Linux shell編程——if條件判斷

時間 2019-11-17

標籤 linux shell 編程條件判斷欄目 Linux 简体版

原文原文鏈接

if 語句格式
web

if 條件
then
Command
else
Command
fi 別忘了這個結尾

If語句忘告終尾fi
test.sh: line 14: syntax error: unexpected end of fi

if 的三種條件表達式

if command then if 函數 then	命令執行成功，等於返回0 （好比grep ,找到匹配）執行失敗，返回非0 （grep,沒找到匹配）
if [ expression_r_r_r ] then	表達式結果爲真，則返回0，if把0值引向then
if test expression_r_r_r then	表達式結果爲假，則返回非0，if把非0值引向then

[ ] && ——快捷if

[ -f "/etc/shadow" ] && echo "This computer uses shadow passwors"

&& 能夠理解爲then
若是左邊的表達式爲真則執行右邊的語句

shell的if與c語言if的功能上的區別

shell if	c語言if
0爲真，走then	正好相反，非0走then
不支持整數變量直接if 必須:if [ i –ne 0 ] 但支持字符串變量直接if if [ str ] 若是字符串非0	支持變量直接if if (i )

=================================以command做爲if 條件===================================

以多條command或者函數做爲if 條件

echo –n 「input:」
read user

if
多條指令,這些命令之間至關於「and」（與）
grep $user /etc/passwd >/tmp/null
who -u | grep $user
then             上邊的指令都執行成功,返回值$?爲0，0爲真，運行then
echo "$user has logged"
else     指令執行失敗，$?爲1，運行else
echo "$user has not logged"
fi

# sh test.sh
input : macg
macg     pts/0        May 15 15:55   .          2075 (192.168.1.100)
macg has logged

# sh test.sh
input : ddd
ddd has not logged

以函數做爲if條件 (函數就至關於command,函數的優勢是其return值能夠自定義)

if
以函數做爲if條件，
getyn
then   函數reture值0爲真，走then
echo " your answer is yes"
else  函數return值非0爲假，走else
echo "your anser is no"
fi

if command 等價於 command+if $?

$ vi testsh.sh #!/bin/sh if cat 111-tmp.txt \| grep ting1 then echo found else echo "no found" fi	$ vi testsh.sh #!/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

========================================以條件表達式做爲 if條件=============================

傳統if 從句子——以條件表達式做爲 if條件
if [ 條件表達式 ]
then
command
command
command
else
command
command
fi

條件表達式

文件表達式

if [ -f  file ]    若是文件存在
if [ -d ...   ]    若是目錄存在
if [ -s file  ]    若是文件存在且非空
if [ -r file  ]    若是文件存在且可讀
if [ -w file  ]    若是文件存在且可寫
if [ -x file  ]    若是文件存在且可執行
shell

整數變量表達式

if [ int1 -eq int2 ]    若是int1等於int2
if [ int1 -ne int2 ]    若是不等於
if [ int1 -ge int2 ]       若是>=
if [ int1 -gt int2 ]       若是>
if [ int1 -le int2 ]       若是<=
if [ int1 -lt int2 ]       若是<

express

字符串變量表達式

If  [ $a = $b ]                 若是string1等於string2
                                字符串容許使用賦值號作等號
if  [ $string1 !=  $string2 ]   若是string1不等於string2
if  [ -n $string  ]             若是string 非空(非0），返回0(true)
if  [ -z $string  ]             若是string 爲空
if  [ $sting ]                  若是string 非空，返回0 (和-n相似)

    條件表達式引用變量要帶$
less

if [ a = b ] ;then
echo equal
else
echo no equal
fi

[macg@machome ~]$ sh test.sh
input a:
5
input b:
5
no equal （等於表達式沒比較$a和$b,而是比較和a和b,天然a!=b)

改正：

if [ $a = $b ] ;then
echo equal
else
echo no equal
fi

[macg@machome ~]$ sh test.sh
input a:
5
input b:
5
equal

-eq -ne -lt -nt只能用於整數，不適用於字符串，字符串等於用賦值號=

[macg@machome ~]$ vi test.sh
echo -n "input your choice:"
read var
if [ $var -eq "yes" ]
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
指望整數形式，即-eq不支持字符串

=放在別的地方是賦值,放在if [ ] 裏就是字符串等於,shell裏面沒有==的,那是c語言的等於

無空格的字符串，能夠加" ",也能夠不加

[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

= 做爲等於時，其兩邊都必須加空格，不然失效
等號也是操做符，必須和其餘變量，關鍵字，用空格格開 (等號作賦值號時正好相反，兩邊不能有空格）

[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" ] 在等號兩邊加空格 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 輸錯了也走then,都走then,爲何? 由於if把$var="yes"連讀成一個變量，而此變量爲空，返回1，則走else	[macg@machome ~]$ sh test.sh input your choice: y y input error [macg@machome ~]$ sh test.sh input your choice: no no input error 一切正常

If [ $ANS ] 等價於 if [ -n $ANS ]
若是字符串變量非空（then） , 空(else)

echo "input your choice:"
read ANS

if [ $ANS ]
then
echo no empty
else
echo empth
fi

[macg@machome ~]$ sh test.sh
input your choice:                       回車

empth                                    說明「回車」就是空串
[macg@machome ~]$ sh test.sh
input your choice:
34
no empty

