CODEWITHSUNDEEP
bash
Eric Renouf
Eric Renouf

Reputation: 14490

Why do I get unexpected results when using a pipe and a herestring?

When trying to both pipe the output of a command into another, and also have the second command read from a herestring, I do not get the results I expected. For example:

echo "a" | grep -f - <<<abc

I would expect to produce the output

abc

but I get nothing instead

Upvotes: 1

Views: 32

Answers (1)

Eric Renouf
Eric Renouf

Reputation: 14490

Both the pipe and the herestring are trying to become stdin for the second command, here grep. Using strace on it shows that only the herestring, in this case, is actually available to grep. It then gets an empty "file" to search, and finds no matches. Here is a portion of strace when the search space is a file instead of a herestring:

read(0, "a\n", 4096)    = 2
read(0, "", 4096)       = 0
<snip>
read(3, "abc\n", 32768) = 4
read(3, "", 32768)      = 0

but with the herestring we instead see:

read(0, "abc\n", 4096)  = 4
read(0, "", 4096)       = 0
<snip>
read(0, "", 32768)      = 0

so we never read the values from the pipe that we expected to have been our pattern space.

Using a process substitution does get around this problem, because then either the pattern space or the search space are not coming from file handle 0:

echo "a" | grep -f - <(echo "abc")

for example produces the exected abc output

Upvotes: 4

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