On Unix-like operating systems, the ln command creates links between files, associating file names with file data.
This document covers the GNU/Linux version of ln.
ln creates a link to file TARGET with the name LINKNAME. If LINKNAME is omitted, a link to TARGET is created in the current directory, using the name of TARGET as the LINKNAME.
ln creates hard links by default, or symbolic links if the -s (--symbolic) option is specified. When creating hard links, each TARGET must exist.
Before we discuss the ln command, let's first discuss the link command, as well as what a link is and how it relates to files as we know them.
A link is an entry in your file system which connects a file name to the actual bytes of data on the disk. More than one file name can "link" to the same data. Here's an example. Let's create a file named file1.txt:
echo "This is a file." > file1.txt
This command echoes the string "This is a file". Normally this would echo to our terminal, but the > operator redirects the string's text to a file, in this case file1.txt. We can check that it worked by using cat to display the contents of the file:
cat file1.txt
This is a file.
When this file was created, the operating system wrote the bytes to a location on the disk and also linked that data to a file name, file1.txt so that we can refer to the file in commands and arguments. If you rename the file, the contents of the file are not altered; only the information that points to it. The file name and the file's data are two separate entities.
Here's an illustration of the file name and the data to help you visualize it:
Using the link command
What the link command does is allow us to manually create a link to file data that already exists. So, let's use link to create our own link to the file data recently created. In essence, we'll create another file name for the data that already exists.
Let's call our new link file2.txt. How do we create it?
The general form of the link command is: "link file_name linkname". Our first argument is the name of the file whose data we're linking to; the second argument is the name of the new link we're creating.
link file1.txt file2.txt
Now both file1.txt and file2.txt point to the same data on the disk:
cat file1.txt
This is a file.
cat file2.txt
This is a file.
The important thing to realize is that we did not make a copy of this data. Both file names point to the same bytes of data on the disk. Here's an illustration to help you visualize it:
If we change the contents of the data pointed to by either one of these files, the other file's contents are changed as well. Let's append a line to one of them using the >> operator:
echo "It points to data on the disk." >> file1.txt
Now let's look at the contents of file1.txt:
cat file1.txt
This is a file.
It points to data on the disk.
... and now let's look at the second file, the one we created with the link command:
cat file2.txt
This is a file.
It points to data on the disk.
Both files show the change because they share the same data on the disk. Changes to the data of either one of these files will change the contents of the other.
But what if we delete one of the files? Will both files be deleted?
No. If we delete one of the files, we're deleting one of the links to the data. Because we created another link manually, we still have a pointer to that data; we still have a way, at the user-level, to access the data we put in there. So if we use the rm command to remove our first file:
rm file1.txt
...it no longer exists as a file with that name:
cat file1.txt
cat: file1.txt: No such file or directory
...but the link to the data we manually created still exists, and still points to the data:
cat file2.txt
This is a file.
It points to data on the disk.
As you can see, the data stays on the disk even after the "file" (which is actually a link to the data) is removed. We can still access that data as long as there is a link to it. This is important to know when you're removing files — "removing" a file makes the data inaccessible by unlink-ing it. The data still exists on the storage media, somewhere, inaccessible to the system, and that space on disk is marked as being available for future use.
The type of link we've been working with here is sometimes called a "hard" link. A hard link and the data it links to must always exist on the same filesystem; you can't, for instance, create a hard link on one partition to file data stored on another partition. You also can't create a hard link to a directory. Only symbolic links may link to a directory; we'll get to that in a moment.
The difference between ln and link
So what about ln? That's why we're here, right?
ln, by default, creates a hard link like link does. So this ln command:
ln file1.txt file2.txt
...is the same as the following link command:
link file1.txt file2.txt
...because both commands create a hard link named file2.txt which links to the data of file1.txt.
However, we can also use ln to create symbolic links with the -s option. So the command:
ln -s file1.txt file2.txt
Create a symbolic link to file1.txt named file2.txt. In contrast to our hard link example, here's an illustration to help you visualize our symbolic link:
What are symbolic links?
