§ chmod

chmod [ugoa] [-+=] [rwxXst]

users
u : the user who owns it
g : user in the files' group
o : other users not in the file's group
a : all uers.

perms
rwx : read (4), write (2), execute (1)

§ Other than rwx

• setuid / setgid : whether the program can set its user id / group id. If the program is a script then this bit is ignored (for security reasons)
• sticky bit:

Filemode defs¹

#include <sys/stat.h>
int fchmod(int fildes, mode_t mode);

mode bits: (man 2 fchmod)

S_ISUID  (04000)
    set-user-ID (set process effective user ID on execve(2))

S_ISGID  (02000)
    set-group-ID  (set  process  effective  group  ID on execve(2); mandatory
    locking, as described in fcntl(2); take a new file's group from parent
    directory, as described in chown(2) and mkdir(2))

S_ISVTX  (01000)
    sticky bit (restricted deletion flag, as described in unlink(2))

S_IRUSR  (00400)
     read by owner

S_IWUSR  (00200)
    write by owner

S_IXUSR  (00100)
    execute/search by owner ("search" applies for directories, and means that
    entries within the directory can be accessed)

S_IRGRP  (00040)
    read by group

S_IWGRP  (00020)
    write by group

S_IXGRP  (00010)
    execute/search by group

S_IROTH  (00004)
    read by others

S_IWOTH  (00002)
    write by others

S_IXOTH  (00001)
    execute/search by others

struct file

struct file {
    atomic_long_t                   f_count;
    spinlock_t                      f_lock;
    fmode_t                         f_mode;
    const struct file_operations   *f_op;
    struct address_space           *f_mapping;
    void                           *private_data;
    struct inode                   *f_inode;
    unsigned int                    f_flags;
    unsigned int                    f_iocb_flags;
    const struct cred              *f_cred;
    struct path                     f_path;
    union {
        // regular files (with FMODE_ATOMIC_POS) and directories
        struct mutex f_pos_lock;
        // pipes
        u64 f_pipe;
    };
    loff_t                          f_pos;
    void                           *f_security; // ifdef CONFIG_SECURITY
    struct fown_struct              *f_owner;
    errseq_t                        f_wb_err;
    errseq_t                        f_sb_err;
    struct hlist_head              *f_ep;       // ifdef CONFIG_EPOLL

#endif
    union {
        struct callback_head    f_task_work;
        struct llist_node   f_llist;
        struct file_ra_state    f_ra;
        freeptr_t       f_freeptr;
    };
    /* --- cacheline 3 boundary (192 bytes) --- */
}

§ refs:

• https://wiki.archlinux.org/title/File_permissions_and_attributes