Bash – Assign multiple environment variables to a single variable and expand them on command

bashenvironment-variables

Let's say I want to repeat the same string of environment variables before running various incantations of a command

if [[ some_thing ]]; then
    TZ=GMT LC_ALL=C LONG_ENV_VAR=foo my_command
elif [[ some_other_thing ]]; then
    TZ=GMT LC_ALL=C LONG_ENV_VAR=foo my_command --with-arg
else
    TZ=GMT LC_ALL=C LONG_ENV_VAR=foo my_command --with-other-arg
fi

Is there a way to combine those? Some options

Set them via export
```
export TZ=GMT
export LC_ALL=C
export LONG_ENV_VAR=foo
if [[ ]] # ...
```
This works but I would rather not have them continue to be set in the environment.
Attempt to create a variable variable!
```
local cmd_args="TZ=GMT LC_ALL=C LONG_ENV_VAR=foo"
```
Unfortunately when I tried to run this via:
```
$cmd_args my_command
```
I got TZ=GMT: command not found.
Just list them all out every time.

I also tried Googling for this, but "environment variable variable" isn't the easiest term to search for and I didn't get anywhere. Is there a fix for what I'm trying to do in #2? or am I stuck with some version of #1 and unsetting the vars afterwards?

Best Answer

I might use a subshell for this:

(
  export TZ=GMT LC_ALL=C LONG_ENV_VAR=foo
  if [[ some_thing ]]; then
    exec my_command
  …
  fi
)

That allows you to clearly set the variables once; have them present for anything you run inside the subshell, and also not be present in the main shell's environment.

Related Solutions

Bash – Multiple path environment variable setup lines with bash

You can do:

export PATH="A"
export PATH="$PATH:B"
export PATH="$PATH:C"

Each subsequent line appends onto the previously defined path. This is generally a good habit, as it avoids trashing the existing path. If you want the new component to take precedence, swap the order:

export PATH="A"
export PATH="B:$PATH"
export PATH="C:$PATH"

Alternatively, you might be able to do:

export PATH=A:\
B:\ 
C

where \ marks a line continuation. Haven't tested this method.

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.

Best Answer

Related Solutions

Bash – Multiple path environment variable setup lines with bash

Bash – How to Set Environment Variables Dynamically

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