Symbolic links, sometimes called "soft" links, are different than "hard" links. Instead of linking to the data of a file, they link to another link. So in the example above, file2.txt points to the link file1.txt, which in turn points to the data of the file.
This has several potential benefits. For one thing, symbolic links (also called "symlinks" for short) can link to directories. Also, symbolic links can cross file system boundaries, so a symbolic link to data on one drive or partition can exist on another drive or partition.
You should also be aware that, unlike hard links, removing the file (or directory) that a symlink points to will break the link. So if we create file1.txt:
echo "This is a file." > file1.txt
...and create a symbolic link to it:
ln -s file1.txt file2.txt
...we can cat either one of these to see the contents:
cat file1.txt
This is a file.
cat file2.txt
This is a file.
...but if we remove file1.txt:
rm file1.txt
...we can no longer access the data it contained with our symlink:
cat file2.txt
cat: file2.txt: No such file or directory
This error message might be confusing at first, because file2.txt still exists in your directory. It's a broken symlink, however — a symbolic link which points to something that no longer exists. The operating system tries to follow the symlink to the file that's supposed to be there (file1.txt), but finds nothing, and so it returns the error message.
While hard links are an essential component of how the operating system works, symbolic links are generally more of a convenience. You can use them to refer, in any way you'd like, to information already on the disk somewhere else.
Creating symlinks to directories
To create a symbolic link to a directory, specify the directory name as the target. For instance, let's say we have a directory named documents, which contains one file, named file.txt.
Let's create a symbolic link to documents named dox. This command will do the trick:
ln -s documents/ dox
We now have a symlink named dox which we can refer to as if it is the directory documents. For instance, if we use ls to list the contents of the directory, and then to list the contents of the symlinked directory, they will both show the same file:
ls documents
file.txt
ls dox
file.txt
When we work in the directory dox now, we will actually be working in documents, but we will see the word dox instead of documents in all pathnames.
Symbolic links are a useful way to make shortcuts to long, complicated pathnames. For instance, this command:
...will save us a lot of typing; now, instead of changing directory with the following command:
cd documents/work/budgets/Engineering/2014/April
...we can do this, instead:
cd aprbudge
Normally, you remove directories (once they're empty) with the rmdircommand. But our symbolic link is not actually a directory: it's a file that points to a directory. So to remove our symlink, we use the rm command:
rm aprbudge
This will remove the symlink, but the original directory and all its files are not affected.
Here are the options that can be passed to the ln command.
--backup[=CONTROL]
Use this option to additionally create a backup of each existing destination file. The style of backup is optionally defined by the value of CONTROL. See below for more information.
-b
This functions like --backup, but you cannot specify the CONTROL; the default style (simple) is used.
-d, -F, --directory
This option allows the superuser to attempt to hard link directories (although it will probably fail due to system restrictions, even for the superuser).
-f, --force
If the destination file or files already exist, overwrite them.
-i, --interactive
Prompt the user before overwriting destination files.
-L, --logical
Dereference TARGETs that are symbolic links. In other words, if you are trying to create a link (or a symlink) to a symlink, link to what it links to, not to the symlink itself.
-n, --no-dereference
Treat LINKNAME as a normal file if it is a symbolic link to a directory.
-P, --physical
Make hard links directly to symbolic links, rather than dereferencing them.
-r, --relative
Create symbolic links relative to link location.
-s, --symbolic
Make symbolic links instead of hard links.
-S, --suffix=SUFFIX
Use the file suffix SUFFIX rather than the default suffix "~".
-t, --target-directory=DIRECTORY
Specify the DIRECTORY in which to create the links.
-T, --no-target-directory
Always treat LINKNAME as a normal file.
-v, --verbose
Operate verbosely; print the name of each linked file.
--help
Display a help message, and exit.
--version
Display version information, and exit.
The --backup option
When using the --backup (or -b) option, the default file suffix for backups is '~'. You can change this, however, using the --suffix option or setting the SIMPLE_BACKUP_SUFFIX environment variable.
