Re-use value in piped command line

Question

有没有办法设置一个变量，然后在整个管道bash命令中重复使用它，例如

> set v <VALUE> | comm1 v | comm2 v

Answer 1

I am not sure what you expect to flow through the first pipe but I guess the following solves your problem anyway. Asuming you use bash :

v=<VALUE>; comm1 $v | comm2 $v

Note that around = no spaces are allowed.

Answer 2

The way you seem to have described the problem, no. If you assign a value just before a command, that value gets exported to the command's environment, but doesn't affect the current (shell's) environment.

$ echo $x

$ x=123 bash -c 'echo $x'
123
$ echo $x

$

When you use a pipe you're creating subshells of the current shell, so only names assigned in the current shell and subsequent subshells will be recognized.

$ a=123; echo wat | echo $a
123
$ #                 ^ subshell ^

Of course, it's easy to do this if you just do the assignments as a separate step, as in…

a=123; foo | ( b=123; bar ) | baz …

but you need to manually wrap the subshell so the assignment becomes part of each pipe chunk.

Alternatively you can just create full shells as children:

a=123 bash -c 'echo $a | bar | baz'

But I can't think of any way in which that's a better approach than just assigning values when you need them.