shell編程——if語(yǔ)句 if -z -n -f -eq -ne -lt

2019-11-11 01:03:09

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shell編程中條件表達(dá)式的使用

if 條件then Commandelse Commandfi 別忘了這個(gè)結(jié)尾

If語(yǔ)句忘了結(jié)尾fitest.sh: line 14: syntax error: unexpected end of fi

if 的三種條件表達(dá)式

ifcommandthenif 函數(shù)then	命令執(zhí)行成功，等于返回0 （比如grep ,找到匹配）執(zhí)行失敗，返回非0 （grep,沒(méi)找到匹配）
if [ exPRession_r_r_r ]then	表達(dá)式結(jié)果為真，則返回0，if把0值引向then
if test expression_r_r_rthen	表達(dá)式結(jié)果為假，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"

&& 可以理解為then 如果左邊的表達(dá)式為真則執(zhí)行右邊的語(yǔ)句

shell的if與c語(yǔ)言if的功能上的區(qū)別

shell if	c語(yǔ)言if
0為真，走then	正好相反，非0走then
不支持整數(shù)變量直接if必須:if [ i –ne 0 ]但支持字符串變量直接ifif [ str ] 如果字符串非0	支持變量直接ifif (i )

=================================以command作為if 條件=================================== 以多條command或者函數(shù)作為if 條件

echo –n “input:”read userif多條指令,這些命令之間相當(dāng)于“and”（與）grep $user /etc/passwd >/tmp/null who -u | grep $userthen 上邊的指令都執(zhí)行成功,返回值$?為0，0為真，運(yùn)行then echo "$user has logged"else 指令執(zhí)行失敗，$?為1，運(yùn)行else echo "$user has not logged"fi

# sh test.shinput : macgmacg pts/0 May 15 15:55 . 2075 (192.168.1.100)macg has logged # sh test.shinput : dddddd has not logged

以函數(shù)作為if條件 (函數(shù)就相當(dāng)于command,函數(shù)的優(yōu)點(diǎn)是其return值可以自定義)

if以函數(shù)作為if條件，getynthen 函數(shù)reture值0為真，走thenecho " your answer is yes"else 函數(shù)return值非0為假，走elseecho "your anser is no"fi

if command 等價(jià)于 command+if $?

$ vi testsh.sh#!/bin/shifcat 111-tmp.txt \| grep ting1thenecho foundelseecho "no found"fi	$ vi testsh.sh#!/bin/shcat 111-tmp.txt \| grep ting1if [ $? -eq 0 ]thenecho $?echo foundelseecho $?echo "no found"fi
$ sh testsh.shno found	$ sh testsh.sh1no found
$ vi 111-tmp.txtthat is 222filethisting1 is 111file$ sh testsh.shthisting1 is 111filefound	$ vi 111-tmp.txtthat is 222filethisting1 is 111file$ sh testsh.shthisting1 is 111file0found

========================================以條件表達(dá)式作為 if條件============================= 傳統(tǒng)if 從句子——以條件表達(dá)式作為 if條件if [ 條件表達(dá)式 ]then command command commandelse command commandfi 條件表達(dá)式

文件表達(dá)式

if [ -f file ] 如果文件存在if [ -d ... ] 如果目錄存在if [ -s file ] 如果文件存在且非空 if [ -r file ] 如果文件存在且可讀if [ -w file ] 如果文件存在且可寫if [ -x file ] 如果文件存在且可執(zhí)行

整數(shù)變量表達(dá)式

if [ int1 -eq int2 ] 如果int1等于int2 if [ int1 -ne int2 ] 如果不等于 if [ int1 -ge int2 ] 如果>=if [ int1 -gt int2 ] 如果>if [ int1 -le int2 ] 如果<=if [ int1 -lt int2 ] 如果<

字符串變量表達(dá)式

If [ $a = $b ] 如果string1等于string2 字符串允許使用賦值號(hào)做等號(hào)if [ $string1 != $string2 ] 如果string1不等于string2 if [ -n $string ] 如果string 非空(非0），返回0(true) if [ -z $string ] 如果string 為空if [ $sting ] 如果string 非空，返回0 (和-n類似) 條件表達(dá)式引用變量要帶$

if [ a = b ] ;then echo equalelseecho no equalfi

[macg@machome ~]$ sh test.shinput a:5input b:5no equal （等于表達(dá)式?jīng)]比較$a和$b,而是比較和a和b,自然a!=b)

改正：

if [ $a = $b ] ;then echo equalelseecho no equalfi

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

-eq -ne -lt -nt只能用于整數(shù)，不適用于字符串，字符串等于用賦值號(hào)=

[macg@machome ~]$ vi test.shecho -n "input your choice:"read varif [ $var -eq "yes" ]thenecho $varfi[macg@machome ~]$ sh -x test.shinput your choice:ytest.sh: line 3: test: y: integer expression_r_r_r expected 期望整數(shù)形式，即-eq不支持字符串

=放在別的地方是賦值,放在if [ ] 里就是字符串等于,shell里面沒(méi)有==的,那是c語(yǔ)言的等于無(wú)空格的字符串，可以加" ",也可以不加

[macg@machome ~]$ vi test.shecho "input a:"read aecho "input is $a"if [ $a = 123 ] ; thenecho equal123fi

[macg@machome ~]$ sh test.shinput a:123input is 123equal123

= 作為等于時(shí)，其兩邊都必須加空格，否則失效等號(hào)也是操作符，必須和其他變量，關(guān)鍵字，用空格格開 (等號(hào)做賦值號(hào)時(shí)正好相反，兩邊不能有空格）