The CONTROL argument to the --backup option specifies the version control method. Alternatively, it can be specified by setting the VERSION_CONTROL environment variable. Here are the values to use for either one:
none, off
Never make backups (even if --backup is given).
numbered, t
Make numbered backups.
existing, nil
Numbered if numbered backups exist, simple otherwise.
simple, never
Always make simple backups.
If you use -b instead of --backup, the CONTROL method is always simple.
If you specify the -s option (which symlinks), ln ignores the -L and -P options. Otherwise (if you are making hard links), the last option specified controls behavior when a TARGET is a symbolic link. The default is to act as if -P was specified.
Create a hard link to the file public_html/myfile1.txt in the current directory.
ln -s public_html/myfile1.txt
Create a symbolic link to the file public_html/myfile1.txt in the current directory.
ln -s public_html/ webstuff
Create a symbolic link to the directory public_html named webstuff.
ln -s -b file1.txt file2.txt
Creates a symbolic link to the file file1.txt named file2.txt. If file2.txt already exists, it is renamed to file2.txt~ before the new file2.txt symlink is created.
ln (在文件之間創建鏈接，將文件名與文件數據相關聯)
在類似Unix的操作系統上，ln命令在文件之間創建鏈接，將文件名與文件數據相關聯。 本文檔介紹ln的GNU / Linux版本。
On Unix-like operating systems, the ln command creates links between files, associating file names with file data. This document covers the GNU/Linux version of ln.
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1 ln 運行系統環境
2 ln 說明
3 什么是link？
4 ln 語法
5 ln 例子
ln 運行系統環境
Linux
ln 說明
ln創建指向名稱為LINKNAME的文件TARGET的鏈接。如果省略LINKNAME，則在當前目錄中創建指向TARGET的鏈接，并使用TARGET的名稱作為LINKNAME。
ln默認創建硬鏈接，如果指定-s（-- symbolic）選項，則創建符號鏈接。創建硬鏈接時，每個TARGET必須存在。
ln creates a link to file TARGET with the name LINKNAME. If LINKNAME is omitted, a link to TARGET is created in the current directory, using the name of TARGET as the LINKNAME.
ln creates hard links by default, or symbolic links if the -s (--symbolic) option is specified. When creating hard links, each TARGET must exist.
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什么是link？
在討論ln命令之前，讓我們首先討論link命令，以及鏈接是什么以及它與我們所知道的文件之間的關系。
一個鏈接在你的項目文件系統其中一個連接文件名實際字節的數據的上盤。一個以上的文件名可以“鏈接”到相同的數據。這是一個例子。讓我們創建一個名為file1.txt的文件：
該命令回顯字符串 “ This is a file ”。通常，這會回顯到我們的終端，但是>運算符會將字符串的文本重定向到文件，在這種情況下為file1.txt。我們可以使用cat顯示文件內容來檢查它是否起作用：
