bash vs sh

Recently I came across some shell scripting errors:

sh: [[ not found error sh: Syntax error: unexpected ')'

I found that error weird, because I ran that script locally and all worked out fine. So that led me on a google search for the differences between bash and sh.

First let's see what we get when we use which on them

$ which sh /bin/sh $ which bash /bin/bash

Ooo, so my first thought was: huh?! they are different things?! I somehow always thought that there were synonymous.

First we have to understand shell

Shell is an interface between a user and the OS.

• It is usually through GUI or CLI.

sh (Bourne shell) is a command line interpreter for Unix OS.

• programming language described by POSIX standards
• have many implementations (bash, ash, k shell, z shell)
• denoted by the shebang: #!/bin/sh
• can be ran with sh script.sh

bash (Bourne again shell) is a shell replacement for Bourne shell.

• it is a superset of sh.
• it supports sh
• denoted by the shebang: #!/bin/bash
• can be ran with bash script.sh

However there are differences between bash and sh:

• [[ is available in bash but not in sh, only [
• sh does not have arrays
• bash keywords like local, source, let, declare, select is not portable to sh

Ok then what about zsh

So the first thing I did on my mac was to download zsh. I didn't even properly know what that was till now.

It is the Z shell, extended from sh with improvements, containing features of bash, ksh, tcsh.

Running the script

sh script.sh bash script.sh

This runs the script using sh / bash interpreter respectively. Does not require executable bit.

. ./script.sh

First dot is a source. Runs the script in current shell process. Does not require executable bit.

./script.sh

Uses the shebang to determine how to execute. Requires executable bit (`chmod u+x script.sh) in order to run. Else you'd be left with permission denied.

Wow, mind boggled. So cool! Took many resources from various google searches / stackoverflows.