How to create a symbolic link with SCons? - scons

Question

I'm using SCons for building a project and need to add a symbolic link to a file it is installing via env.Install. What command(s) will make a link that's the equivalent of running ln -s on the command line?

Answer 1

SCons doesn't have a dedicated symbolic link command, but you can use os.symlink(src, dst) from Python's os module:
import os
env = Environment()
def SymLink(target, source, env):
os.symlink(os.path.abspath(str(source[0])), os.path.abspath(str(target[0])))
env.Command("file.out", "file.in", SymLink)
This may not work correctly on Windows, I've only tried it on Linux.

Answer 2

There seems to be little advancement in the SCons core code for symbolic link support and I wasn't satisfied any one solution I found on the web. Here is a potential builder which incorporates aspects of both Nick's and richq's answers. Additionally, it will catch name changes (due to the emitter method) and is as platform-agnostic as I could get it.
I prefer this builder because it will make links relative to the directory in which they are installed. One could add an option to force the link to be absolute I suppose, but I have not needed or wanted that yet.
Currently, if the OS doesn't support symlinks, I just pass and do nothing, but one could use os.copytree() for example however the dependency becomes messy if the source is a directory so the emitter would need to do something fancy. I'm up for any suggestions here.
One can put the following code into the file site_scons/site_tools/symlink.py (with blank _init_.py files in the appropriate places). Then do this in the SConstruct file:
SConstruct:
env = Environment()
env.Tool('symlink')
env.SymLink('link_name.txt', 'real_file.txt')
symlink.py:
import os
from os import path
from SCons.Node import FS
from SCons.Script import Action, Builder
def generate(env):
'''
SymLink(link_name,source)
env.SymLink(link_name,source)
Makes a symbolic link named "link_name" that points to the
real file or directory "source". The link produced is always
relative.
'''
bldr = Builder(action = Action(symlink_builder,symlink_print),
target_factory = FS.File,
source_factory = FS.Entry,
single_target = True,
single_source = True,
emitter = symlink_emitter)
env.Append(BUILDERS = {'SymLink' : bldr})
def exists(env):
'''
we could test if the OS supports symlinks here, or we could
use copytree as an alternative in the builder.
'''
return True
def symlink_print(target, source, env):
lnk = path.basename(target[0].abspath)
src = path.basename(source[0].abspath)
return 'Link: '+lnk+' points to '+src
def symlink_emitter(target, source, env):
'''
This emitter removes the link if the source file name has changed
since scons does not seem to catch this case.
'''
lnk = target[0].abspath
src = source[0].abspath
lnkdir,lnkname = path.split(lnk)
srcrel = path.relpath(src,lnkdir)
if int(env.get('verbose',0)) > 3:
ldir = path.relpath(lnkdir,env.Dir('#').abspath)
if rellnkdir[:2] == '..':
ldir = path.abspath(ldir)
print ' symbolic link in directory: %s' % ldir
print ' %s -> %s' % (lnkname,srcrel)
try:
if path.exists(lnk):
if os.readlink(lnk) != srcrel:
os.remove(lnk)
except AttributeError:
# no symlink available, so we remove the whole tree? (or pass)
#os.rmtree(lnk)
print 'no os.symlink capability on this system?'
return (target, source)
def symlink_builder(target, source, env):
lnk = target[0].abspath
src = source[0].abspath
lnkdir,lnkname = path.split(lnk)
srcrel = path.relpath(src,lnkdir)
if int(env.get('verbose',0)) > 4:
print 'target:', target
print 'source:', source
print 'lnk:', lnk
print 'src:', src
print 'lnkdir,lnkname:', lnkdir, lnkname
print 'srcrel:', srcrel
if int(env.get('verbose',0)) > 4:
print 'in directory: %s' % path.relpath(lnkdir,env.Dir('#').abspath)
print ' symlink: %s -> %s' % (lnkname,srcrel)
try:
os.symlink(srcrel,lnk)
except AttributeError:
# no symlink available, so we make a (deep) copy? (or pass)
#os.copytree(srcrel,lnk)
print 'no os.symlink capability on this system?'
return None

Answer 3

This creates a builder to perform the job:
mylib = env.SharedLibrary("foobar", SRCS)
builder = Builder(action = "ln -s ${SOURCE.file} ${TARGET.file}", chdir = True)
env.Append(BUILDERS = {"Symlink" : builder})
mylib_link = env.Symlink("_foobar.so", mylib)
env.Default(mylib)
env.Default(mylib_link)
Again, this solution is for Linux.

Answer 4

If you wanted to issue the command directly to the shell and know the OS, subprocess can be used as well.
E.g.: subprocess.call(['ln', '-s', '</src/path>', '</dest/path>'])

Answer 5

In addition to Nicks solution, you can add a directory symlink by using a file as a directory name carrier. It's not the cleanest solution and debugging path names is a pain, but this works well:
def symlink_last(target_source_env):
src = os.path.basename(os.path.dirname(str(source[0])))
link = "deliverables/last"
print "Symlinking "+ src + "as" + link
os.symlink(src, link)
BUILD_TARGETS.append('link')
install_dir = "deliverables/subdir"
carrier_file = "filename"
builder = Builder(action = symlink_last, chdir=False)
env.Append(BUILDERS={ "Symlink" : builder })
env.Alias(target="link", source=env.Symlink(dir="deliverables", source = install_dir + carrier_file)
This will make a link to deliverables/subdir named deliverables/last, provided that a file deliverables/subdir/filename exists.