創建此文件后，操作系統將字節寫入磁盤上的某個位置，還將該數據鏈接到文件名file1.txt，以便我們可以在command和arguments中引用該文件。如果重命名文件，則文件的內容不會更改；僅指向它的信息。文件名和文件數據是兩個單獨的實體。
這是文件名和數據的圖解，以幫助您形象化：
使用鏈接命令
什么是鏈接命令的作用是讓我們手動創建一個鏈接到已存在的文件數據。因此，讓我們使用link創建我們自己的指向最近創建的文件數據的鏈接。本質上，我們將為已經存在的數據創建另一個文件名。
我們將其稱為新鏈接file2.txt。我們如何創建它？
link命令的一般形式是：“ link file_name linkname ”。我們的第一個參數是我們要鏈接到其數據的文件的名稱；第二個參數是我們正在創建的新鏈接的名稱。
現在，file1.txt和file2.txt都指向磁盤上的相同數據：
要意識到的重要一點是我們沒有復制這些數據。這兩個文件名都指向磁盤上相同的數據字節。這是幫助您形象化的例證：
如果我們更改其中一個文件指向的數據的內容，則另一個文件的內容也將更改。讓我們使用>>運算符向其中之一添加一行：
現在讓我們看一下file1.txt的內容：
...，現在讓我們看第二個文件，我們使用鏈接命令創建的文件：
這兩個文件都顯示更改，因為它們共享磁盤上的相同數據。對這些文件之一的數據進行更改將更改另一個文件的內容。
但是，如果我們刪除其中一個文件怎么辦？兩個文件都將被刪除嗎？
否。如果我們刪除其中一個文件，則會刪除到數據的鏈接之一。因為我們手動創建了另一個鏈接，所以仍然有一個指向該數據的指針。我們仍然有一種在用戶級別上訪問我們放入其中的數據的方法。因此，如果我們使用rm命令刪除第一個文件：
...它不再作為具有該名稱的文件存在：
...但是指向我們手動創建的數據的鏈接仍然存在，并且仍然指向該數據：
如您所見，即使在刪除“文件”（實際上是數據的鏈接）之后，數據仍保留在磁盤上。只要有鏈接，我們仍然可以訪問該數據。了解何時刪除文件非常重要-“刪除”文件通過取消鏈接使數據不可訪問。數據仍然存在于系統無法訪問的某個位置的存儲介質上，并且磁盤上的該空間被標記為可供將來使用。
我們在這里使用的鏈接類型有時稱為“硬”鏈接。硬鏈接及其鏈接到的數據必須始終存在于同一文件系統上。例如，您不能在一個分區上創建到存儲在另一分區上的文件數據的硬鏈接。您也無法創建指向目錄的硬鏈接。只有符號鏈接可以鏈接到目錄。我們稍后會解決。
ln和link之間的區別
那么ln呢？那就是為什么我們在這里，對嗎？
ln默認情況下會像link一樣創建一個硬鏈接。因此，此ln命令：
...與以下link命令相同：
...因為這兩個命令都創建了一個名為file2.txt的硬鏈接，該鏈接鏈接到file1.txt的數據。
但是，我們也可以使用ln通過-s選項創建符號鏈接。所以命令：
創建一個名為file2.txt的符號鏈接到file1.txt。與我們的硬鏈接示例相反，以下示例可幫助您可視化我們的符號鏈接：
什么是符號鏈接？
符號鏈接有時也稱為“軟”鏈接，與“硬”鏈接不同。它們沒有鏈接到文件的數據，而是鏈接到另一個link。因此，在上面的示例中，file2.txt指向鏈接file1.txt，而鏈接又指向文件的數據。
這有幾個潛在的好處。一方面，符號鏈接（簡稱“符號鏈接”）可以鏈接到目錄。同樣，符號鏈接可以跨越文件系統邊界，因此到一個驅動器或分區上的數據的符號鏈接可以存在于另一驅動器或分區上。
您還應該意識到，與硬鏈接不同，刪除符號鏈接指向的文件（或目錄）會破壞鏈接。因此，如果我們創建file1.txt：
...并創建指向它的符號鏈接：
...我們能貓其中任一一個看到的內容：
...但是如果我們刪除file1.txt：
...我們無法再通過符號鏈接訪問其包含的數據：
首先，此錯誤消息可能會造成混淆，因為file2.txt仍存在于您的目錄中。但是，這是一個斷開的符號鏈接-指向不再存在的符號鏈接。操作系統嘗試遵循符號鏈接到應該存在的文件（file1.txt），但是什么也沒找到，因此它返回錯誤消息。
雖然硬鏈接是操作系統工作方式的重要組成部分，但符號鏈接通常更方便。您可以使用它們以任何方式引用磁盤上其他地方已經存在的信息。
創建目錄的符號鏈接
要創建指向目錄的符號鏈接，請指定目錄名稱作為目標。例如，假設我們有一個名為documents的目錄，其中包含一個名為file.txt的文件。
讓我們創建一個指向名為dox的documents的符號鏈接。這個命令可以解決問題：
現在，我們有了一個名為dox的符號鏈接，可以將其稱為目錄documents。例如，如果我們使用ls列出目錄的內容，然后列出符號鏈接目錄的內容，則它們都將顯示相同的文件：
現在，當我們在目錄dox中工作時，實際上將在document中工作，但是在所有路徑名中都將看到單詞dox而不是document。
符號鏈接是用于快捷方式生成長而復雜的路徑名的有用方法。例如，此命令：
...將為我們節省很多打字時間；現在，而不是使用以下命令更改目錄：
...我們可以這樣做，而不是：
通常，使用rmdir命令刪除目錄（一旦目錄為空）。但是我們的符號鏈接實際上不是目錄：它是指向目錄的文件。因此，要刪除符號鏈接，我們使用rm命令：
這將刪除符號鏈接，但是原始目錄及其所有文件均不受影響。
Before we discuss the ln command, let's first discuss the link command, as well as what a link is and how it relates to files as we know them.
