The baffling source and sh

Hello everyone!

Ever tried source and sh on some script and it worked with source but not with sh? I had this trouble sometime back. I was working on a python project and was supposed to activate virtual environment before starting off. However, not being very habitual of using virtual environments (I know its a bad practice, I take care now.), I did not remember how to activate it. Though, I knew there was an activate script that needed to be run, so I located it and did

sh activate

Didn’t work. Hmm. :/ Permission issues.

chmod +x activate

Still won’t work. Then, I looked up the contents of file, it said,

# This file must be used with "source bin/activate" *from bash*
# you cannot run it directly

This made me look for differences between some very basic utilities I use everyday.

  • ./

The file can be a binary executable, or it can start with a shebang line.

When you run ./executable, what shell will be run essentially depends on the shebang written at the beginning of the executable script.

  • sh

sh initiates a sub-process that runs a new session of /bin/sh, which is usually a symbolic link to bash .

  • .
. filename [arguments]
source filename [arguments]
       Read  and  execute  commands  from filename in the current shell
       environment and return the exit status of the last command  exe
       cuted from filename.  If filename does not contain a slash, file
       names in PATH are used to find the  directory  containing  file
       name.   The  file  searched  for in PATH need not be executable.
       When bash is  not  in  posix  mode,  the  current  directory  is
       searched  if no file is found in PATH.  If the sourcepath option
       to the sh opt builtin command is turned  off,  the  PATH  is  not
       searched.   If any arguments are supplied, they become the posi
       tional parameters when  filename  is  executed.   Otherwise  the
       positional  parameters  are unchanged.  The return status is the
       status of the last command exited within the  script  (0  if  no
       commands  are  executed),  and false if filename is not found or
       cannot be read.
  • source

When a script is run using source it runs within the existing shell, any variables created or modified by the script will remain available after the script completes.

$ type source
source is a shell built-in

There’s no difference between source and . (this is not a full stop)

So, that’s what happened. The script activate remained inside the same shell only when invoked with source (or .) which is exactly what was needed.

Thanks for reading.



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