Setting environment variables for multiple commands in bash one-liner

Parentheses spawn a new process, where you can set its own variables:

( MYENVVAR=myfolder; echo  1: $MYENVVAR; ); echo  2: $MYENVVAR;
1: myfolder
2:

Wrapping the commands into a string and using eval on them is one way not yet mentioned:

a=abc eval 'echo $a; echo $a'
a=abc eval 'echo $a && echo $a'

Or, if you want to use a general-purpose many-to-many mapping between environment variables and commands, without the need to quote your commands, you can use my trap-based function below:

envMulti()
{
shopt -s extdebug;
PROMPT_COMMAND="$(trap -p DEBUG | tee >(read -n 1 || echo "trap - DEBUG")); $(shopt -p extdebug); PROMPT_COMMAND=$PROMPT_COMMAND";
eval "trap \"\
[[ \\\"\\\$BASH_COMMAND\\\" =~ ^trap ]] \
|| { eval \\\"$@ \\\$BASH_COMMAND\\\"; false; }\" DEBUG";
}

Usage:

envMulti a=aaa b=bbb; eval 'echo $a'; eval 'echo $b'

Note: the eval 'echo...'s above have nothing to do with my script; you can never do a=aaa echo $a directly, because the $a gets expanded too early.

Or use it with env if you prefer (it actually prefixes any commands with anything):

echo -e '#!/bin/bash\n\necho $a' > echoScript.sh
chmod +x echoScript.sh
envMulti env a=aaa; ./echoScript.sh; ./echoScript.sh

Note: created a test script just to demonstrate usage with env, which can't accept built-ins like eval as used in the earlier demo.

Oh, and the above were all intended for running your own shell commands by-hand. If you do anything other than that, make sure you know all the cautions about using eval -- i.e. make sure you trust the source of the commands, etc.

Did you consider using export like

export MYENVVAR=myfolder

then type your commands like echo $MYENVVAR (that would work even in sub-shells) etc