A link is an entry in your file system which connects a file name to the actual bytes of data on the disk. More than one file name can "link" to the same data. Here's an example. Let's create a file named file1.txt:
This command echoes the string "This is a file". Normally this would echo to our terminal, but the > operator redirects the string's text to a file, in this case file1.txt. We can check that it worked by using cat to display the contents of the file:
When this file was created, the operating system wrote the bytes to a location on the disk and also linked that data to a file name, file1.txt so that we can refer to the file in commands and arguments. If you rename the file, the contents of the file are not altered; only the information that points to it. The file name and the file's data are two separate entities.
Here's an illustration of the file name and the data to help you visualize it:
Using the link command
What the link command does is allow us to manually create a link to file data that already exists. So, let's use link to create our own link to the file data recently created. In essence, we'll create another file name for the data that already exists.
Let's call our new link file2.txt. How do we create it?
The general form of the link command is: "link file_name linkname". Our first argument is the name of the file whose data we're linking to; the second argument is the name of the new link we're creating.
Now both file1.txt and file2.txt point to the same data on the disk:
The important thing to realize is that we did not make a copy of this data. Both file names point to the same bytes of data on the disk. Here's an illustration to help you visualize it:
If we change the contents of the data pointed to by either one of these files, the other file's contents are changed as well. Let's append a line to one of them using the >> operator:
Now let's look at the contents of file1.txt:
... and now let's look at the second file, the one we created with the link command:
Both files show the change because they share the same data on the disk. Changes to the data of either one of these files will change the contents of the other.
But what if we delete one of the files? Will both files be deleted?
No. If we delete one of the files, we're deleting one of the links to the data. Because we created another link manually, we still have a pointer to that data; we still have a way, at the user-level, to access the data we put in there. So if we use the rm command to remove our first file:
...it no longer exists as a file with that name:
...but the link to the data we manually created still exists, and still points to the data:
As you can see, the data stays on the disk even after the "file" (which is actually a link to the data) is removed. We can still access that data as long as there is a link to it. This is important to know when you're removing files — "removing" a file makes the data inaccessible by unlink-ing it. The data still exists on the storage media, somewhere, inaccessible to the system, and that space on disk is marked as being available for future use.
The type of link we've been working with here is sometimes called a "hard" link. A hard link and the data it links to must always exist on the same filesystem; you can't, for instance, create a hard link on one partition to file data stored on another partition. You also can't create a hard link to a directory. Only symbolic links may link to a directory; we'll get to that in a moment.
The difference between ln and link
So what about ln? That's why we're here, right?
ln, by default, creates a hard link like link does. So this ln command:
...is the same as the following link command:
...because both commands create a hard link named file2.txt which links to the data of file1.txt.
However, we can also use ln to create symbolic links with the -s option. So the command:
Create a symbolic link to file1.txt named file2.txt. In contrast to our hard link example, here's an illustration to help you visualize our symbolic link:
What are symbolic links?
Symbolic links, sometimes called "soft" links, are different than "hard" links. Instead of linking to the data of a file, they link to another link. So in the example above, file2.txt points to the link file1.txt, which in turn points to the data of the file.
This has several potential benefits. For one thing, symbolic links (also called "symlinks" for short) can link to directories. Also, symbolic links can cross file system boundaries, so a symbolic link to data on one drive or partition can exist on another drive or partition.
