结构化命令
if-then语句
if command
then
command
fi
不同于其他编程语言在if之后的对象是一个等式，这个等式结果为TRUE或FALSE,bash shell的if语句会运行if后面的命令，如果该命令退出状态码为0,则运行then如果不为0则then后不执行
例：
#!/bin/bash
#testing the if statement
if pwd
then
echo "It worked"
fi
例：
#!/bin/bash
#testing a bad command
if IamNotaCommand
then
echo "It worked"
fi
echo "We are outside the if statement"
例：
#!/bin/bash
#testing multiple commands in the then section
testuser=keynes
if grep $testuser /etc/passwd
then
echo "This is my first command"
echo "This is my second command"
echo "I can even put in other commands besides echo"
ls -a /home/$testuser/.b*
fi
设置一个不存在的账户
例：
#!/bin/bash
#testing multiple commands in the then section
testuser=NoSuchUser
if grep $testuser /etc/passwd
then
echo "This is my first command"
echo "This is my second command"
echo "I can even put in other commands besides echo"
ls -a /home/$testuser/.b*
fi
if-then-else语句
if command
then
commands
else
commands
fi
例：
#!/bin/bash
#testing the els section
testuser=NoSuchUser
if grep $testuser /etc/passwd
then
echo "The bash fiels for user $testuser are:"
ls -a /home/$testuser/.b*
echo
else
echo "The user $testuser does not exist on this system."
echo
fi
嵌套if
例：
#!/bin/bash
#Testing nested ifs
testuser=NoSuchUser
if grep $testuser /etc/passwd
then
echo "The user $testuser exists on this system"
else
echo "The user $testuser does not exist on this system"
if ls -d /home/$testuser/
then
echo "However,$testuser has a directory"
fi
test命令
test命令提供了if-then语句测试,如果test命令中列出条件成立,test命令便退出并返回状态码0,否则返回非零状态码,使得if-then语句不会再被执行
例：
if test condition
then
commands
fi
bash shell 提供了另一种条件测试方法,无需声明test,
if [ condition ]
then
commands
fi
例：
#!/bin/bash
#Testing the test command
if test
then
echo "No expression returns a True"
else
echo "No expression returns a False"
fi
例：
#!/bin/bash
#Testing the test command
my_variable="Full"
if test $my_variable
then
echo "The $my_variable expression returns a True"
else
echo "The $my_variable expression returns a False"
fi
例：
#!/bin/bash
#Testing the test command
my_variable=""
if test $my_variable
then
echo "The $my_variable expression returns a True"
else
echo "The $mys_variable expression returns a False"
fi
test命令的数值比较功能
数值比较
| 比较 | 描述 |
|---|
| n1 -eq n2 | 检查n1是否与n2相等 |
| n1 -ge n2 | 检查n1是否大于或等于n2 |
| n1 -gt n2 | 检查n1是否大于n2 |
| n1 -le n2 | 检查n1是否小于等于n2 |
| n1 -lt n2 | 检查n1是否小于n2 |
| n1 -ne n2 | 检查n1是否不等于n2 |
例：
#!/bin/bash
#Using numeric test evaluations
value1=10
value2=11
if [ $value1 -gt 5 ]
then
echo "The test value $value1 is greater than 5"
fi
if [ $value1 -eq $value2 ]
then
echo "The values are equal"
else
echo "The values are different"
fi
注:变量只可以存储浮点值
字符串比较
| 比较 | 描述 |
|---|
| str1 = str2 | 检查str1是否和str2相同 |
| str1 !=str2 | 检查str1是否和str2不同 |
| str1 < str2 | 检查str1是否比str2小 |
| str1 > str2 | 检查str1是否比str2大 |
| -n str1 | 检查str1的长度是否非零 |
| -z str1 | 检查str1的长度是否为0 |
字符串相等性
例：
#!/bin/bash
# testing string equality
testuser=keynes
if [ $USER=$testuser ]
then
echo "Welcome $testuser"
fi
例：
#!/bin/bash