整數條件表達式，大於，小於，s hell裏沒有> 和< ,會被看成尖括號，只有-ge,-gt,-le,lt

[macg@machome ~]$ vi test.sh

echo "input a:"
read a
if [ $a -ge 100 ] ; then
echo 3bit
else
echo 2bit
fi

[macg@machome ~]$ sh test.sh
input a:
123
3bit
[macg@machome ~]$ sh test.sh
input a:
20
2bit

整數操做符號-ge,-gt,-le,-lt, 別忘了加-

if test $a ge 100 ; then

[macg@machome ~]$ sh test.sh
test.sh: line 4: test: ge: binary operator expected

if test $a -ge 100 ; then

[macg@machome ~]$ sh test.sh
input a:
123
3bit

============================邏輯表達式=========================================

    邏輯非 !                   條件表達式的相反
if [ ! 表達式 ]
if [ ! -d $num ]    若是不存在目錄$num

    邏輯與 –a 條件表達式的並列
if [ 表達式1 –a 表達式2 ]

邏輯或 -o           條件表達式的或
if [ 表達式1 –o 表達式2 ]

   邏輯表達式

表達式與前面的= != -d –f –x -ne -eq -lt等合用

邏輯符號就正常的接其餘表達式，沒有任何括號（），就是並列

if [ -z "$JHHOME" -a -d $HOME/$num ]
socket

注意邏輯與-a與邏輯或-o很容易和其餘字符串或文件的運算符號搞混了

最多見的賦值形式，賦值前對=兩邊的變量都進行評測
左邊測變量是否爲空，右邊測目錄(值)是否存在（值是否有效）
函數

[macg@mac-home ~]$ vi test.sh
:
echo "input the num:"
read num
echo "input is $num"

if [ -z "$JHHOME" -a -d $HOME/$num ]   若是變量$JHHOME爲空，且$HOME/$num目錄存在
then
JHHOME=$HOME/$num                      則賦值
fi

echo "JHHOME is $JHHOME"

-----------------------
[macg@mac-home ~]$ sh test.sh
input the num:
ppp
input is ppp
JHHOME is

目錄-d $HOME/$num 不存在，因此$JHHOME沒被then賦值

[macg@mac-home ~]$ mkdir ppp
[macg@mac-home ~]$ sh test.sh
input the num:
ppp
input is ppp
JHHOME is /home/macg/ppp

一個-o的例子，其中卻揭示了」=」必須兩邊留空格的問題

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
爲何輸入不是yes,結果還是y(走then）
由於=被連讀了，成了變量$ANS="Yes"，而變量又爲空，因此走else了

[macg@machome ~]$ vi test.sh

echo "input your choice:"
read ANS 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:
no
n
[macg@machome ~]$ sh test.sh
input your choice:
yes
y
[macg@machome ~]$ sh test.sh
input your choice:
y
y

===================以 test 條件表達式做爲if條件===================================

if test $num -eq 0      等價於   if [ $num –eq 0 ]

  test  表達式,沒有 [  ]
if test $num -eq 0
then
echo "try again"
else
echo "good"
fi

    man test

[macg@machome ~]$ man test
[(1)                             User Commands                            [(1)

SYNOPSIS
       test EXPRESSION
       [ EXPRESSION ]

       [-n] STRING
              the length of STRING is nonzero          -n和直接$str都是非0條件

       -z STRING
              the length of STRING is zero

       STRING1 = STRING2
              the strings are equal

       STRING1 != STRING2
              the strings are not equal

       INTEGER1 -eq INTEGER2
              INTEGER1 is equal to INTEGER2

       INTEGER1 -ge INTEGER2
              INTEGER1 is greater than or equal to INTEGER2

       INTEGER1 -gt INTEGER2
              INTEGER1 is greater than INTEGER2

       INTEGER1 -le INTEGER2
              INTEGER1 is less than or equal to INTEGER2

       INTEGER1 -lt INTEGER2
              INTEGER1 is less than INTEGER2

       INTEGER1 -ne INTEGER2
              INTEGER1 is not equal to INTEGER2

       FILE1 -nt FILE2
              FILE1 is newer (modification date) than FILE2

       FILE1 -ot FILE2
              FILE1 is older than FILE2

       -b FILE
              FILE exists and is block special

       -c FILE
              FILE exists and is character special

       -d FILE
              FILE exists and is a directory

       -e FILE
              FILE exists                                 文件存在

       -f FILE
              FILE exists and is a regular file     文件存在且是普通文件

       -h FILE
              FILE exists and is a symbolic link (same as -L)

       -L FILE
              FILE exists and is a symbolic link (same as -h)

       -G FILE
              FILE exists and is owned by the effective group ID

       -O FILE
              FILE exists and is owned by the effective user ID

       -p FILE
              FILE exists and is a named pipe

       -s FILE
              FILE exists and has a size greater than zero

       -S FILE
              FILE exists and is a socket

       -w FILE
              FILE exists and is writable

       -x FILE
FILE exists and is executable

======================if簡化語句=================================

最經常使用的簡化if語句

&& 若是是「前面」，則「後面」
[ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid 檢查文件是否存在，若是存在就刪掉

|| 若是不是「前面」，則後面
[ -f /usr/sbin/dhcpd ] || exit 0 檢驗文件是否存在，若是存在就退出

用簡化 if 和$1,$2,$3來檢測參數，不合理就調用help
[ -z "$1" ] && help 若是第一個參數不存在（-z 字符串長度爲0 ）
[ "$1" = "-h" ] && help 若是第一個參數是-h,就顯示help

例子
#!/bin/sh

[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0
cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0

[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1
cp ifcfg-eth1.bridge /etc/sysconfig/network-scripts/ifcfg-eth1

[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0:1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0:1