [in WORDS … ] ; do COMMANDS; done 2.普通for循环：for ((: for (( exp1; exp2; exp3 )); do COMMANDS; done

输出1~5：

[root@node1 shell]# for((a=1;a<=5;a++));do echo $a;done
1
2
3
4
5

[root@node1 shell]# for i in 1 2 3 4 5;do echo $i;((i++));done
1
2
3
4
5

[root@node1 shell]# for i in `seq 5`;do echo $i;((i++));done
1
2
3
4
5

练习题：

用户给定路径
输出文件大小最大的文件
递归子目录

思路指引：du -a /tmp/ |sort -nr(忘记sort命令的可以点此查看！)

#! /bin/bash
oldIFS=$IFS
IFS=$'\n'
for i in `du -a $1 | sort -nr`;doecho $i
done
IFS=$oldIFS

接下来我们要判断file并输出：

#! /bin/bash
oldIFS=$IFS
IFS=$'\n'
for i in `du -a $1 | sort -nr`;dofilename=`echo $i | awk '{print $2}'`if [ -f $filename ];thenecho $filenamebreakfi
done
IFS=$oldIFS

最后来一个案例练习！

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