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

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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 shell script into the current shell 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 often a symbolic link to bash). In this case, environment variables set by the sub-script would be dropped when the sub-script terminate.

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. 😉

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