shell程式設計——if語句 if -z -n -f -eq -ne -lt
轉載於:https://loongembedded.blog.csdn.net/article/details/52678998 感謝作者。
shell程式設計——if語句 if -z -n -f -eq -ne -lt
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shell程式設計中條件表示式的使用
if條件 then Command else Command fi別忘了這個結尾 |
If語句忘了結尾fi test.sh: line 14: syntax error:unexpected end of fi |
if 的三種條件表示式
if command then if 函式 then |
命令執行成功,等於返回0 (比如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 |
表示式結果為真,則返回0,if把0值引向then |
if test expression_r_r_r then |
表示式結果為假,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 如果左邊的表示式為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為真,走then | 正好相反,非0走then |
不支援整數變數直接if 必須:if [ i –ne 0 ] 但支援字串變數直接if if [ str ] 如果字串非0 |
支援變數直接if if (i ) |
=================================以command作為if 條件===================================
以多條command或者函式作為if 條件
echo –n “input:” read user if 多條指令,這些命令之間相當於“and”(與) grep $user /etc/passwd >/tmp/null who -u | grep $user then上邊的指令都執行成功,返回值$?為0,0為真,執行then echo "$user has logged" else指令執行失敗,$?為1,執行else echo "$user has not logged" fi |
# sh test.sh input : macg macgpts/0May 15 15:55.2075 (192.168.1.100) macg has logged # sh test.sh input : ddd ddd has not logged |
以函式作為if條件(函式就相當於command,函式的優點是其return值可以自定義)
if 以函式作為if條件, getyn then函式reture值0為真,走then echo " your answer is yes" else函式return值非0為假,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 |
========================================以條件表示式作為 if條件=============================
傳統if 從句子——以條件表示式作為 if條件
if [ 條件表示式 ]
then
command
command
command
else
command
command
fi
條件表示式
- 檔案表示式
if [ -ffile ]如果檔案存在
if [ -d ...]如果目錄存在
if [ -s file]如果檔案存在且非空
if [ -r file]如果檔案存在且可讀
if [ -w file] 如果檔案存在且可寫
if [ -x file] 如果檔案存在且可執行
- 整數變量表達式
if [ int1 -eq int2 ]如果int1等於int2
if [ int1 -ne int2 ]如果不等於
if [ int1 -ge int2 ]如果>=
if [ int1 -gt int2 ]如果>
if [ int1 -le int2 ]如果<=
if [ int1 -lt int2 ]如果<
- 字串變量表達式
If[ $a = $b ]如果string1等於string2
字串允許使用賦值號做等號
if[ $string1 !=$string2 ]如果string1不等於string2
if[ -n $string]如果string 非空(非0),返回0(true)
if[ -z $string]如果string 為空
if[ $sting ]如果string 非空,返回0 (和-n類似)
條件表示式引用變數要帶$
if [ a = b ] ;then echo equal else echo no equal fi |
[macg@machome ~]$ sh test.sh input a: 5 input b: 5 no equal(等於表示式沒比較$a和$b,而是比較和a和b,自然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,不適用於字串,字串等於用賦值號=
[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 期望整數形式,即-eq不支援字串 |
=放在別的地方是賦值,放在if [ ] 裡就是字串等於,shell裡面沒有==的,那是c語言的等於
無空格的字串,可以加" ",也可以不加
[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 |
= 作為等於時,其兩邊都必須加空格,否則失效
等號也是操作符,必須和其他變數,關鍵字,用空格格開 (等號做賦值號時正好相反,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,為什麼? 因為if把$var="yes"連讀成一個變數,而此變數為空,返回1,則走else |
[macg@machome ~]$ 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 ]
如果字串變數非空(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 |
整數條件表示式,大於,小於,shell裡沒有> 和< ,會被當作尖括號,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 |
整數操作符號-ge,-gt,-le,-lt, 別忘了加-
iftest $age100 ; then [macg@machome ~]$ sh test.sh test.sh: line 4: test: ge: binary operator expected |
iftest $a -ge 100 ; then [macg@machome ~]$ sh test.sh input a: 123 3bit |
============================邏輯表示式=========================================
邏輯非 !條件表示式的相反
if [ ! 表示式 ]
if [ ! -d $num ]如果不存在目錄$num
邏輯與 –a條件表示式的並列
if [ 表示式1–a表示式2 ]
邏輯或 -o條件表示式的或
if [ 表示式1–o 表示式2 ]
邏輯表示式
- 表示式與前面的=!= -d –f –x -ne -eq -lt等合用
- 邏輯符號就正常的接其他表示式,沒有任何括號( ),就是並列
if [ -z "$JHHOME" -a -d $HOME/$num ]
- 注意邏輯與-a與邏輯或-o很容易和其他字串或檔案的運算子號搞混了
最常見的賦值形式,賦值前對=兩邊的變數都進行評測
左邊測變數是否為空,右邊測目錄(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 ]如果變數$JHHOME為空,且$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,所以$JHHOME沒被then賦值 |
[macg@mac-home ~]$ 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 為什麼輸入不是yes,結果仍是y(走then) 因為=被連讀了,成了變數$ANS="Yes",而變數又為空,所以走else了 |
[macg@machome ~]$ vi test.sh echo "input your choice:" read ANSecho "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 條件表示式 作為if條件===================================
if test $num -eq 0等價於if [ $num –eq 0 ]
test表示式,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檔案存在 -f FILE FILE exists and is a regular file檔案存在且是普通檔案 -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=================================
最常用的簡化if語句
&& 如果是“前面”,則“後面” [ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid檢查 檔案是否存在,如果存在就刪掉 |
||如果不是“前面”,則後面 [ -f /usr/sbin/dhcpd ] || exit 0檢驗檔案是否存在,如果存在就退出 |
用簡化 if 和$1,$2,$3來檢測引數,不合理就呼叫help
[ -z "$1" ] && help如果第一個引數不存在(-z字串長度為0 )
[ "$1" = "-h" ] && help如果第一個引數是-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