You should also be aware that, unlike hard links, removing the file (or directory) that a symlink points to will break the link. So if we create file1.txt:
...and create a symbolic link to it:
...we can cat either one of these to see the contents:
...but if we remove file1.txt:
...we can no longer access the data it contained with our symlink:
This error message might be confusing at first, because file2.txt still exists in your directory. It's a broken symlink, however — a symbolic link which points to something that no longer exists. The operating system tries to follow the symlink to the file that's supposed to be there (file1.txt), but finds nothing, and so it returns the error message.
While hard links are an essential component of how the operating system works, symbolic links are generally more of a convenience. You can use them to refer, in any way you'd like, to information already on the disk somewhere else.
Creating symlinks to directories
To create a symbolic link to a directory, specify the directory name as the target. For instance, let's say we have a directory named documents, which contains one file, named file.txt.
Let's create a symbolic link to documents named dox. This command will do the trick:
We now have a symlink named dox which we can refer to as if it is the directory documents. For instance, if we use ls to list the contents of the directory, and then to list the contents of the symlinked directory, they will both show the same file:
When we work in the directory dox now, we will actually be working in documents, but we will see the word dox instead of documents in all pathnames.
Symbolic links are a useful way to make shortcuts to long, complicated pathnames. For instance, this command:
...will save us a lot of typing; now, instead of changing directory with the following command:
...we can do this, instead:
Normally, you remove directories (once they're empty) with the rmdircommand. But our symbolic link is not actually a directory: it's a file that points to a directory. So to remove our symlink, we use the rm command:
This will remove the symlink, but the original directory and all its files are not affected.
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ln 語法
選件
這是可以傳遞給ln命令的選項。
--backup選項
使用--backup（或-b）選項時，備份的默認文件后綴為' ? '。但是，您可以使用--suffix選項或設置SIMPLE_BACKUP_SUFFIX 環境變量來更改此設置。
--backup選項的CONTROL參數指定版本控制方法。或者，可以通過設置VERSION_CONTROL環境變量來指定它。這是用于其中任何一個的值：
如果使用-b而不是--backup，則CONTROL方法始終很簡單。
如果指定-s選項（符號鏈接），則ln忽略-L和-P選項。否則（如果您正在建立硬鏈接），則當TARGET是符號鏈接時，指定的最后一個選項控制行為。缺省情況是充當-P被指定。
Options
Here are the options that can be passed to the ln command.
The --backup option
When using the --backup (or -b) option, the default file suffix for backups is '~'. You can change this, however, using the --suffix option or setting the SIMPLE_BACKUP_SUFFIX environment variable.
The CONTROL argument to the --backup option specifies the version control method. Alternatively, it can be specified by setting the VERSION_CONTROL environment variable. Here are the values to use for either one:
If you use -b instead of --backup, the CONTROL method is always simple.
If you specify the -s option (which symlinks), ln ignores the -L and -P options. Otherwise (if you are making hard links), the last option specified controls behavior when a TARGET is a symbolic link. The default is to act as if -P was specified.
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ln 例子
在當前目錄中創建指向文件public_html / myfile1.txt的硬鏈接。
在當前目錄中創建指向文件public_html / myfile1.txt的符號鏈接。
創建一個符號鏈接到名為webstuff的目錄public_html。
創建指向文件file1.txt的符號鏈接，名為file2.txt。如果file2.txt已經存在，則在創建新的file2.txt符號鏈接之前將其重命名為file2.txt?。
Create a hard link to the file public_html/myfile1.txt in the current directory.
Create a symbolic link to the file public_html/myfile1.txt in the current directory.
Create a symbolic link to the directory public_html named webstuff.
Creates a symbolic link to the file file1.txt named file2.txt. If file2.txt already exists, it is renamed to file2.txt~ before the new file2.txt symlink is created.
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其他命令行
link | ldd | last | login | ls | lsmod | lpstat | losetup | lprm | lpr | lpq | lp | lpc | locate | logname | lpadmin |