“ env” bash中“ echo”的奇怪行为

Question

When I am trying echo variable inside 'env' command I got nothing, but I can see it using 'printenv' command:

root@devel:~# env xxx=23 echo $xxx

root@devel:~# env xxx=23 printenv | grep xxx
xxx=23

what's wrong here?

Answer 1

env xxx=23 echo $xxx

In the above, the shell evaluates $xxx before env is executed. Thus, nothing is echoed.

In more detail, the shell sees the four words env , xxx=23 , echo and $xxx . It interprets env as a command name and xxx=23 , echo , and $xxx as three arguments which will be passed to the command env . It evaluates $xxx before passing it to the command env .

By contrast, in the following, there are no shell variables for the shell to evaluate. Instead env is executed with two arguments, xxx=23 and printenv . env sets the environment variable xxx and then executes printenv :

$ env xxx=23 printenv | grep xxx
xxx=23

Similarly, observe:

$ env xxx=23 sh -c 'echo $xxx'
23

Since $xxx is inside single-quotes, the shell does not evaluate it. Instead is runs env with four arguments: xxx=23 , sh , -c , and echo $xxx . After env sets environment variable xxx , it executes sh with arguments -c and echo $xxx . The $xxx is evaluated when sh is executed and hence it sees the variable xxx .

Answer 2

When you run env xxx=23 echo $xxx , the variable xxx=23 becomes visible to the echo process during its execution. But in echo $xxx the value of $xxx is not evaluated by echo , it is evaluated by the currently executing shell. And since the env ... invocation doesn't affect the current shell, the value of $xxx is whatever it was before you executed this command (probably unset).

echo is not a good way to test the effect of env , because you cannot make the echo command print a specific value defined in its environment. Your example with printenv is better, because it dumps the content of environment variables it knows about. Another good test is what @john wrote in his answer, invoking another shell and make the shell print a chosen environment variable. Any program with the ability to print the content of environment variables would work.