* Added backward compatibility/support for python 2.3 or older.

* Added more general-purpose (some coreutils, grep, less, etc.) Linux commands
  into the namespace for ease of use in regular python.

shell_tools.py

@@ -1,4 +1,4 @@
-# $Id: shell_tools.py,v 1.3 2010-01-20 17:25:54 wirawan Exp $
+# $Id: shell_tools.py,v 1.4 2010-01-20 21:27:41 wirawan Exp $
 #
 # wpylib.shell_tools
 # Created: 20100106
@@ -9,30 +9,35 @@
 #
 
 import os
-import subprocess
+import os.path
 import sys
 
+try:
+  import subprocess
+  has_subprocess = True
+except:
+  if "has_subprocess" not in globals():
+    print >>sys.stderr, "Newer subprocess module does not exist, using older interfaces."
+  has_subprocess = False
+
 def mcd(subdir):
   # Assuming we have GNU coreutils' mkdir
-  cmd = ["mkdir", "-p", subdir]
+  mkdir("-p", subdir)
-  try:
-    retcode = subprocess.call(cmd, shell=False)
-    if retcode == 0:
   os.chdir(subdir)
-      return
 
-    print >>sys.stderr, "mcd " + subdir + ": ",
-    if retcode < 0:
-      print >>sys.stderr, "mkdir was terminated by signal", -retcode
-    else:
-      print >>sys.stderr, "mkdir returned", retcode
-    raise RuntimeError, "Directory creation failure"
-  except OSError, e:
-    print >>sys.stderr, "mcd failed:", e
-    raise
 
+def provide_file(dest, src):
+  """Checks if file `dest' exists. If it does not, provide for it by means
+  of a softlink from `src'."""
+  if not os.path.exists(dest):
+    # strip trailing /'s just in case it exists
+    os.symlink(src, dest.rstrip("/"))
+
+
+# Low-level utilities:
 
 def errchk(cmd, args, retcode):
+  """Checking for error after the invocation of an external command."""
   if retcode == 0: return
 
   print >>sys.stderr, "Error executing ", cmd, " ".join(args)
@@ -43,12 +48,30 @@ def errchk(cmd, args, retcode):
   raise RuntimeError, err
 
 
+def quote_cmdline(seq):
+  """Quotes the strings in seq for feeding to shell.
+  This is a severe protection to prevent:
+  - variable, command, or other substitutions
+  - shell expansions (parameter, wildcard)
+  - word splitting
+  - invocation of shell builtin (!!!)
+  """
+  # Python 2.6's subprocess.py has list2cmdline, but I don't like it because
+  # it still allows the shell to interpret wildcards. We have to quote wildcards
+  # (*, [], {}, ?) and $ as well.
+  rslt = []
+  for i in seq:
+    inew = '"' + i.replace("\\", "\\\\").replace('"', '\\"').replace('$', '\\$').replace('`', '\\`') + '"'
+    rslt.append(inew)
+  return " ".join(rslt)
+
+if has_subprocess:
+
   def run(prg, args):
-  retcode = subprocess.call((prg,) + args)
+    retcode = subprocess.call((prg,) + tuple(args))
     errchk(prg, args, retcode)
     return 0
 
-
   def pipe_out(args, split=False, shell=False):
     """Executes a shell command, piping out the stdout to python for parsing.
     This is my customary shortcut for backtick operator.
@@ -60,9 +83,51 @@ def pipe_out(args, split=False, shell=False):
     else:
       return retval.splitlines()
 
+else:
+
+  def run(prg, args=()):
+    # Python < 2.4 does not have subprocess, so we use spawnvp
+    retcode = os.spawnvp(os.P_WAIT, prg, (prg,) + tuple(args))
+    errchk(prg, args, retcode)
+    return 0
+
+  def pipe_out(args, split=False, shell=False):
+    """Executes a shell command, piping out the stdout to python for parsing.
+    This is my customary shortcut for backtick operator.
+    The result is either a single string (if split==False) or a list of strings
+    with EOLs removed (if split==True)."""
+    if shell or isinstance(args, str):
+      # BEWARE: args should be a string in this case
+      p = os.popen(args, "r")
+    else:
+      args = quote_cmdline(args)
+      p = os.popen(args, "r")
+    retval = p.read()
+    status = p.close()
+    if not split:
+      return retval
+    else:
+      return retval.splitlines()
 
-# coreutils
 
+# coreutils
+# and other common utilities
+
+CMD = ['cat', 'cp', 'head', 'grep', 'less', 'ls', 'mkdir', 'mv', 'rm', 'tail']
+CMD_NAME = {}
+for n in CMD:
+  CMD_NAME[n] = n
+  s = """def %(cmd)s(*args): run(CMD_NAME['%(cmd)s'], args)"""
+  exec(s % {'cmd': n })
+
+def import_commands(namespace, cmds=None):
+  if cmds == None: cmds = CMD
+  thismod = globals()
+  for n in cmds:
+    n_act = thismod[n]
+    namespace.setdefault(n, n_act)
+
+"""
 def cp(*args):
   run('cp', args)
 
@@ -71,4 +136,5 @@ def mkdir(*args):
 
 def mv(*args):
   run('mv', args)
+"""