if 条件测试语句 thenaction
[elif 条件action
elseaction
]
fi

# a=5;if [[ a -gt 4 ]] ;then echo 'ok';fi;
ok

#!/bin/sh
scores=40;
if [[ $scores -gt 90 ]]; thenecho "very good!";
elif [[ $scores -gt 80 ]]; thenecho "good!";
elif [[ $scores -gt 60 ]]; thenecho "pass!";
elseecho "no pass!";
fi;

1.for … in 语句
for 变量 in seq字符串；doaction
done
说明：seq字符串 只要用空格字符分割，每次for…in 读取时候，就会按顺序将读到值，给前面的变量。
实例(testfor.sh)：（结果分行输出1，2，3，4，5，6，7，8，9，10）
#!/bin/sh
for i in $(seq 10); doecho $i;
done;

for i in python-devel readline-develsqlite-devel bzip2-devel openssl-devel ncurses-devel zlib-devel readline sqlite bzip2 openssl libdbi ncurseszlib;doresult=`rpm -qa $i`if [ ! -n "$result" ]thenyum -y install $i&>/dev/nullelseecho "--$i is install--"fi
done

filelist=`ls -l|awk {'print $9'}`
for file in $filelist
doif [ -f $file ]thenecho "echo ${file}"fi
done

2.for((赋值；条件；运算语句))for((赋值；条件；运算语句))；doaction
done;实例(testfor2.sh)：（结果分行输出1，2，3，4，5，6，7，8，9，10）
#!/bin/sh
for((i=1;i<=10;i++));doecho $i;
done;

while语句结构while 条件语句；doaction
done;实例1:
#!/bin/sh
i=10;
while [[ $i -gt 5 ]];doecho $i;((i--));
done;
运行结果：sh testwhile1.sh
10
9
8
7
6
实例2：(循环读取文件内容：)
#!/bin/sh
while read line;doecho $line;
done < /etc/hosts;
运行结果：sh testwhile2.sh
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 centos6 localhost.localdomain localhost

语法结构：
until 条件；doaction
done
意思是：直到满足条件，就退出。否则执行action.实例(testuntil.sh)：
#!/bin/sh
a=10;
until [[ $a -lt 0 ]];doecho $a;((a—));
done;
运行：sh testuntil.sh （分行输出 10 9 8 7 6 5 4 3 2 1 0 中的一个数字）

语法结构
case $arg in  pattern | sample) # arg in pattern or sample  ;;  pattern1) # arg in pattern1  ;;  *) #default  ;;
esac
说明：pattern1 是正则表达式,可以用下面字符：*       任意字串?       任意字元[abc]   a, b, 或c三字元其中之一[a-n]   从a到n的任一字元|       多重选择
实例：
#!/bin/sh
case $1 in
start | begin)echo "start something"  ;;
stop | end)echo "stop something"  ;;
*)echo "Ignorant"  ;;
esac
运行：testcase.sh start
start something

语法：
select 变量name  in seq变量
doaction
done
实例：
#!/bin/sh
select ch in "begin" "end" "exit"
do
case $ch in
"begin")echo "start something"  ;;
"end")echo "stop something"  ;;
"exit")echo "exit"  break;;;
*)echo "Ignorant"  ;;
esac
done;

sh testselect.sh
1) begin
2) end
3) exit
#? begin
Ignorant
#? 1
start something
#? 2
stop something
#? 3
exit 

使用方法：test EXPRESSION，如：
[root@localhost ~]# test 1 = 1 && echo 'ok'
ok
[root@localhost ~]# test -d /etc/ && echo 'ok'
ok
[root@localhost ~]# test 1 -eq 1 && echo 'ok'
ok
[root@localhost ~]# if test 1 = 1 ; then echo 'ok'; fi
ok

[] 表达式[root@localhost ~]# [ 1 -eq 1 ] && echo 'ok'
ok[root@localhost ~]# [ 2 < 1 ] && echo 'ok'
-bash: 2: No such file or directory[root@localhost ~]# [ 2 \< 1 ] && echo 'ok'[root@localhost ~]# [ 2 -gt 1 -a 3 -lt 4 ] && echo 'ok'
ok    [root@localhost ~]# [ 2 -gt 1 && 3 -lt 4 ] && echo 'ok'
-bash: [: missing `]'

[[]] 表达式[root@localhost ~]# [ 1 -eq 1 ] && echo 'ok'
ok
[root@localhost ~]$ [[ 2 < 3 ]] && echo 'ok'
ok
[root@localhost ~]$ [[ 2 < 3 && 4 > 5 ]] && echo 'ok'
ok

# time (for m in {1..100000}; do test -d .;done;)
real    0m0.658s
user    0m0.558s
sys     0m0.100s

# time (for m in {1..100000}; do [ -d . ];done;)
real    0m0.609s
user    0m0.524s
sys     0m0.085s

# time (for m in {1..100000}; do [[ -d . ]];done;)
real    0m0.311s
user    0m0.275s
sys     0m0.036s