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if 语句格式| if then else fi |
| If语句忘了结尾fi test.sh: line 14: syntax error: unexpected end of fi |
| if command then if then | 执行失败，返回非0 （grep,没找到匹配） |
| if [ expression_r_r_r then | |
| if test expression_r_r_r then | |
| [ -f "/etc/shadow" ] && echo "This computer uses shadow passwors" |
| |
| | |
| 0为真，走then | |
| 必须:if [ i –ne 0 ] 但支持字符串变量直接if if [ str ] 如果字符串非0 | if (i ) |
=================================以command作为if 条件===================================
| echo –n “input:” read user if 多条指令,这些命令之间相当于“and”（与） grep $user /etc/passwd >/tmp/null who -u | grep $user then else fi |
| # sh test.sh input : macg macg macg has logged # sh test.sh input : ddd ddd has not logged |
| if 以函数作为if条件， getyn then echo " your answer is yes" else echo "your anser is no" fi |
| $ vi testsh.sh #!/bin/sh if cat 111-tmp.txt | grep ting1 then echo found else echo "no found" fi | #!/bin/sh cat 111-tmp.txt | grep ting1 if [ $? -eq 0 ] then echo $? echo found else echo $? echo "no found" fi |
| $ sh testsh.sh no found | $ sh testsh.sh 1 no found |
| $ vi 111-tmp.txt that is 222file thisting1 is 111file $ sh testsh.sh thisting1 is 111file found | $ vi 111-tmp.txt that is 222file thisting1 is 111file $ sh testsh.sh thisting1 is 111file 0 found |
========================================以条件表达式作为 if条件=============================
if [ 条件表达式 ]
then
else
fi
- 文件表达式
if [ -d ...
if [ -s file
if [ -r file
if [ -w file
if [ -x file
- 整数变量表达式
if [ int1 -ne int2 ]
if [ int1 -ge int2 ]
if [ int1 -gt int2 ]
if [ int1 -le int2 ]
if [ int1 -lt int2 ]
-
字符串变量表达式
if
if
if
if
| if [ a = b ] ;then echo equal else echo no equal fi |
| [macg@machome ~]$ sh test.sh input a: 5 input b: 5 no equal |
| if [ $a = $b ] ;then echo equal else echo no equal fi |
| [macg@machome ~]$ sh test.sh input a: 5 input b: 5 equal |
| [macg@machome ~]$ vi test.sh echo -n "input your choice:" read var if then echo $var fi [macg@machome ~]$ sh -x test.sh input your choice: y test.sh: line 3: test: y: integer expression_r_r_r expected |
| [macg@machome ~]$ vi test.sh echo "input a:" read a echo "input is $a" if [ $a = 123 ] ; then echo equal123 fi |
| [macg@machome ~]$ sh test.sh input a: 123 input is 123 equal123 |
等号也是操作符，必须和其他变量，关键字，用空格格开 (等号做赋值号时正好相反，两边不能有空格）
| [macg@machome ~]$ vi test.sh echo "input your choice:" read var if [ $var="yes" ] then echo $var echo "input is correct" else echo $var echo "input error" fi | [macg@machome ~]$ vi test.sh echo "input your choice:" read var if [ $var = "yes" ] then echo $var echo "input is correct" else echo $var echo "input error" fi |
| [macg@machome ~]$ sh test.sh input your choice: y y input is correct [macg@machome ~]$ sh test.sh input your choice: n n input is correct 输错了也走then,都走then,为什么? 因为if把$var="yes"连读成一个变量，而此变量为空，返回1，则走else | input your choice: y y input error [macg@machome ~]$ sh test.sh input your choice: no no input error 一切正常 |
| echo "input your choice:" read ANS if [ $ANS ] then echo no empty else echo empth fi |
| [macg@machome ~]$ sh test.sh input your choice: empth [macg@machome ~]$ sh test.sh input your choice: 34 no empty |
| [macg@machome ~]$ vi test.sh echo "input a:" read a if echo 3bit else echo 2bit fi |
| [macg@machome ~]$ sh test.sh input a: 123 3bit [macg@machome ~]$ sh test.sh input a: 20 2bit |
| if [macg@machome ~]$ sh test.sh test.sh: line 4: test: ge: binary operator expected |
| if [macg@machome ~]$ sh test.sh input a: 123 3bit |
============================逻辑表达式=========================================
if [ ! 表达式 ]
if [ ! -d $num ]
if [ 表达式1
if [ 表达式1
-
表达式与前面的= != -d –f –x -ne -eq -lt等合用
-
逻辑符号就正常的接其他表达式，没有任何括号（ ），就是并列
-
注意逻辑与-a与逻辑或-o很容易和其他字符串或文件的运算符号搞混了
左边测变量是否为空，右边测目录(值)是否存在（值是否有效）
| |
| [macg@mac-home ~]$ vi test.sh : echo "input the num:" read num echo "input is $num" if [ -z "$JHHOME" -a -d $HOME/$num ] then JHHOME=$HOME/$num fi echo "JHHOME is $JHHOME" |
| ----------------------- [macg@mac-home ~]$ sh test.sh input the num: ppp input is ppp JHHOME is 目录-d $HOME/$num |
| [macg@mac-home ~]$ mkdir ppp [macg@mac-home ~]$ sh test.sh input the num: ppp input is ppp JHHOME is /home/macg/ppp |
| echo "input your choice:" read ANS if [ $ANS="Yes" -o $ANS="yes" -o $ANS="y" -o $ANS="Y" ] then ANS="y" else ANS="n" fi echo $ANS |
| [macg@machome ~]$ sh test.sh input your choice: n y [macg@machome ~]$ sh test.sh input your choice: no y 为什么输入不是yes,结果仍是y(走then） 因为=被连读了，成了变量$ANS="Yes"，而变量又为空，所以走else了 |
| [macg@machome ~]$ vi test.sh echo "input your choice:" read ANS read ANS if [ $ANS = "Yes" -o $ANS = "yes" -o $ANS = "y" -o $ANS = "Y" ] then ANS="y" else ANS="n" fi echo $ANS |
| [macg@machome ~]$ sh test.sh input your choice: no n [macg@machome ~]$ sh test.sh input your choice: yes y [macg@machome ~]$ sh test.sh input your choice: y y |
===================以
if test $num -eq 0
then
echo "try again"
else
echo "good"
fi
| [macg@machome ~]$ man test [(1) SYNOPSIS |