What are syntax differences between bash and dash?

Question

I just realized that in current Ubuntu version /bin/sh is no more symlinked to bash (as it used to be for many years), but to dash instead. I wonder what are the actual syntax differences between those two shells and how big is the probability that a shell script written with bash in mind won't work under dash. Can anybody point me to a good and clear description of differences between these two?

Does this answer your question? What is the point of sh being linked to dash? — Nmath, Sep 26 '20 at 14:18
DashAsBinSh is a good start; a more thorough reference for a POSIX compliant shell is The Open Group Base Specifications Issue 6: Shell Command Language — steeldriver, Sep 26 '20 at 14:24
@Nmath: None of the answers there does clearly specify differences between those shells. What I would like is some form of a detailed list of bash commands or other syntax features that are not supported in dash. — raj, Sep 26 '20 at 14:25
Don't know about dash, but at least the root user should initially have a shell with very minimal requirements to hardware and software. In case of emergency, a fat shell might not be able to even start up. — Gyro Gearloose, Sep 26 '20 at 14:30
Don't know dash, but use fish and most bash commands work. Most differences are when combining commands with &&. Probably a few others. — crip659, Sep 26 '20 at 15:17
FYI if you want to test a particular script for sh/POSIX compatibility, the devscripts package provides a checkbashisms script for that — steeldriver, Sep 26 '20 at 17:18
Does this answer your question? What is the difference between #!/bin/sh and #!/bin/bash? — KK Patel, Oct 11 '20 at 07:10
@KK Patel the link you specified is totally unrelated to my question. I obviously understand that bash and dash are two different shells. I was asking about the exact set of syntax differences. — raj, Oct 11 '20 at 22:02

score 8 · Answer 1 · answered Sep 26 '20 at 18:39

A simple rule of thumb is: if your script was written in bash, do not assume it will work in dash. A full list of differences is beyond the scope of a simple Q&A, but essentially, dash is a POSIX shell, so it implements what is described in the POSIX specification for the shell language and only that.

Here are the common bashisms I most often fall afoul of:

[[: the [[ condition ]] construct isn't supported by dash, you need to use [ ] instead.
== : to test if two values are equal, use = in dash since == is not supported.
source: the POSIX command for sourcing a script is .. The source builtin is a bash alias to the standard ., so always use . file instead of source file.
shopt: this is a bash builtin that sets certain non-standard options. Not supported by dash.
$RANDOM: this is set to a random number on each use in bash, but doesn't work in dash.

By far the most common problem is the lack of [[ support. You can find a more comprehensive list on the Ubuntu Wiki: https://wiki.ubuntu.com/DashAsBinSh

as a supplementation, 'function' is a keyword in bash to declare a function but not available in dash. check description here https://stackoverflow.com/questions/7917018/what-is-the-function-keyword-used-in-some-bash-scripts — Kevin Chan, Jun 23 '22 at 02:01
Believe it or not, until today I've been Google searching for bourne shell instead of dash any time I'm trying to find supported syntax by /bin/sh — Sridhar Sarnobat, Nov 04 '22 at 22:36

What are syntax differences between bash and dash?

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