for循环有2种常用语法格式

for i in `ls /`;do
    echo $i;
done

for((i=1;i<10;i++));do
    echo $i;
done

while循环有一种常用语法

while read line;do
    echo $line
done < /etc/passwd

case 语句有一种常见用法

echo -n "type your machine:"
read i
case $i in
unix)
echo "your machine is unix";
;;
linux)
echo "echo your machine is linux";
;;
*)
echo "your machine is other";
esac

---

Bash shell的各种循环

首先确定该shell为Bash：

# echo $SHELL

• C语法格式的for循环

for((i=0; i<10; i++));do
       echo $i
done

• 使用seq命令

for i in $(seq 10);do
       echo $i
done

• while-do格式的循环

i=1
while(($i<10));do
       echo $i
       i=`expr $i + 1`
done

• for…i…in格式的循环

for i in {1..10};do
       echo $i
done

for i in {01..10};do
       echo $i
done

for i in {a..z};do
       echo $i
done

• for格式的另一种循环（命令执行结果/手动指定）

for i in $(ls);do
       echo $i
done

for i in `ls`;do
       echo $i
done

for i in f1 f2 f3;do
       echo $i
done

• cat和while...read格式的循环

cat /tmp/drops.txt | while read url title
do
    title=$(echo $title | tr ' </' '_')
    curl -s $url | sed -n '/<div id="content">/,/entry-tags/p' >/tmp/tmp.html
    cat /tmp/top.html /tmp/tmp.html /tmp/bottom.html >/tmp/$title.html
done

while read url title
do
    title=$(echo $title | tr ' </' '_')
    curl -s $url | sed -n '/<div id="content">/,/entry-tags/p' >/tmp/tmp.html
    cat /tmp/top.html /tmp/tmp.html /tmp/bottom.html >/tmp/$title.html
done < /tmp/drops.txt

• 嵌套循环

dir=(papers tips tools news web pentesting database binary)
dir_num=${#dir[@]}
for((i=0;i<$dir_num;i++))
do
       for j in $(seq 1 15)
       do
       if [ "$j" == 1 ]
       then
              echo http://drops.wooyun.org/category/${dir[i]}
       else
              echo http://drops.wooyun.org/category/${dir[i]}/page/$j
       fi
       done
done

arr=("a" "b" "c")
echo "arr is (${arr[@]})"

arr=(a b c)
for i in ${arr[@]}
do
       echo $i
done

echo "参数 $* 表示脚本输入的所有参数："$*
for i in $* ; do
       echo $i
done

Bash在循环读取文件内容时跳过空行/注释行

搜索关键字：linux bash skip blank comment line

参考解答：

• http://www.linuxquestions.org/questions/linux-newbie-8/how-can-i-cat-or-grep-a-file-to-ignore-lines-starting-with-654683/
• http://stackoverflow.com/questions/11267015/how-to-ignore-blank-lines-and-comment-lines-using-awk
• http://stackoverflow.com/questions/17392869/how-to-print-a-file-excluding-comments-and-blank-lines-using-grep-sed
• http://stackoverflow.com/questions/114814/count-non-blank-lines-of-code-in-bash

cat filename | grep -v '^#' | grep -v '^$'
cat filename | grep -Ev '^(#|$)'
cat filename | grep -Ev '(^#|^\s*$)'
cat filename | grep -v '^$\|^\s*\#'

awk '!/^ *#/ && NF' filename

cat filename | sed '/^\s*#/d;/^\s*$/d'

=EOF=