# testing string equality
testuser=baduser
if [ $USER !=$testuser ]
then
echo "This is not $testuser"
else
echo "Welcome $testuser"
fi
字符串顺序
例：
#!/bin/bash
#mis-using string comparisons
val1=baseball
val2=hockey
if [ $var1 \>$val2 ]
then
echo "$val1 is greater then $val2"
else
echo "$varl1 is less then $val2"
fi
字符串大小
-n 判断变量字符串长度非0
-z 判断变量字符串长度为0
例：
#!/bin/bash
#testing string length
val1=testing
val2=''
if [ -n $val1 ]
then
echo "The string '$val1' is not empty"
else
echo "The string '$val1' is empty"
fi
if [ -z $val2 ]
then
echo "The string '$val2' is empty"
else
echo "The string '$val2' is not empty"
fi
if [ -z $val3 ]
then
echo "The string '$var3' is empty"
else
echo "The string '$var3' is not empty"
fi
文件比较
| 比较 | 描述 |
|---|
| -d file | 检查file是否存在且是个目录 |
| -e file | 检查file是否存在 |
| -f file | 检查file是否存在且是个文件 |
| -r file | 检查file是否存在且可读 |
| -s file | 检查file是否存在且非空 |
| -w file | 检查file是否存在且可写 |
| -x file | 检查file是否存在且可执行 |
| -O file | 检查file是否存在且属当前用户所有 |
| -G file | 检查file是否存在且默认组与当前用户相同 |
| file1 -nt file2 | 检查file1是否比file2新 |
| file1 -ot file2 | 检查file1是否比file2旧 |
检查目录
例：
#!/bin/bash
#Look before you leap
jump_directory=/home/keynes
if [ -d $jump_directory ]
then
echo "The $jump_directory directory exists"
cd $jump_directory
ls
else
echo "The $jump_directory directory does not exist"
fi
检查对象是否存在
例：
#!/bin/bash
# Check if either a directory or file exists
location=$HOME
file_name="dd"
if [ -e $location ]
then
echo "OK on the filename"
echo "Now checking on the file,$file_name"
if [ -e $location/$file_name ]
then
echo "OK on the filename"
echo "Updateing Current Date..."
date >> $location/$file_name
else
echo "File does not exist"
echo "Nothing to update"
fi
else
echo "The %location directory does not exist"
echo "Nothing to update"
fi
检查文件
例：
#!/bin/bash
#Check if either a directory or file exist
item_name=$HOME
echo
echo "The item being checked:$item_name"
echo
if [ -e $item_name ]
then
echo "The item,$item_name,does exist."
echo "But is it a file?"
echo
if [ -f $item_name ]
then
echo "Yes,$item_name is a file"
else
echo "No,$item_name is not a file"
fi
else
echo "The item,$item_name,does not exist."
echo "Nothing to update"
fi
检查是否可读
例：
#!/bin/bash
#testing if you can read a file
pwfile=/etc/shadow
if [ -f $pwfile ]
then
if [ -r $pwfile ]
then
tail $pwfile
else
echo "Sorry,I am unable to read the $pwfile file"
fi
else
echo "Sorry,the file $pwfile does not exist"
fi
检查空文件
例：
#!/bin/bash
#Testing if a file is empty
file_name=$HOME/sentinel
if [ -f $file_name ]
then
if [ -s $file_name ]
then
echo "The $file_name file exists and has data in it"
echo "Will not remove this file."
else
echo "The $file_name file exists,but is empty"
echo "Deleting empty file..."
rm $file_name
fi
else
echo "File,$file_name does not exist."
fi
检查是否可写
例：
#!/bin/bash
#Check if a file is writable
item_name=$HOME/sentinel
echo
echo "The item being checked:$item_name"
echo
if [ -f $item_name ]
then
echo "Yes,$item_name is a file."
echo "But is it writable?"
