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Generally, shell scripts contain the following comment at the first line of the script file: #!/bin/sh. According to the researches that I made, this is called "hash bang" and it is conventional comment. This comment informs Unix that this file is executed by the Bourne Shell under the directory /bin.

My question begins in that point. Up to now I have not seen this comment like #!/bin/bash. It is always #!/bin/sh. However, Ubuntu distributions do not have the Bourne Shell program. They have the Bourne Again Shell (bash).

In that point, is it correct to place the comment #!/bin/sh in shell scripts written in Ubuntu distributions?

Yes you can use #!/bin/sh in a script because /bin/sh is (hopefully) provided for on such systems, usually via a link of some sort that makes bash behave (more or less) like a sh would. Here's a Centos7 system, for example, that links sh to bash:

-bash-4.2$ ls -l /bin/sh

lrwxrwxrwx 1 root root 4 Dec  4 16:48 /bin/sh -> bash

-bash-4.2$ 

You could also use #!/bin/bash if you are writing a bash script only for that system and want to use bash features. However, such scripts will suffer from portability problems, for example on OpenBSD where bash is only installed if the admin takes the trouble to install it (I do not) and then it is installed to /usr/local/bin/bash, not /bin/bash. A strictly POSIX #!/bin/sh script should be more portable.

#!/bin/sh should work on all Unix and Unix-like distributions. It is generally thought of as the most portable hashbang so long as your script is kept POSIX compliant.

#!/bin/sh is normally just a link now as the Bourne shell is no longer maintained. On many Unix systems /bin/sh will be a link to /bin/ksh, on many Linux systems it will be a link to /bin/bash (bash invoked as sh is equivalent to running bash --posix) however on Ubuntu it is a link to /bin/dash.

It is an important placeholder though because it allows for much greater portability than other methods, so long as your script is strictly POSIX compliant (repeated to stress importance).

Note: When bash is invoked in POSIX mode it will still allow some non-POSIX things like [[, arrays, and more. Those things will fail on a non-bash system.

You asked

is it correct to place the comment #!/bin/sh in shell scripts written in ubuntu distributions ?

The answer depends on what you write in the shell script.

• If you strictly use portable POSIX-compliant scripts, and don't use any bash-specific commands, then you can use /bin/sh.




• If you know that you are only ever using the script on a machine with bash, and you want to use bash-specific syntax, then you should use /bin/bash




• If you're want to be sure that the script will work on an assortment of unix machines, then you should use only POSIX-compliant syntax, and /bin/sh




• If you regularly use another shell (e.g. ksh, zsh or tcsh), and want to use that syntax in your script, then you should use the appropriate interpreter (like /bin/ksh93, /bin/zsh, or /bin/tcsh)

The "#!" comment does not always use /bin/bash or /bin/sh. It just lists whatever the interpreter should be, not just for shell scripting. For example my python scripts usually start with #!/usr/bin/env python.

Now the difference between #!/bin/sh and #!/bin/bash is that /bin/sh is not always a symlink to /bin/bash. Often but not always. Ubuntu is a notable exception here. I have seen scripts working fine on CentOS but failing on Ubuntu because author used bash-specific syntax with #!/bin/sh.