Introduction to shell programming

1、realpath Get file absolute path

[email protected]:~/crystalmedia/scripts$ realpath activeMq_docker.sh 
/home/kylin/crystalmedia/scripts/activeMq_docker.sh
[email protected]:~/crystalmedia/scripts$ 

2、if sentence
if Mainly pay attention to the space ,if And [ then [] A space before and after

if [ ! -f "${filename}" ]
then
  echo "File '${filename}' NOT exists!"
  exit 1
fi

Or this way of writing

if [ ! -f "${filename}" ];then
  echo "File '${filename}' NOT exists!"
  exit 1
fi

remarks ：

 Basically the same as other scripting languages . There's no big difference . But here's the thing .[] The conditions inside .
1、 String judgment 

str1 = str2　　　　　　 When two strings have the same content 、 Length is true 
str1 != str2　　　　　  Dangchuang str1 and str2 It's true when you don't wait 
-n str1　　　　　　　  When the length of the string is greater than 0 Time is true ( The string is not empty )
-z str1　　　　　　　  When the length of the string is 0 Time is true ( Empty string )
str1　　　　　　　　    Dangchuang str1 If it's not empty, it's true 

2、 The judgment of numbers 

int1 -eq int2　　　　 It is true that two numbers are equal 
int1 -ne int2　　　　 Two numbers are not equal to true 
int1 -gt int2　　　　int1 Greater than int2 It's true 
int1 -ge int2　　　　int1 Greater than or equal to int2 It's true 
int1 -lt int2　　　　int1 Less than int2 It's true 
int1 -le int2　　　　int1 Less than or equal to int2 It's true 

3、 Judgment of documents 

-r file　　　　　 User readable as true 
-w file　　　　　 Users can write true 
-x file　　　　　 User executable is true 
-f file　　　　　 The file is normal and the file is true 
-d file　　　　　 File is true directory is true 
-c file　　　　　 File is character special file is true 
-b file　　　　　 Files are blocks, special files are true 
-s file　　　　　 The file size is not 0 Time is true 
-t file　　　　　 When file descriptor ( The default is 1) True when the specified device is a terminal 

4、 Complex logical judgment 

-a 　 　　　　　  And 
-o　　　　　　　  or 
!　　　　　　　　 Not 

3、pushd and popd
pushd and popd It is to operate on a directory stack , and dirs Is to display the contents of the directory stack . The directory stack is a stack structure that stores directories , The current directory is always stored at the top of the stack structure （ Here we can see further from below ）.

dirs There are two commonly used parameters ：

Options
meaning
-p
Each line shows a record
-v
Each line shows a record , At the same time, show the record in the stack index
-c
Empty the directory stack

There is only one directory in the directory stack ( Of course, at the top of the stack ), Is the current directory . After switching directories , The directory in the stack also changes . adopt -c Option can clear other directories in the directory stack except the current directory , Because the directory stack in the above example only contains the current directory , So there's no change .

Every time pushd After the command is executed , By default, a dirs Command to display the contents of the directory stack .pushd There are mainly the following usages ：
pushd Catalog

pushd If you use it directly after the directory , Will switch to the directory and put the directory on the top of the directory stack .( Always remember , The current directory is always stored at the top of the directory stack . If the current directory changes , Then the top element of the directory stack must also change ; In turn, , If the element at the top of the stack changes , Then the current directory must have changed .) Here is an example ：

[email protected]:~$ pushd crystalmedia/test/
~/crystalmedia/test ~
[email protected]:~/crystalmedia/test$ pushd /opt/
/opt ~/crystalmedia/test ~
[email protected]:/opt$ popd
~/crystalmedia/test ~
[email protected]:~/crystalmedia/test$ popd
~
[email protected]:~$ 

From the above example we can see that , use pushd To replace cd It's very useful .

pushd and popd Of the two +n
pushd +n
pushd +n Switch to the... In the directory stack n A catalog ( there n Namely dirs -v Command displayed index), And push the directory to the top of the stack in a stack loop .
popd +n
Put the... In the directory stack n Elements delete ( there n It's an order dirs -v Displayed directory index)
We usually don't want to dirs Show it , So pass

 pushd  Catalog  >/dev/null