22/02/15 study notes

One 、 Some instruction sets

Count the number of lines in a file ：
Instructions ：

wc -l  file name 

Count the number of files in a folder ：
Instructions ：

ls -l|grep "^-"|wc -l

Count the number of directories ：

• Only check the number of directories under the current directory
  Instructions ：

ls -l|grep "^d"|wc -l

• When viewing the number of directories under the current directory, include the child directories
  Instructions ：

ls -lR|grep "^d"|wc -l

Homework ：
1. Query the number of all destination files with the specified prefix name in the current path

Two 、liunx user 、 Group 、 jurisdiction

1.ls And ll

Use ll When viewing directories and files in the current directory , The meaning of the first column above ：

The first letter represents the file type ;
The last nine characters correspond to the corresponding owner 、 Generic group 、 other
Character meaning

Empowerment ：chmod、chgrp、chown
Authority modification

Switching users ：su

3、 ... and 、shell Programming

1. first shell Program

Use vi The editor enters the writer in it firstshell, And then use ./firstshell Calling procedure ;

among ,#/bin/bash It's an interpreter ,echo Is the output keyword ;
adopt ./firstshell You can call the program directly , Output helloworld;

2. Variable

（1） data type

shell There are only two data types in , String and integer ;
stay shell You can add... Before the variable readonly Keyword to create a variable equivalent to a constant （ After adding the read-only keyword, the save can only be read , Cannot be modified ）

（2） Delete variables

To delete a variable, we can use unset To delete variables ;
for example ：

give the result as follows ：

It is not a report but an error, but there is no output on it , It is reasonable to guess that the variable also exists, but its memory is modified to be empty ;
ok , Query the information to know , Obviously wrong , No error is reported because when using variables as output , It was originally unset The released variable has a variable with the same name created , That's it ;
Pass the test , You can see that directly calling variables as output can also directly output , The result is the same as above ;

（3） Keyboard entry

You can enter variables through the keyboard , Use read Variable To read a row of data , Enter to end ;

3. Arithmetic operator

Format example ：

#/bin/bash
sum=`expr 1 + 1`
sub=`expr 5 - 2`
chu=`expr 4 / 2`
mul=`expr 3 \* 3`
ehco ${sum}

Output example ：

• Be careful ： When doing the four operations, we should pay attention to the multiplication * You need to use an escape character to escape , That is to use \*, In addition, the remainder operation can also be performed in shell Use in （%）.

4.shell Programming structure

（1） Branching structure

form ：

#!/bin/bash
if ((5>7))
then
	echo " Five and seven "
else 
	echo " Five less than seven "
fi

（2） Loop structure

Format example ：

#!/bin/bash
sum=0
for((i=1;i<=100;i++))
do
        ((sum+=i))
        #let sum+=i
        #sum=`expr $sum + $i`
        #sum=$((sum+i))
        #sum=$(($sum+$i))
        #((sum=$i+$sum))
done
echo $sum

Homework ： Output 20 The Fibonacci series ;

#!/bin/bash
p=0
j=1
sum=0
for((i=1;i<=20;i++))
do
        echo ${j}
        ((sum=p+j))
        #sum=`expr $p + $j`
        p=$j
        j=$sum
done