[macg@machome ~]$ vi test.shecho "input your choice:"read varif [ $var="yes" ]thenecho $varecho "input is correct"elseecho $varecho "input error"fi	[macg@machome ~]$ vi test.shecho "input your choice:"read varif [ $var = "yes" ] 在等號(hào)兩邊加空格thenecho $varecho "input is correct"elseecho $varecho "input error"fi
[macg@machome ~]$ sh test.shinput your choice:yyinput is correct[macg@machome ~]$ sh test.shinput your choice:n ninput is correct 輸錯(cuò)了也走then,都走then,為什么?因?yàn)閕f把$var="yes"連讀成一個(gè)變量，而此變量為空，返回1，則走else	[macg@machome ~]$ sh test.shinput your choice:yyinput error[macg@machome ~]$ sh test.shinput your choice:no noinput error一切正常

If [ $ANS ] 等價(jià)于 if [ -n $ANS ] 如果字符串變量非空（then） , 空(else)

echo "input your choice:"read ANSif [ $ANS ]thenecho no emptyelseecho empthfi

[macg@machome ~]$ sh test.shinput your choice: 回車 empth 說(shuō)明“回車”就是空串[macg@machome ~]$ sh test.shinput your choice:34no empty

整數(shù)條件表達(dá)式，大于，小于，shell里沒(méi)有> 和< ,會(huì)被當(dāng)作尖括號(hào)，只有-ge,-gt,-le,lt

[macg@machome ~]$ vi test.shecho "input a:"read aif [ $a -ge 100 ] ; thenecho 3bitelseecho 2bitfi

[macg@machome ~]$ sh test.shinput a:1233bit[macg@machome ~]$ sh test.shinput a:202bit

整數(shù)操作符號(hào)-ge,-gt,-le,-lt, 別忘了加-

if test $a ge 100 ; then[macg@machome ~]$ sh test.shtest.sh: line 4: test: ge: binary Operator expected

if test $a -ge 100 ; then[macg@machome ~]$ sh test.shinput a:1233bit

============================邏輯表達(dá)式========================================= 邏輯非 ! 條件表達(dá)式的相反if [ ! 表達(dá)式 ]if [ ! -d $num ] 如果不存在目錄$num 邏輯與 –a 條件表達(dá)式的并列if [ 表達(dá)式1 –a 表達(dá)式2 ] 邏輯或 -o 條件表達(dá)式的或if [ 表達(dá)式1 –o 表達(dá)式2 ] 邏輯表達(dá)式

表達(dá)式與前面的= != -d –f –x -ne -eq -lt等合用邏輯符號(hào)就正常的接其他表達(dá)式，沒(méi)有任何括號(hào)（），就是并列

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

注意邏輯與-a與邏輯或-o很容易和其他字符串或文件的運(yùn)算符號(hào)搞混了

最常見(jiàn)的賦值形式，賦值前對(duì)=兩邊的變量都進(jìn)行評(píng)測(cè)左邊測(cè)變量是否為空，右邊測(cè)目錄(值)是否存在（值是否有效）

[macg@mac-home ~]$ vi test.sh:echo "input the num:"read numecho "input is $num"if [ -z "$JHHOME" -a -d $HOME/$num ] 如果變量$JHHOME為空，且$HOME/$num目錄存在thenJHHOME=$HOME/$num 則賦值fiecho "JHHOME is $JHHOME"

-----------------------[macg@mac-home ~]$ sh test.shinput the num:pppinput is pppJHHOME is目錄-d $HOME/$num 不存在，所以$JHHOME沒(méi)被then賦值

[macg@mac-home ~]$ mkdir ppp[macg@mac-home ~]$ sh test.shinput the num:pppinput is pppJHHOME is /home/macg/ppp

一個(gè)-o的例子，其中卻揭示了”=”必須兩邊留空格的問(wèn)題

echo "input your choice:"read ANSif [ $ANS="Yes" -o $ANS="yes" -o $ANS="y" -o $ANS="Y" ]thenANS="y"elseANS="n"fiecho $ANS

[macg@machome ~]$ sh test.shinput your choice:ny[macg@machome ~]$ sh test.shinput your choice:noy為什么輸入不是yes,結(jié)果仍是y(走then）因?yàn)?被連讀了，成了變量$ANS="Yes"，而變量又為空，所以走else了

[macg@machome ~]$ vi test.shecho "input your choice:"read ANS echo "input your choice:"read ANSif [ $ANS = "Yes" -o $ANS = "yes" -o $ANS = "y" -o $ANS = "Y" ]thenANS="y"elseANS="n"fiecho $ANS

[macg@machome ~]$ sh test.shinput your choice:non[macg@machome ~]$ sh test.shinput your choice:yesy[macg@machome ~]$ sh test.shinput your choice:yy

===================以 test 條件表達(dá)式作為if條件=================================== if test $num -eq 0 等價(jià)于 if [ $num –eq 0 ] test 表達(dá)式,沒(méi)有 [ ]if test $num -eq 0 thenecho "try again"elseecho "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 FILEFILE exists and is executable

======================if簡(jiǎn)化語(yǔ)句================================= 最常用的簡(jiǎn)化if語(yǔ)句

&& 如果是“前面”，則“后面”[ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid 檢查文件是否存在，如果存在就刪掉

|| 如果不是“前面”，則后面[ -f /usr/sbin/dhcpd ] || exit 0 檢驗(yàn)文件是否存在，如果存在就退出

用簡(jiǎn)化 if 和$1,$2,$3來(lái)檢測(cè)參數(shù)，不合理就調(diào)用help[ -z "$1" ] && help 如果第一個(gè)參數(shù)不存在（-z 字符串長(zhǎng)度為0 ）[ "$1" = "-h" ] && help 如果第一個(gè)參數(shù)是-h,就顯示help例子#!/bin/sh[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1cp 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

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