echo
if [ -w $item_name ]
then
echo "Writing current time to $item_name"
date +%H%M >> $item_name
else
echo "Unable to write to $item_name"
fi
else
echo "No,$item_name is not a file"
fi
检查文件是否可以执行
例：
#!/bin/bash
#testing file execution
if [ -x es16.sh ]
then
echo "You can run the script:"
./es16.sh
else
echo "Sorry,you are unable to execute the script"
fi
检查所属关系
例：
#!/bin/bash
#check file ownership
if [ -O /etc/passwd ]
then
echo "You are the owner of the /etc/passwd file"
else
echo "Sorry,you are not the owner of the /etc/passwd file"
fi
检查默认属组关系
例：
#!/bin/bash
#check file group test
if [ -G $HOME/sentinel ]
then
echo "You are in the same group as the file"
else
echo "The file is not owned by your group"
fi
检查文件日期
例：
#!/bin/bash
#testing file dates
if [ es19.sh -nt es18.sh ]
then
echo "The es19 file is newer than es18"
else
echo "The es18 file is newer than es19"
fi
if [ es17.sh -ot es18.sh ]
then
echo "The es17 file is older than es18"
else
echo "The es18 file is older than es17"
fi
比较的前提是文件必须存在,否则将返回一个错误答案
例：
#!/bin/bash
#testing file dates
if [ badfile1 -nt badfile2 ]
then
echo "The badfile1 file is newer than badfile2"
else
echo "The badfile2 file is newer than badfile1"
fi
复合条件测试
[ condition1 ] && [ condition2 ]
[ condition1 ] || [ condition2 ]
例：
#!/bin/bash
#testing compound comparisons
if [ -d $HOME ] && [ -w $HOME/testing ]
then
echo "The file exists and you can write to it"
else
echo "I cannot write to the file"
fi
使用双括号
双括号命令用来使用高级数学表达式
双括号命令符号
| 符号 | 描述 |
|---|
| val++ | 后增 |
| val– | 后减 |
| ++val | 先增 |
| –val | 先减 |
| ! | 逻辑求反 |
| ~ | 位求反 |
| ** | 幂运算 |
| << | 左位移 |
| >> | 右位移 |
| & | 位布尔和 |
| | | 位布尔或 |
| && | 逻辑和 |
| || | 逻辑或 |
例：
#!/bin/bash
#using double parenthesis
val1=10
if (($val1**2>90))
then
((val2=$val1**2))
echo "The square of $val1 is $val2"
fi
使用双方括号
双方括号针对字符串比较
例：
#!/bin/bash
#using pattern matching
if [[ $USER == k* ]]
then
echo "Hello $USER"
else
echo "Sorry,I do not know you"
fi
case命令
例：
#!/bin/bash
#looking for a possible value
if [ $USER = "rich" ]
then
echo "Welcome $USER"
echo "Please enjoy your visit"
elif [ $USER = "barbara" ]
then
echo "Welcome $USER"
echo "Please enjoy your visit"
elif [ $USER = "testing" ]
then
echo "Special testing account"
elif [ $USER = "jessica" ]
then
echo "Do not forget to logout when you're done"
else
echo "Sorry,you are not allowed here"
fi
for命令
for var in list
do
commands
done
例：
#!/bin/bash
#basic for command
for test in Alabama Alaska Arizona Arkansas California Colorado
do
echo The next state is $test
done
例：
#!/bin/bash
#testing the for variable after the looping
for test in Alabama Alaska Arizona Arkansas California Colorado
do
echo "The next state is $test"
done
echo "The last state we visited was $test"
test=Connecticut
echo "Wait,now we're visiting $test"
使用转义字符来定义转义,使用双引号定义单引号的值
例：
#!/bin/bash
#another example of how not to use the for command
for test in I don\'t know if "this'll" work
do
echo "word:$test"
done
如果在单独的数据值中有空格,就必须用""将这些值圈起来
例：
#!/bin/bash
#an example of how to properly define values
for test in Nevada "New Hampshire" "New Mexico" "New York"
do
echo "Now going to $test"
done
将一些列值集中存储到一个变量中
例：
#!/bin/bash
#using a variable to hold the list
list="Alabama Alask Arizona Arkansas Colorado"
list=$list" Connecticut"
for state in $list
do
echo "Have you ever visited $state"
done
从命令读取值
例：
#!/bin/bash
#reading values from a file
file="tt"
for state in $(cat $file)
do
echo "Visit beautiful $state"
done
更改字段分隔符(相当于split的分隔符)更改IFS环境变量的值
例：
#!/bin/bash
#reading values from a file
file="tt"
IFS=$'\n'
for state in $(cat $file)
do
echo "Visit beautiful $state"
done
用通配符读取目录
例：
#!/bin/bash
#iterate through all the files in a directory
for file in /home/keynes/*
do
if [ -d $file ]
then
echo "$file is a directory"
elif [ -f $file ]
