Practical shell knowledge

Put this picture , Nothing else .

Basic knowledge of

./XXX.sh

It works like this

---

Shell Variable

your_name="XXX"

1、 There can be no spaces between variable names and equal signs .
2、 Only English letters can be used for naming , Numbers and underscores , The first character cannot begin with a number .
3、 No spaces in between , You can use underscores _.
4、 You can't use punctuation .
5、 Out of commission bash Keywords in （ You can use help Command to view reserved keywords ）.

Using variables

Use a defined variable , Just precede the variable name with a dollar sign , Such as ：

your_name="qinjx"
echo $your_name
echo ${
    your_name}

Maybe one day you'll see someone who didn't add ${}, Don't be surprised .

---

Shell character string

The string is shell The most commonly used and useful data types in programming （ Besides numbers and strings , There is no other type to use ）, Strings can be in single quotes , You can also use double quotes , You can also use no quotes .

Single quotation marks

str='this is a string'

Restrictions on single quoted string ：

 Any character in a single quotation mark will be output as is , Variables in a single quoted string are invalid ;
 A single quotation mark cannot appear in a single quotation mark string （ You can't escape a single quotation mark ）, But in pairs , Use as string concatenation .

Double quotes

your_name="runoob"
str="Hello, I know you are \"$your_name\"! \n"
echo -e $str

The output is ：

Hello, I know you are "runoob"! 

The advantages of double quotes ：

 You can have variables in double quotes 
 Escape characters can appear in double quotes 

String concatenation

your_name="runoob"
#  Using double quotation marks 
greeting="hello, "$your_name" !"
greeting_1="hello, ${your_name} !"
echo $greeting  $greeting_1
#  Using single quotation marks 
greeting_2='hello, '$your_name' !'
greeting_3='hello, ${your_name} !'
echo $greeting_2  $greeting_3

The output is ：

hello, runoob ! hello, runoob !
hello, runoob ! hello, ${your_name} !

Get string length

string="abcd"
echo ${
    #string} # Output  4

Extract substring

The following example starts from the string No 2 Characters begin to intercept 4 Characters ：

string="runoob is a great site"
echo ${string:1:4} #  Output  unoo

---

Shell Array

stay Shell in , Use parentheses to represent arrays , For array elements " Space " Symbol split . The general form of defining an array is ：

 Array name =( value 1  value 2 ...  value n)

for example ：

array_name=(value0 value1 value2 value3)

perhaps

array_name=(
value0
value1
value2
value3
)

You can also define the components of an array separately ：

array_name[0]=value0
array_name[1]=value1
array_name[n]=valuen

Can not use consecutive subscripts , And there is no limit to the scope of subscripts .

Read array

The general format for reading array element values is ：

${ Array name [ Subscript ]}

Use @ The symbol can get all the elements in the array

echo ${array_name[@]}

Gets the length of the array

 The way to get the array length is the same as the way to get the string length , for example ：

#  Get the number of array elements 
length=${
    #array_name[@]}
#  perhaps 
length=${
    #array_name[*]}
#  Get the length of a single element of an array 
lengthn=${
    #array_name[n]}

---

Shell notes

With # The first line is the comment , Will be ignored by the interpreter .

Multiline comments can also use the following format ：

:<<EOF  The comment ...  The comment ...  The comment ... EOF

EOF You can also use other symbols :

:<<'  The comment ...  The comment ...  The comment ... '

---

Shell Pass parameters

We can execute Shell Script time , Pass parameters to script , The format of obtaining parameters in the script is ：$n.n Represents a number ,1 For executing the first parameter of the script ,2 For executing the second parameter of the script , And so on ……

echo "Shell  Pass parameter instance ！";
echo " File name of execution ：$0";
echo " The first parameter is zero ：$1";
echo " The second parameter is ：$2";
echo " The third parameter is zero ：$3";

$ ./test.sh 1 2 3

in addition , There are also a few special characters used to pass parameters ：

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Operator

Relational operator

Logical operators

String operators

File test operators

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echo

For string output .

echo string

echo "It is a test"

echo -e "OK! \n" # -e  Open escape 
echo "It is a test"

---

Process control

Branch

Judge whether the execution of the previous statement is successful ：

shell Using symbols in “$?” To display the return value of the last command execution , If 0 On behalf of the successful implementation , Other indication of failure .

if [ $? -ne 0 ]; then
    echo "failed"
else
    echo "succeed"
fi

if condition
then
    command1 
    command2
    ...
    commandN 
fi

if condition
then
    command1 
    command2
    ...
    commandN
else
    command
fi

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

---

for loop

for var in item1 item2 ... itemN
do
    command1
    command2
    ...
    commandN
done

Write in a line ：

for var in item1 item2 ... itemN; do command1; command2… done;

When variable values are in the list ,for A loop executes all the commands at once , Use the variable name to get the current value in the list . The command can be any valid shell Commands and statements .in The list can contain substitutions 、 String and file name .

in List is optional , If you don't use it ,for Loop through the positional parameters of the command line .

while sentence

while Loop is used to execute a series of commands continuously , Also used to read data from the input file . Its grammatical form is ：

while condition
do
    command
done

Out of the loop

During the cycle , Sometimes it's necessary to force the loop out before the loop end condition is reached ,Shell Use two commands to do this ：break and continue.
break command

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Shell Input / Output redirection

---

Practical script collection

Determine whether the instruction is successfully executed

if [ $? -ne 0 ]; then
    echo "failed"
else
    echo "succeed"
fi

I have been using this today , Because the log must be written for successful or unsuccessful execution , And write logs separately .

---

Whether the batch test file exists

[[email protected] ~]# cat testfile.sh 
#!/bin/bash 
 
 
for file in test*.sh 
do 
  if [ -f $file ];then 
     echo "$file existed." 
  fi 
done 
 
[[email protected] ~]# ./testfile.sh 
test.sh existed. 
test1.sh existed. 
test2.sh existed. 
test3.sh existed. 
test4.sh existed. 
test5.sh existed. 
test78.sh existed. 
test_dev_null.sh existed. 
testfile.sh existed.  

Supplement after use .