What do Unix and Linux file permissions mean?

File Permissions

USER##S_IWUSR##S_IXUSR2WRITE 1EXEC Other users write2WRITE # #S_IXOTHchmod 755. The above table can be divided into three groups , the maximum value of each group is 7, indicating read, write and execute permissions. Let’s use C code to see the specific values:

#include <stdio.h>
#include <sys/stat.h>

/*
S_IRUSR: 使用者读权限, READ USER 4
S_IWUSR: 使用者写权限, WRITE USER 2
S_IRGRP: 组用户读权限, READ GROUP 4
S_IROTH: 其他用户读权限, READ OTHER 4
*/


int main(int argc, char const *argv[]) {
    printf(
        "S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH=%o, %o, %o, %o, (S_IRUSR | S_IWUSR "
        "| S_IRGRP | S_IROTH)=%o\n",
        S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH,
        (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH));
    return 0;
}

##st_mode	Meaning	Octal value	English note
##S_IRUSR ## User Read	4	READ
User writes	2	WRITE USER
User execution	1	##EXEC USER	S_IRGRP
Group Reading	4	READ GROUP	##S_IWGRP
Group Write		GROUP	S_IXGRP
Group execution		GROUP	##S_IROTH	Read by other users	4	##READ OTHER
##S_IWOTH			OTHER
Other users execute	##1	# #EXEC OTHER	This also corresponds to our common

Result:

S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH=400, 200, 40, 4, (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)=644

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