The chmod command is typically used to modify the six permission bits.

The chmod command allows one to specify u for user (owner), g for group, and o for other.

The three categories (read, write, and execute) are used in various ways by different functions with the chmod command.

Whenever we want to open any type of file by name, we must have execute permission in each directory mentioned in the name, including the current directory, if it is implied.

The excute-permission bit for a directory is often called the search bit.

The "ln" command creates a symbolic link. Here a symbolic link is created to point a file named "myfile."

The file "myfile" does exist, yet cat says there is no such file, because "myfile" is a symbolic link.

Here the file pointed to by the symbolic link doesn’t exist.

The -l option to "ls" command gives us one hint: the first character is an l, which means a symbolic link.

The -l option to "ls" command gives us one hint: the sequence -> also indicates a symbolic link.

The ls command with option (-F) that appends an at-sign (@) to filenames that are symbolic links.

Like the octal format with umask command, the symbolic format specifies which permissions are to be denied (i.e., clear in the file creation mask) instead of which ones are to be denied (i.e., set in the file creation mask).

Users can set the umask value to control the default permissions on the files they create.

The umask value is expressed in octal, with one bit representing one permission to be masked off.

With umask value, the permissions can be denied by setting the corresponding bits.

The Single UNIX Specification requires that the umask command support a symbolic mode of operation.

Given a pathname, the function nstat
obtains the information about the named file.

Given a pathname, the stat function returns a structure of information about the named file.

Given a pathname, the fstat function obtains the information about the file that is already open on the descriptor.

Given a pathname, the lstat function is similar to stat, but when the named file is a symbolic link, lstat returns the information about the symbolic link, not the file referenced by the symbolic link.

Given a pathname, the fstatat function provides a way to return the file statistics for a pathname relative to an open directory represented by the fd argument.

Symbolic link

Regular file

Directory file

Block Special file

Character Special file

FIFO

Socket

umask

access

chmod

mask

mode

Any user can create a hard or symbolic link to a directory.

There are two types of links: hard link and symbolic link.

A symbolic link is an indirect pointer to a file.

A hard link points directly to the i-node of the file.

Hard links normally require that the link and the file reside in the same file system.

the sticky bit

the directory access bit

the user write bit

the user execute bit

the special bit

To ensure that anyone can read a file, we should set the umask to 0777.

Most users of UNIX systems never deal with their umask value.

umask value is usually set once, on login, by the shell’s start-up file, and never changed.

Writing programs that create new files, if we want to ensure that specific access permission bits are enabled, we must modify the umask value while the process is running.

The umask value that is in effect when our process is running can cause permission bits to be turned off.

To delete an existing file, we need only the write permission for the file itself.

The read permission for a file determines whether we can open an existing file for reading: the O_RDONLY and O_RDWR flags for the open function.

The write permission for a file determines whether we can open an existing file for writing: the O_WRONLY and O_RDWR flags for the open function.

We must have write permission for a file to specify the O_TRUNC flag in the open function.

We cannot create a new file in a directory unless we have write permission and execute permission in the directory.

Execute permission for a file must be on if we want to execute the file using any of the seven exec functions. The file also has to be a regular file.

the sticky bit

the directory access bit

the user write bit

the user execute bit

the special bit

In all modern systems, chown does not follow symbolic links.

Symbolic links are typically used to ‘‘move’’ a file or an entire directory hierarchy to another location on a system.

If the function follows a symbolic link, a pathname argument to the function refers to the file pointed to by the symbolic link.

There are no file system limitations on a symbolic link and what it points to.

Anyone can create a symbolic link to a directory.

If the process is executing the file, the user-execute bit must be on if the process group ID of the process equals the group ID of the file (i.e., the process group owns the file).

If the effective user ID of the process is 0 (the superuser), access is allowed. This gives the superuser free rein throughout the entire file system.

If the effective user ID of the process equals the owner ID of the file (i.e., the process owns the file), access is allowed if the appropriate user access permission bit is set. Otherwise, permission is denied.

If the effective group ID of the process or one of the supplementary group IDs of the process equals the group ID of the file, access is allowed if the appropriate group access permission bit is set. Otherwise, permission is denied.

If the appropriate other access permission bit is set, access is allowed. Otherwise, permission is denied.

v-node number

file type & mode (permissions)

i-node number (serial number)

number of links

user ID of owner

size in bytes, for regular files

time of last access

time of last modification

The function which removes a directory entry is always called "delete."

Using mkdir command, a directory ("testdir") has been just created under a directory. This will create two new links: one link pointing to "dot" directory entry and the other for "dot-dot" directory entry.

Every i-node has a link count that contains the number of directory entries that point to it.

Only when the link count of i-node goes to 0 can the file be deleted, thereby releasing the data blocks associated with the file.

The operation of "unlinking a file" does not always mean "deleting the blocks associated with the file."

The umask value 0700 is to allow the user (owner) to read, write, or execute the files

The umask value 0000 is to allow anyone to do read, write, or execute the files

The umask value 0002 is to prevent others from writing the files

The umask value 0022 is to prevent group members and others from writing the files

The umask value 0027 is to prevent group members from writing your files and others from reading, writing, or executing the files.

The real user ID and the real group ID

The effective user ID and the effective group ID

The saved set-user-ID and the saved set-group-ID

The virtual user ID and the virtual group ID

The system user ID and the system group ID

Read permission lets us read the directory, obtaining and searching through a list of all the filenames in the directory.

To open the file /usr/include/stdio.h, we need execute permission in the directory /, execute permission in the directory /usr, and execute permission in the directory /usr/include.

If the current directory is /usr/include, then we need execute permission in the current directory to open the file stdio.h.

Execute permission lets us pass through the directory when it is a component of a pathname that we are trying to access (in order to search the directory to look for a specific filename).

If the PATH environment variable specifies a directory that does not have execute permission enabled, the shell will never find executable files in that directory.

The effective user ID and the effective group ID

The real user ID and the real group ID

The saved set-user-ID and the saved set-group-ID

The virtual user ID and the virtual group ID

The system user ID and the system group ID