mjhjmtu

I have a tmp.txt file containing variables to be exported, for example:

a=123
b="hello world"
c="one more variable"

How can I export all these variables using the export command, so that they can later be used by child processes?

source tmp.txt
export a b c
./child ...

Judging by your other question, you don't want to hardcode the variable names:

source tmp.txt
export $(cut -d= -f1 tmp.txt)

test it:

$ source tmp.txt
$ echo "$a $b $c"
123 hello world one more variable
$ perl -E 'say "@ENVqw(a b c)"'

$ export $(cut -d= -f1 tmp.txt)
$ perl -E 'say "@ENVqw(a b c)"'
123 hello world one more variable

set -a
. ./tmp.txt
set +a

set -a causes variables¹ defined from now on to be automatically exported. It's available in any Bourne-like shell. . is the standard and Bourne name for the source command so I prefer it for portability (source comes from csh and is now available in most modern Bourne-like shells including bash though (sometimes with a slightly different behaviour)).

In POSIX shells, you can also use set -o allexport as a more descriptive alternative way to write it (set +o allexport to unset).

^{¹ In bash, be aware that, while allexport is on, all functions that have been declared (which are also variables) will also be exported to the environment (as BASH_FUNC_myfunction%% environment variables that are then imported by all bash shells run in that environment, even when running as sh).}