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I'm making this test script to help me reverse engineer scripts in charge of setting the dev environment.

The script:

#env-changes
#!/bin/bash
TESTED_SCRIPT=$1
shift
ENV_BEFORE=$(env | sort)
. $TESTED_SCRIPT
ENV_AFTER=$(env | sort)
diff <(printf '%s\n' "$ENV_BEFORE") <(printf '%s\n' "$ENV_AFTER")

Dummy env script:

#!/bin/bash
export I_AM_NEW=hello

Usage:

$ ./env-changes dummy-env-script.sh 
21a22
> I_AM_NEW=hello

Currently it only works with bash. Suggestions on how to make it more POSIX compilant?

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  • 3
    Why does it need to be more POSIX? Are you targetting a platform that does not have bash? Don't waste your time if this is not a real requirement for you. Commented Mar 14, 2016 at 13:38
  • @glennjackman I had a general feeling that that is what one should always aim for :) . It isn't a real requirement so far. Commented Mar 14, 2016 at 13:46
  • 1
    BTW, the #! line should be the FIRST line of the script. If your first line is #env-changes as you've posted it, it is only being run by bash because /bin/sh is the default interpreter and /bin/sh is bash on your system. It's not uncommon even on linux systems for /bin/sh to be dash or ash or some other minimalist posix sh (partly because they're much smaller and often faster than bash, and because some people see the huge size of bash as a potential security risk due to the larger "attack surface" of large programs vs small programs). Commented Mar 15, 2016 at 2:07

1 Answer 1

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First of all, the POSIX source command will only source files from directories in the PATH. To get around that, you will need to use a full path:

sh ./env-changes ./dummy-env-script.sh

Also, the portable version of source is .:

. "$1"

Then, the <() construct is not POSIX, so you'll need to work around that. For example, you can use temp files:

tmp1=$(mktemp)
tmp2=$(mktemp)
env | sort > "$tmp1"
. "$1"
env | sort > "$tmp2"
diff "$tmp1" "$tmp2"
rm  "$tmp1" "$tmp2"

Or, a different approach:

ENV_BEFORE=$(env)
. "$1"
ENV_AFTER=$(env)
printf -- '%s\n%s\n' "$ENV_BEFORE" "$ENV_AFTER" | sort | uniq -u

Here, we print both variables, sort them, pass them through uniq -u which will only print unique lines, printing only the new variable:

$ sh ./env-changes ./dummy-env-script.sh 
I_AM_NEW=hello
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