Bash – Use env with environment variables from file

bashenvironment-variablesshell

Let's say I have a file (called e) with this content:

a=3
b="5 6"

How can I get each one to be interpreted as a separate argument to env? That is, to have it equivalent to:

$ env a=3 b="5 6" ruby -e 'p ENV["a"]; p ENV["b"]'
"3"
"5 6"
$ env a=3 "b=5 6" ruby -e 'p ENV["a"]; p ENV["b"]'
"3"
"5 6"

This one obviously doesn't work because of the space:

env $(cat e) ruby -e 'p ENV["a"]; p ENV["b"]'

But neither does this one:

IFS=$'\n' env $(cat e) ruby -e 'p ENV["a"]; p ENV["b"]'

And quoting $(cat e) will result in:

"1\nb=2 3"
nil

Best Answer

This is an example of not using the right tool for the job. env isn't the only tool that sets environment variables and then chain loads another program. And it doesn't read variable data from file.

Of course, the . a.k.a. source command is not the right tool for this job, either.

It permits the file to unexpectedly contain shell commands other than simple variable assignments, which in certain circumstances (depending from what the configuration file is) can be a massive security hole.
A series of assignments as in the example in the question, sourced by ., sets shell variables, not environment variables. They aren't automatically exported.

The right tools are ordinary chain-loading tools from several toolsets of such tools.

One such is read-conf from the nosh toolset, designed to read exactly this kind of configuration file (such as /etc/rc.conf from — say — PC-BSD) and set environment variables without implementing and opening up configuration data files to an entire shell language syntax:

jdebp % uname
Linux
jdebp % cat e
a=3
b="5 6"
jdebp % read-conf e printenv | tail -n 3
USER=jdebp
a=3
b=5 6
jdebp %

jdebp % uname
FreeBSD
jdebp % read-conf /etc/rc.conf printenv | head -n 2
cron_enable=NO
sendmail_msp_queue_enable=NO
jdebp %

Another such tool is Wayne Marshall's runenv from perp.

Related Solutions

Bash – Arithmetic operations with expr and variables

bash cannot do floating point operations, only integers. I suggest to use bc -l instead:

$ mPercent='2.72'
$ fPercent='3.14'
$ bc -l <<< $(echo "$mPercent / 100 "*" .482 + $fPercent / 100 "*" .518")
.02937560000000000000

Shell – pipeline as find’s -exec argument

find . -type f -name "CMakeLists*" -exec grep lib /dev/null {} +

This finds files in the current directory whose basename begins or is the string CMakeLists. The argument is escaped (double quoted) so that the shell doesn't expand it before find runs.

There is no need to add cat with a pipe to grep --- it's a useless process with useless IO, here. Adding /dev/null insures that grep will report the filename along with the matching line(s) when there is more than one file to match.

By using {} + as the terminating sequence to the -exec argument, multiple filenames are passed to each invocation of the grep command. Had we used {} \; then a grep process would have been spawned for every file found. Needless process instantiation is expensive if done hundreds or thousands of times.

To use a pipe with a find -exec argument, you need to invoke the shell. A contrived example could be to grep for the string "one" but only if the string "two" isn't present too. This could be done as:

find . -type f -name "CMakeLists*" -exec sh -c 'grep one "$@"|grep -v two' sh {} +

This is predicated on the comments, below, by @muru, @Serg and @Scott, with thanks.

Best Answer

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Bash – Arithmetic operations with expr and variables

Shell – pipeline as find’s -exec argument

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