How do I give write permission to file in Linux?

Question

How can I (programmatically) give write permission on a file to a particular user in Linux? Like, for example, its owner? Everyone has read access to this file.

Answer 1

In a shell or shell script simply use:

chmod u+w <filename>

This only modifies the write bit for the user, all other flags remain untouched.

If you want to do it in a C program, you need to use:

int chmod(const char *path, mode_t mode);

First query the existing mode via

int stat(const char *path, struct stat *buf);

... and just set the write bit by doing newMode = oldMode | S_IWUSR. See man 2 chmod and man 2 stat for details.

Answer 2

The octal mode 644 will give the owner read and write permissions, and just read permissions for the rest of the group, as well as other users.

read     = 4
write    = 2
execute  = 1
owner    = read | write = 6
group    = read         = 4
other    = read         = 4

The basic syntax of the command to set the mode is

chmod 644 [file name]

In C, that would be

#include <sys/stat.h>

chmod("[file name]", 0644);