Bash Builtins – What Do ‘set’ and ‘export’ Do?

bashshell

I am at a bit of a loss as to the purpose of set and export in Bash (and I guess probably other shells too).

I would think it is for setting environment variables, but that can be done just with VARIABLE=VALUE, right?

Also typing set and export on their own show different values.

So what is their purpose?

Best Answer

export exports to children of the current process, by default they are not exported. For example:

$ foo=bar
$ echo "$foo"
bar
$ bash -c 'echo "$foo"'

$ export foo
$ bash -c 'echo "$foo"'
bar

set, on the other hand, sets shell attributes, for example, the positional parameters.

$ set foo=baz
$ echo "$1"
foo=baz

Note that baz is not assigned to foo, it simply becomes a literal positional parameter. There are many other things set can do (mostly shell options), see help set.

As for printing, export called with no arguments prints all of the variables in the shell's environment. set also prints variables that are not exported. It can also export some other objects (although you should note that this is not portable), see help export.

Related Solutions

Bash – How to Set Environment Variables Dynamically

export $key=$val should work just fine in bash. I suspect your issue is the pipe (|). All commands in a pipeline are executed in a subshell. In your example, the instance of bash that is running your script will fork off 2 subshells: one for cat $1 and another for the while read .... The variables are assigned and exported in the subshell running the while loop, and then all of them are promptly thrown out when the subshell exits. One way to fix this is to not spawn subshells at all. Instead of a useless use of cat, try redirection instead:

while read ...; do
   ...
done < "$1"

BashFAQ 24 explains this in much greater detail.

An alternate approach is to just source the file, with exports thrown in:

. <(sed '/^export/!s/^/export /' "$1")

^{The <( ) is process substitution.}

As an additional note of caution, since you are reading environment variables from an argument, you must make sure that the argument file $1 is a trusted source so it can't do bad things to your script.

To Export or Not to Export Bash PS1 Variable

That's correct: PS1 is only meaningful in interactive instances of bash, so it should be set in ~/.bashrc and should not be exported. PS1 is also meaningful in other shells, but it has a different meaning, because prompt expansions differ between shells. In fact, even between instances of bash, PS1 can have different meanings, since the meaning depends on shell options (at least promptvars).

Exporting PS1 to the environment from .profile is a throwback to the 1970s, when there was only one shell that used it (the Bourne shell) and it didn't have a configuration file. It still works today if you always use the same shell and never configure it differently. But all modern shells that aren't designed purely for scripting (csh, ksh, bash, zsh, …) read a configuration file when started interactively (.cshrc, .kshrc, .bashrc, .zshrc, …), so the 1970s method is no longer necessary. Setting PS1 and other shell-specific settings in a shell-specific file, and not exporting it to the environment, avoids breaking things when you use a different shell configuration or a different shell or a different terminal that isn't capable of showing your usual prompt fanciness. Setting PS1 in a shell-specific file works all the time, whereas setting it in .profile and exporting it only works in “simple” cases, so there's no reason not to do it the right way, but there are plenty of bad tutorials around the web and even bad default configurations in distributions. C'est la vie.

Best Answer

Related Solutions

Bash – How to Set Environment Variables Dynamically

To Export or Not to Export Bash PS1 Variable

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