Setting an environment variable before a command in bash not working for second command in a pipe

Question

In a given shell, normally I'd set a variable or variables and then run a command. Recently I learned about the concept of prepending a variable definition to a command:

FOO=bar somecommand someargs

This works... kind of. It doesn't work when you're changing a LC_* variable (which seems to affect the command but NOT its arguments, e.g., '[a-z]' char ranges) or when piping output to another command thusly:

FOO=bar somecommand someargs | somecommand2  # somecommand2 is unaware of FOO

I can prepend somecommand2 with "FOO=bar" as well, which works but which adds unwanted duplication, and it doesn't help with arguments that are interpreted depending on the variable (e.g. '[a-z]')

So, what's a good way to do this on a single line? I'm thinking something on the order of:

FOO=bar (somecommand someargs | somecommand2)  # Doesn't actually work

Edit: I got lots of good answers! The goal is to keep this a one-liner, preferably without using "export". The method using a call to bash was best overall, though the parenthetical version with "export" in it was a little more compact. The method of using redirection rather than a pipe is interesting as well.

Answer 1

FOO=bar bash -c 'somecommand someargs | somecommand2'

Answer 2

How about exporting the variable, but only inside the subshell?:

(export FOO=bar && somecommand someargs | somecommand2)

Keith has a point, to unconditionally execute the commands, do this:

(export FOO=bar; somecommand someargs | somecommand2)

Answer 3

You can also use eval:

FOO=bar eval 'somecommand someargs | somecommand2'

---

Since this answer with eval doesn't seem to please everyone, let me clarify something: when used as written, with the single quotes, it is perfectly safe. It is good as it will not launch an external process (like the accepted answer) nor will it execute the commands in an extra subshell (like the other answer).

As we get a few regular views, it's probably good to give an alternative to eval that will please everyone, and has all the benefits (and perhaps even more!) of this quick eval “trick”. Just use a function! Define a function with all your commands:

mypipe() {
    somecommand someargs | somecommand2
}

and execute it with your environment variables like this:

FOO=bar mypipe

Answer 4

Use env.

For example, env FOO=BAR command. Note that the environment variables will be restored/unchanged again when command finishes executing.

Just be careful about about shell substitution happening, i.e. if you want to reference $FOO explicitly on the same command line, you may need to escape it so that your shell interpreter doesn't perform the substitution before it runs env.

$ export FOO=BAR
$ env FOO=FUBAR bash -c 'echo $FOO'
FUBAR
$ echo $FOO
BAR

Answer 5

How about using a shell script?

#!/bin/bash
# myscript
FOO=bar
somecommand someargs | somecommand2

> ./myscript