Diffrence between $(git ls-files -s | wc -l ) and $(git ls-files -s >out && wc -l <out)

Question

$(git ls-files -s | wc -l)和$(git ls-files -s >out && wc -l <out)这两个命令是一样的$(git ls-files -s | wc -l)最终得到错误。

Answer 1

When you pipe the output of one program into the input of another, as in:

$(git ls-files -s | wc -l)

...the programs run concurrently. wc will start counting lines as soon as it receives them. The pipe also directs the output of git to the input of wc without any intermediate file.

Note that in this case, wc will run even if the git command fails for some reason, so you'll get the wc output (in most cases, 0 ).

In your second example:

$(git ls-files -s >out && wc -l <out)

...the git command runs first, and stores its results in a file called out . Then, if that was successful, wc runs and counts the lines. Because of && , if the git command fails, wc won't run at all. In either case, you'll have a file named out laying around with the results of the git command in it.

Piping is generally better; it'll run faster and if you don't need to keep the intermediate results, it won't have any side effects.