Shell script basic syntax

Mac Under the computer Bash, yes Unix shell A kind of , Developers through shell Script middleware calls the system kernel , So it is necessary for Apple developers to learn .

1 Create and export

1-1 establish Shell file

command ：touch hello.sh

1-2 Definition file declaration

#!/bin/bash
#!： Indicates the Convention mark , He will tell the system what kind of interpreter this script needs to execute , It is both a script embodiment .'#' notes , Be careful ： stay Shell Script , No multi line gaze , Only single line comments .
echo “Hello world!”

1-3 perform Shell Script hello file

command ：./hello.sh

View and modify file permissions

View command ：ls -l hello.sh
Modify the order ： chmod +x ./hello.sh
-rwxr-xr-x

1-4 Variable operating

• There should be no spaces in the assignment
  name="Kenny" Can not write name = "Kenny"
  Variable names must start with ‘a-z’ or ‘A-Z’ See the letter of , Read only variables cannot be modified , taste readonly name
• Delete variables unset name
• Special variables
  ${0}： File name
  ${?}： Indicates the return value of the execution status of the previous command
  $#： Number of parameters
  $*： parameter list , Composite string
  [email protected]： parameter list
  $$： Go back to the current shell Conduct ID
  $!: Execute last instruction PID

1-5 String splicing 、 Intercept 、 Delete

• Splicing

#!/bin/bash
name='Andy'
age=100
sex=" male "
info="  full name ：'${name}'  Age ：'${age}'  Gender ：'${sex}' " 
echo ${info}                                

• Intercept
  Variable name ： start ： end , result ’have’

name="I have a Dream"
result=${name:2:4}
echo ${result}

Find the length of the variable length=${#name}-1
The end position may not be written , To the end

name="I have a Dream"
result=${name:5:length-1}
echo ${result}

• Delete
  # Grammar 1 ：${ Variable name # Delete string Regular expressions } Delete from the first
  ## Grammar II ：${ Variable name ## Delete string Regular expressions } Delete from the last
  % Grammar 3 ：${ Variable name % Delete string Regular expressions } Find the first of the specified characters , And delete all the previous characters （ Include yourself ）
  %% Grammar 3 ：${ Variable name %% Delete string Regular expressions } Find the last of the specified character , And delete all the previous characters （ Include yourself ）

Delete range ： Delete from left to right
#-> Indicates that the query direction is from left to right
%-> Indicates that the query direction is from right to left

Delete range ： Delete from right to left
##-> Indicates that the query direction is from right to left
%%-> Indicates that the query direction is from left to right

Case a ： Find the first of the specified characters , And delete all the following characters （ Include yourself ）

# Look for the first one from the left , And delete all the previous characters （ Include yourself ）

name="I have a Dream"
result=${name#*a}
echo ${result}

result

ve a Dream

## Look for the first one from the right , And delete all the previous characters （ Include yourself ）

name="I have a Dream"
result=${name##*a}
echo ${result}

result

m

Case 2 ： Find the first of the specified characters , And delete all the following characters （ Include yourself ）

% Look for the first one from the right , And delete all the following characters （ Include yourself ）

name="I have a Dream"
result=${name%a*}
echo ${result}

result

I have a Dre

%% Look for the first one from the left , And delete all the following characters （ Include yourself ）

name="I have a Dream"
result=${name%%a*}
echo ${result}

result

I h