Use of Linux ln command

1. Command Introduction

The ln command is used to create links for files, which are divided into hard links and soft links (symbolic links). Hard links are created by default. If you want to create soft links, you must use the -s option. This article describes the implementation of the GNU version; other versions (such as the POSIX version) may have different implementations.

Notice:

(1) A hard link is not an independent file, but just a file name. A file can have multiple file names, and the file can only be deleted by deleting the last file name from the disk;
(2) Soft links can cross file systems, but hard links cannot cross file systems because hard links are just aliases of files, not independent files.
(3) You cannot create a hard link to a directory because a hard link to a directory may cause the directory's inode and physical block to form a loop. At this point, if you delete the directory, the directory entity block will become inaccessible to the system, resulting in an isolated directory (no longer accessible from the root directory);
(4) When creating a hard link, each target must exist. When creating a soft link, the target file does not need to exist;
(5) A soft link is an independent file that contains path information, similar to a Windows shortcut. Many of its properties depend on the original file, so it is meaningless to set permissions for the soft link file.

2. Command format

ln [OPTION]... [-T] TARGET LINK_NAME (1st form)
ln [OPTION]... TARGET (2nd form)
ln [OPTION]... TARGET... DIRECTORY (3rd form)
ln [OPTION]... -t DIRECTORY TARGET... (4th form)

The first format creates a link with the specified name for the specified target file. This is the most commonly used format.
The second format creates a link with the same name in the current directory for the specified target file;
The third and fourth formats create links with the same name in the specified directory for each target file.

3. Option Description

Mandatory arguments for long options are mandatory for short options as well.

--backup[=CONTROL]
	Back up every existing target file -b
	Similar to --backup, but does not accept the -d, -F, --directory parameters
	Allow the superuser to attempt to create hard links for directories (note: even the superuser may fail due to system limitations)
-f, --force
	Forcefully create a link to a file or directory. The file or directory with the same name as the link will be overwritten -i, --interactive
	Ask the user before overwriting existing files -L, --logical
	When creating a hard link, if the target file is a soft link, dereference it to point to the target file of the soft link -n, --no-dereference
	Treat soft links as normal files and do not dereference them -P, --physical
	When creating a hard link, point directly to the soft link itself, rather than to the target file of the soft link (default)
-r, --relative
	Create a symbolic link relative to the link location -s, --symbolic
	Create a soft link instead of a hard link -S, --suffix=SUFFIX
	Modify the backup file suffix. After backing up the target file with the -b parameter, the backup file suffix defaults to ~
-t, --target-directory=DIRECTORY
	Specify the directory where the link file is stored -T, --no-target-directory
	Treat LINK_NAME as a link file rather than a directory containing the link file -v, --verbose
	Display the command execution process --help
	show help and exit --version
	Display version and exit

The parameter CONTROL of the option --backup controls the way the version of the file is generated after the backup. The possible values ​​are as follows:

none, nil
	Do not back up numbered, t
	Use a numeric suffix to scroll. The backup file name suffix increases in sequence ~1~,
existing, nil
	If a numeric suffix is ​​used, use the number, otherwise use the simple backup method, that is, only back up once simple, never
	Use only simple backup methods

When using option -s to generate a soft link, options -L and -P will be ignored, and a hard link will be created. The -P option will be used by default to point the hard link to the soft link itself, which is equivalent to giving the soft link an alias.

4. Common Examples

(1) Create a soft link for the file /etc/passwd.

ln -s /etc/passwd passwdSoftLink

ll passwdSoftLink
lrwxrwxrwx 1 root root 11 Nov 13 22:21 passwdSoftLink -> /etc/passwd

(2) Create multiple soft links for the file /etc/passwd. The names of the soft links are the same, and numbers are used to represent the version numbers of the backup files. For multiple backups, the version number will increase sequentially.

ln -s --backup=numbered /etc/passwd passwdSoftLink

ll passwdSoftLink*
lrwxrwxrwx 1 root root 11 Nov 14 10:36 passwdSoftLink -> /etc/passwd
lrwxrwxrwx 1 root root 11 Nov 14 10:36 passwdSoftLink.~1~ -> /etc/passwd

(3) Create a soft link for a non-existent file.

ln -s nofile nofileSoftLink

When you use the ll command to view soft links, the soft link name is red, and the non-existent target file name flashes continuously in white text on a red background.

After writing content to the soft link nofileSoftLink and saving it, the file nofile will be generated.

(4) Create a hard link for /etc/passwd.

ln /etc/passwd passwdHardLink

ll -i /etc/passwd passwdHardLink
787795 -rw-r--r-- 2 root root 1552 Jan 4 2019 /etc/passwd
787795 -rw-r--r-- 2 root root 1552 Jan 4 2019 passwdHardLink

When you use the ll command to view two files, the inode numbers in the first column are the same, and the number of hard links in the third column is 2, indicating that there are two file names pointing to the data entities of the files.

(5) Create a soft link with the same name for /etc/passwd and place the soft link in the current directory. That is, use the third and fourth command formats to create a link for the file.

ln -s /etc/passwd .

# or ln -s -t . /etc/passwd

# View ll passwd
lrwxrwxrwx 1 root root 11 Nov 14 10:43 passwd -> /etc/passwd

Note that when writing the target file, the path must be relative to the target directory, or use an absolute path, otherwise the soft link cannot point to the target file.

(6) If a file with the same name as the created link file exists, it will be forcibly overwritten without backup.

ln -sf /etc/passwd passwdSoftLink

(7) Modify the soft link to point to the new target file. Point the soft link passwdSoftLink to /usr/bin/passwd, re-establish the soft link, and force overwrite the original soft link passwdSoftLink.

ln -sf /usr/bin/passwd passwdSoftLink

ll passwdSoftLink
lrwxrwxrwx 1 root root 15 Nov 14 10:52 passwdSoftLink -> /usr/bin/passwd

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