Linux learning: system data files and information (time and date functions, password files)

lvan_ linux 2021-01-24 12:53:01
linux learning data files information

brief introduction : In this section we will briefly introduce Linux Some data in the system are .( ad locum 、 Let's just give a few examples )

1:passwd Data files and group files .

a: First of all, let's introduce passwd Data files , This data file is stored in /etc/passwd In the file directory . The data in this file may have the following contents .


notes : Because of different systems , The entries displayed in the key file may be different .
Here's a special example : That's how we organize for login , Here are a few ways .
1>: The shell Switch to /bin/false.
2>: The shell Switch to /bin/nologin.( The user can be allowed to use ftp Etc )
3>: Use command usermod -s /sbin/nologin .
Next 、 Let's introduce some functions about key files .
struct passwd *getpwuid(uid_t uid);
struct passwd* getpwnam(const char* name);
These two functions are based on the user's ID And name to get some information about the user , Please refer to man manual 、 I won't go into details here .
And then again , Let's introduce a few functions about viewing the entire password file .
struct passwd* getpwent();
void setpwent();
void endpwent();
The first function is to get the next entry in the password file , It returns a filled passwd Pointer to structure , The second function resets the file location to the beginning of the file . The third function is to release the resources that the file is .

2: System identification , Get some information about the system .

The function prototype :
int uname(struct utsname *name);
This function gets some information about the system . On the terminal we can use uname Command to get some information .

3: Next 、 We're going to talk about time-dependent functions .

 Here we mainly introduce the functions related to time and date .
time_t time(time_t *calptr);// Return with time and date , If the parameters are not empty rings , Save the time and date to the memory space pointed by the parameter .
int gettimeofday(struct timeval* tp, void* tzp);// Return to a more precise time , What he returns is the execution time of the program .
meanwhile 、 We're getting this time in seconds 、 We also need to convert this kind of time into various forms of time, such as month, day, etc . meanwhile struct tm The structure is represented by year, year, day, hour, minute and second
Time , We can also convert this time into other time . As shown in the figure below , Conversion of various time formats .


ad locum , Let's list all the function prototypes .


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