What is the difference between using `sh` and `source`?

Question

What is the difference between sh and source?

source: source filename [arguments]     Read and execute commands from FILENAME and return.  The pathnames     in $PATH are used to find the directory containing FILENAME.  If any     ARGUMENTS are supplied, they become the positional parameters when     FILENAME is executed. 

And for man sh:

NAME        bash - GNU Bourne-Again SHell  SYNOPSIS        bash [options] [file]  COPYRIGHT        Bash is Copyright (C) 1989-2004 by the Free Software Foundation, Inc.  DESCRIPTION        Bash  is  an sh-compatible command language interpreter that executes commands read from the standard input or from a file.  Bash also incorporates        useful features from the Korn and C shells (ksh and csh).         Bash is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2). 

Answer 1

When you call source or . (the one is an alias to the other. source cmd not POSIX - kind of bashism), you load and execute a bash script into the current bash process. So you can

• read variables set in the sourced script,
• use functions defined within it.
• and even execute forks and/or subprocess if script do this.

When you call sh, you initiate a fork (sub-process or child) that runs a new session of /bin/sh, which is usually a symbolic link to bash. In this case, environment variables set by the sub-script would be dropped when the sub-script finishes.

Caution: sh could be a symlink to another shell.

Practical sample

For example, if you want to change current working directory by a specific manner, you could not do

$ cat <<eof >myCd2Doc.sh #!/bin/sh cd /usr/share/doc eof  $ chmod +x myCd2Doc.sh 

This won't do what you expect:

$ cd /tmp $ pwd /tmp $ ~/myCd2Doc.sh $ pwd /tmp 

because current working dir is part of environment and myCd2Doc.sh would run in a subshell.

But:

$ cat >myCd2Doc.source <<eof # Shell source file myCd2Doc() {     cd /usr/share/doc } eof  $ . myCd2Doc.source $ cd /tmp $ pwd /tmp $ myCd2Doc $ pwd /usr/share/doc 

Have a look at mycd function!! (With bash completion based on Associative Array).

Execution level $SHLVL

$ cd /tmp printf %b '\43\41/bin/bash\necho This is level \44SHLVL.\n' >qlvl.sh  $ bash qlvl.sh  This is level 2.  $ source qlvl.sh  This is level 1. 

Recursion (when a script run from itself)

$ cat <<eoqlvl2 >qlvl2.sh  #!/bin/bash  export startLevel recursionLimit=5 echo This is level $SHLVL started:${startLevel:=$SHLVL}. (( SHLVL < recursionLimit )) && ./qlvl2.sh eoqlvl2 $ chmod +x qlvl2.sh  $ ./qlvl2.sh  This is level 2 started:2. This is level 3 started:2. This is level 4 started:2. This is level 5 started:2.  $ source qlv2.sh  This is level 1 started:1. This is level 2 started:1. This is level 3 started:1. This is level 4 started:1. This is level 5 started:1. 

A little futher

$ sed '$a ps --sid $SID fw' qlvl.sh >qlvl3.sh $ chmod +x qlvl3.sh  $ export SID $ read SID < <(ps ho sid $$) $ echo $SID $$ 8983 8983 

( Current PID ($$ == process Id) are same identifier than SID (session ID). It's not alway true.)

$ ./qlvl3.sh  This is level 2.   PID TTY      STAT   TIME COMMAND  8983 pts/10   Ss     0:00 /bin/bash 10266 pts/10   S+     0:00  \_ /bin/bash ./qlvl3.sh 10267 pts/10   R+     0:00      \_ ps --sid 8983 fw  $ . qlvl3.sh  This is level 1.   PID TTY      STAT   TIME COMMAND  8983 pts/10   Ss     0:00 /bin/bash 10428 pts/10   R+     0:00  \_ ps --sid 8983 fw 

Dot . is an alias of source. So the only difference between two command are slash replaced by space.

And a final test:

$ printf %b '\43\41/bin/bash\necho Ending this.\nsleep 1;exit 0\n' >finalTest.sh  $ bash finalTest.sh  Ending this.  $ source finalTest.sh Ending this. 

... You may notice a different behaviour between the two syntaxes. ;-)