Bash Shell – Difference Between Using Bash and sh to Run a Script

bashshell

Does bash and sh execute a script differently? I executed a shell script with both bash and sh, and got the same output. So what is the difference?

Also, when you just run ./executable, does it use bash or sh?

Best Answer

That depends on the system you are running. One many OSes, especially Linux based ones, sh is a link to bash.

In such case, there are still some differences in behavior where bash try to be more like traditional bourne shell when called sh, but it still accepts most bashisms.

On some other OSes, like Debian based ones, sh is provided by dash, not bash. That makes a much bigger difference as dash doesn't support bashisms, being designed to be a clean POSIX shell implementation.

On proprietary OSes, sh is often either provided by a POSIX compliant ksh88 which like dash, doesn't implement bashisms. On Solaris 10 and older, depending on what your PATH is, sh will likely be a legacy Bourne shell, predating POSIX.

In any case, you likely got the same output with your test simply because your script was not using any bash specific command, option or syntax.

When you run ./executable, what shell will be run essentially depends on the shebang written at the beginning of the .executable script. That will be bash if the shebang specifies it:

#!/bin/bash
....

If there is no shebang and you call the script from a POSIX compliant shell, the script should technically be executed by the first sh found in the PATH. Many shell interpreters like bash, dash and ksh are considering themselves to be POSIX so will interpret the script. Note that the SHELL environment variable is not used here.

Related Solutions

Bash – Difference between sh -x and ./ for script execution

The ./foobar.sh file is started by running it with whatever follows the #! in the first line. If this line reads #!/bin/sh -x then it would be identical to the sh -x foobar.sh case (assuming sh is resolved to /bin/sh from the PATH). Maybe it is not started by sh but bash?

The -x flag prints debug info, i.e. every command before it is executed.

Bash – what is the difference between “./command” and “bash command”

There is a BIG difference.

Lets take the following script called testscript (configured to use /bin/ksh as you see in the hashbang):

#!/bin/ksh
#im testscript
cd /proc/$$
file exe

First lets execute it with ./:

$ ./testscript
exe: symbolic link to /bin/ksh93

Now calling bash:

$ bash testscript
exe: symbolic link to /bin/bash

Do you see the difference? The interpreter used by the script changed in the second command, so if the script depends in some ksh specific code it will be broken, and in other way if the scripts depends on bash specific code, then will broke it with the first command.

Other test you can do is to echo $PATH in the two different cases of execution.

Best Answer

Related Solutions

Bash – Difference between sh -x and ./ for script execution

Bash – what is the difference between “./command” and “bash command”

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