Differences between declare, typeset and local variable in Bash

Question

When typing variables in Bash, what is the difference between declare and typeset? When used inside a function: what is the difference between declare and typeset and local?

The only difference I have come across is that typeset is portable to ksh scripts. Other than that, are there any reasons why one should be preferred over the other?

UPDATE: Added local to the question.

Solution 1

• Difference between typeset and declare:

The former is more portable(e.g. ksh), while the latter is more preferable when portability is not a concern.

• Difference between declare(or typeset) and local when used inside a function:

The former implies the latter, but more powerful. For example, declare -i x makes x have the integer attribute, declare -r x makes x readonly, etc.

Solution 2

As far as bash is concerned, no, there is no difference. In fact, the manpage has them share the same definition

declare [-aAfFilrtux] [-p] [name[=value] ...]
typeset [-aAfFilrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are given then display the values of variables. The -p option will display the attributes and values of each name...

I also found this little tidbit which further substantiates my claim as well as the ksh portability you mentioned.

The declare or typeset builtins, which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the typing [1] available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts.

Solution 3

In the Bash manual under section 4.2 Bash Builtin Commands it states:

'typeset'
typeset [-afFrxi] [-p] [NAME[=VALUE] ...]
The 'typeset' command is supplied for compatibility with the Korn shell; however, it has been deprecated in favor of the 'declare' builtin command.

Comments

[lecodesportif]

"help typeset" tells me that it's obsolete. Deprecated in favor of "declare".

[ormaaj]

local and declare are mostly identical and take all the same arguments with two exceptions: local will fail if not used within a function, and local with no args filters output to print only locals, declare doesn't.

[Hui Zheng]

@ormaaj you're right, I didn't realize that local supports options too. Thanks a lot.

[osirisgothra]

this is a good example of the problems in the bash documentation and theres inconsistency with command line help (using the 'help' command) and the manual pages, (using 'man bash' and going to SHELL BUILTINS) and the builtin page (using 'man builtins'). There are also some typos in there, so be careful

[Alexander Mills]

so a variable declared with typeset or declare is local? seems like that question is the most obvious question to answer.

[Aethalides]

@AlexanderMills Using declare or typeset without the -g modifier inside a function will declare a local variable. local will of course also create a local variable. Any other method of declaring a variable inside a function will create a global variable.

[Roland]

"When used in a function, declare makes each name local, as with the local command, unless the -g option is used." from gnu.org/software/bash/manual/html_node/Bash-Builtins.html

[Paddy Landau]

I know that this is old, but it's useful to note that within a function, declare and local are identical (apart from using local without parameters). If you use local -g, the variable will counterintuitively be global! For me, the only useful distinction is visual, so that the human reader knows that local is intended to be local, whereas declare is intended to be used with -g.