在python中修改符号链接

Question

How do I change a symlink to point from one file to another in Python?

The os.symlink function only seems to work to create new symlinks.

Answer 1

If you need an atomic modification, unlinking won't work.

A better solution would be to create a new temporary symlink, and then rename it over the existing one:

os.symlink(target, tmpLink)
os.rename(tmpLink, linkName)

You can check to make sure it was updated correctly too:

if os.path.realpath(linkName) == target:
    # Symlink was updated

According to the documentation for os.rename though, there may be no way to atomically change a symlink in Windows. In that case, you would just delete and re-create.

Answer 2

A little function for Python2 which tries to symlink and if it fails because of an existing file, it removes it and links again. Check [Tom Hale's answer] for a up-to-date solution.

import os, errno

def symlink_force(target, link_name):
    try:
        os.symlink(target, link_name)
    except OSError, e:
        if e.errno == errno.EEXIST:
            os.remove(link_name)
            os.symlink(target, link_name)
        else:
            raise e

For python3 except condition should be except OSError as e:
[Tom Hale's answer]: https://stackoverflow.com/a/55742015/825924

Answer 3

Given overwrite=True , this function will safely overwrite an existing file with a symlink.

It is cognisant of race conditions, which is why it is not short, but it is safe.

import os, tempfile

def symlink(target, link_name, overwrite=False):
    '''
    Create a symbolic link named link_name pointing to target.
    If link_name exists then FileExistsError is raised, unless overwrite=True.
    When trying to overwrite a directory, IsADirectoryError is raised.
    '''

    if not overwrite:
        os.symlink(target, link_name)
        return

    # os.replace() may fail if files are on different filesystems
    link_dir = os.path.dirname(link_name)

    # Create link to target with temporary filename
    while True:
        temp_link_name = tempfile.mktemp(dir=link_dir)

        # os.* functions mimic as closely as possible system functions
        # The POSIX symlink() returns EEXIST if link_name already exists
        # https://pubs.opengroup.org/onlinepubs/9699919799/functions/symlink.html
        try:
            os.symlink(target, temp_link_name)
            break
        except FileExistsError:
            pass

    # Replace link_name with temp_link_name
    try:
        # Pre-empt os.replace on a directory with a nicer message
        if not os.path.islink(link_name) and os.path.isdir(link_name):
            raise IsADirectoryError(f"Cannot symlink over existing directory: '{link_name}'")
        os.replace(temp_link_name, link_name)
    except:
        if os.path.islink(temp_link_name):
            os.remove(temp_link_name)
        raise

Notes for pedants:

1. If the function fails (eg computer crashes), an additional random link to the target might exist.

2. An unlikely race condition still remains: the symlink created at the randomly-named temp_link_name could be modified by another process before replacing link_name .

I raised a python issue to highlight the issues of os.symlink() requiring the target to not exist, where I was advised to raise my suggestion on the python-ideas mailing list

Credit to Robert Siemer's input .

Answer 4

您可以os.unlink()它，然后使用os.symlink()重新创建以指向新目标。

Answer 5

I researched this question recently, and found out that the best way is indeed to unlink and then symlink . But if you need just to fix broken links, for example with auto-replace, then you can do os.readlink :

for f in os.listdir(dir):
    path = os.path.join(dir, f)
    old_link = os.readlink(path)
    new_link = old_link.replace(before, after)
    os.unlink(path)
    os.symlink(new_link, path)

Answer 6

Don't forget to add a raise command in the case when e.errno != errno.EEXIST You don't want to hide an error then:

if e.errno == errno.EEXIST:
     os.remove(link_name)
     os.symlink(target, link_name)
else:
    raise

Answer 7

A quick and easy solution:

while True:
     try:
         os.symlink(target, link_name)
         break
     except FileExistsError:
         os.remove(link_name)

However this has a race condition when replacing a symlink which should always exist, eg:

 /lib/critical.so -> /lib/critical.so.1.2

When upgrading by:

 my_symlink('/lib/critical.so.2.0', '/lib/critical.so')

There is a point in time when /lib/critical.so doesn't exist.

This answer avoids the race condition.