Bash – Proper way to shut down consumer

bashpipeshell

Say we have a simple pipeline:

set -eo pipefail;
echo 'foo' | cat

how does cat know when to exit? Is it when the STDIN stream ends, or does cat receive a signal? In my case, the producer process receives a SIGINT and then exits with code 1, so how can cat (consumer process) know which exit code the producer process exited with?

Best Answer

cat doesn't know about echo's signals. It receives end-of-file condition per man pipe(7):

If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end- of-file (read(2) will return 0).

For writers to the pipe, however, there is SIGPIPE. See the same man page

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Bash – Conditional execution block with || and parentheses problem

What you're searching is something like this:

[ "$USER" = "x" ] || { echo "Time to bail"; exit 1; }

The { list; } statement executes the commands in the given list in the current shell context. No subshell is created, unlike in the ( list ) notation. An exit call between parentheses will exit that subshell and not the parent shell itself.

The examples in your question with the if-statement on one line or multiple lines are syntactically correct. I cannot reproduce that behavior. It doesn't matter how many lines there arre; the if-statement never starts a subshell in its body.

BTW: I added double quotes to the variable in the condition, because when the variable $USER is empty, the construct would expand to [ = "x" ] which is not valid. With double quotes it expands to [ "" = "x" ].

Bash – How to print and exit bash if a backtick operator fails

(...) is not (primarily) to group commands, but to start a subshell. So the exit in (echo failed && exit 1) only exits the subshell.

To group commands without running them in a subshell, you'd use { ...; } instead:

X=`getinfo.sh` || {
  echo >&2 "failed"
  exit 1
}

Though here, I'd rather use:

if ! x=$(getinfo.sh); then
  echo >&2 failed
  exit 1
fi

Also note that your echo failed && exit 1 could fail to exit if echo itself fails which could happen for intance when stdout is a file on a full filesystem, or a broken pipe with SIGPIPE ignored, or stdout has been closed, or a file size limit has been reached with SIGXFSZ ignored...

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Bash – Conditional execution block with || and parentheses problem

Bash – How to print and exit bash if a backtick operator fails

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