2

I saw this command:

docker inspect --format '{{.State.Running}}' $(docker ps -lq)

and this:

docker ps -l -q|while read cid b; do docker inspect -f '{{ .State.Running }}' $cid; done;

My question is why it cannot be written as:

docker ps -l -q|docker inspect -f '{{.State.Running}}'

If that was because docker inspect does not work with pipe, how can I know which command can and which cannot?

2

Pipe takes the output of the first program and passes it into the next program. In a sense, it pretends to be a user typing input into the second program in a terminal. So, if you can run the program and type in (for example) a list of files to operate on while the program is running, then you can do the same thing with a pipe.

You may see hints to what can be done using pipes in a program's manual. For instance, man wget mentions the -i argument, and that -i - will read files from standard input, also called STDIN. So you could cat list-of-links.txt | wget -i -, or more effectively wget -i - < list-of-links.txt.

  • A caveat to this is that some programs acts differently when the input comes from or goes to a pipe, compared from when connected to a terminal. – Kusalananda Nov 25 '18 at 15:03

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