then
echo "$file is a file"
fi
done
多个目录通配符
例：
#!/bin/bash
#iterating through multiple directories
for file in /home/keynes/.b* /home/keynes/
do
if [ -d "$file" ]
then
echo "$file is a directory"
elif [ -f "$file" ]
then
echo "$file is a file"
else
echo "$file doesn't exist"
fi
done
C语言风格的for命令
例：
#!/bin/bash
#testing the C-style for loop
for((i=1;i<=10;i++))
do
echo "The next number is $i"
done
使用多个变量
例：
#!/bin/bash
#multiple variable
for (( a=1,b=10;a<=10;a++,b--))
do
echo "$a-$b"
done
while命令
例：
#!/bin/bash
#while command test
var1=10
while [ $var1 -gt 0 ]
do
echo $var1
var1=$[ $var1 - 1 ]
done
使用多个测试命令
例：
#!/bin/bash
#testing a multicommand while loop
var1=10
while echo $var1
[ $var1 -ge 0 ]
do
echo "This is inside the loop"
var1=$[ $var1 - 1]
done
until命令
例：
#!/bin/bash
#using the until command
var1=100
until [ $var1 -eq 0 ]
do
echo Inside the loop : $var1
var1=$[ $var1 - 25 ]
done
例：
#!/bin/bash
#using the until c command
var1=100
until echo $var1
[ $var1 -eq 0 ]
do
echo Inside the loop:$var1
var1=$[ $var1 -25 ]
done
嵌套循环
例：
#!/bin/bash
#nesting for loops
for((a=1;a<=3;a++))
do
echo "Starting loop $a:"
for((b=1;b<=3;b++))
do
echo " Inside loop:$b"
done
done
例：
#!/bin/bash
#placing a for loop inside a while loop
var1=5
while [ $var1 -ge 0 ]
do
echo "Outer loop:$var1"
for((var2=1;var2<3;var2++))
do
var3=$[ $var1*$var2 ]
echo " Inner loop:$var1*$var2=$var3"
done
var1=$[$var1-1]
done
例：
#!/bin/bash
#using until and while loops
var1=3
until [ $var1 -eq 0 ]
do
echo "Outer loop:$var1"
var2=1
while [ $var2 -lt 5 ]
do
var3=$(echo "scale=4;$var1/$var2" | bc)
echo " Inner loop:$var1/$var2=$var3"
var2=$[var2+1]
done
var1=$[$var1 - 1]
done
循环处理文件数据
#!/bin/bash
#changing the IFS value
IFS.OLD=$IFS
IFS='\n'
for entry in $(cat /etc/passwd)
do
echo "Value in $entry -"
IFS=:
for value in $entry
do
echo " $value"
done
done
控制循环
break命令
例：
#!/bin/bash
#breaking out of a for loop
for var1 in 1 2 3 4 5 6 7 8 9 10
do
if [ $var1 -eq 5 ]
then
break
fi
echo "Iteration number:$var1"
done
echo "The loop is completed"
例：
#!/bin/bash
#breaking out of a while loop
var1=1
while [ $var1 -lt 10 ]
do
if [ $var1 -eq 5 ]
then
break
fi
echo "Iteration:$var1"
var1=$[ $var1 + 1 ]
done
echo "The while loop is completed"
例：
#!/bin/bash
#breaking out of an inner loop
for((a=1;a<4;a++))
do
echo "Outer loop:$a"
for((b=1;b<100;b++))
do
if [ $b -eq 5 ]
then
break
fi
echo " Inner loop:$b"
done
done
break n 其中n指定要跳出的循环层级
例：
#!/bin/bash
#breaking out of an outer loop
for((a=1;a<4;a++))
do
echo "Outer loop:$a"
for((b=1;b<100;b++))
do
if [ $b -gt 4 ]
then
break 2
fi
echo " Inner loop:$b"
done
done
continue命令
例：
#!/bin/bash
#using the continue command
for((var1=1;var1<15;var1++))
do
if [ $var1 -gt 5 ] && [ $var1 -lt 10 ]
then
continue
fi
echo "Iteration number:$var1"
done
例：
#!/bin/bash
#improperly using the continue command in a while loop
var1=0
while echo "while iteration:$var1"
[ $var1 -lt 15 ]
do
if [ $var1 -gt 5 ] && [ $var1 -lt 10 ]
then
continue
fi
echo " Inside iteration number:$var1"
var1=$[$var1 +1]
done
continue n
例：
#!/bin/bash
#continuing an outer loop
for ((a=1;a<=5;a++))
do
echo "Iteration $a"
for ((b=1;b<3;b++))
do
if [ $a -gt 2 ] && [ $a -lt 4 ]
then
continue 2
fi
var3=$[ $a*$b ]
echo " The result of $a * $b is $var3"
done
done
处理循环的输出
例：
#!/bin/bash
#redirecting the for output to a file
for((a=1;a<10;a++))
do
echo "The number is $a"
done>test23.txt
echo "The command is finished"
例：
#!/bin/bash
#piping a loop to another command
for state in "North Dakota" Connecticut Illinois Alabama Tennessee
do
echo "$state is the next place to go"
done | sort
echo "This completes our travels"
查找可执行文件
例：
#!/bin/bash
#find file in the PATH
IFS=:
for folder in $PATH
do
echo "$folder:"
for file in $folder/*
do
if [ -x $file ]
then
echo " $file"
fi
done
done
