Bash read command and stdin redirection

bashio-redirection

As I understand the bash documentation both | and < redirect stdin. So, cmd | foo and foo < <(cmd) should be more or less equivalent. However, for the bash-internal read command this doesn't seem to be the case. E.g., if I enter

a=""; b=""; read a b < <(echo a b); echo $a $b

it prints out a b, while

a=""; b=""; echo a b | read a b; echo $a $b

prints out nothing. On the other hand, if I enter

a=""; b=""; cat < <(echo a b) >foo1

a=""; b=""; echo a b | cat > foo2

the files foo1 and foo2 are exactly the same. So my question is: what is the difference with both forms when the read command is involved (or in general)? They should see exactly the same redirected input. While the < <(...) form works, I find it pretty unreadable and would much prefer to use the pipe form.

Best Answer

The act of pipelining executes read in a subshell, whereas process substitution does not. For most use cases, this isn't a problem, but in this instance you want to directly affect the shell (by manipulating variables). Because read has been invoked in a subshell, only the subshell is affected. As such, any actions that you might wish to perform on them will need to also be performed in the subshell; for example:

echo a b | { read a b ; echo "$a" "$b" ; }

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Bash – Precedence of stdin and stdout redirection in Bash

The POSIX standard specifies that shell redirection is from left to right; that is, order is significant:

The construct 2>&1 is often used to redirect standard error to the same file as standard output. Since the redirections take place beginning to end, the order of redirections is significant. For example:
ls > foo 2>&1
directs both standard output and standard error to file foo. However:
ls 2>&1 > foo
only directs standard output to file foo because standard error was duplicated as standard output before standard output was directed to file foo.

bash operates in compliance with this part of the standard:

$ ls doesnotexist > foo 2>&1
$ cat foo
ls: cannot access doesnotexist: No such file or directory
$ ls doesnotexist 2>&1 > foo
ls: cannot access doesnotexist: No such file or directory
$ cat foo
$

As for piping:

Because pipeline assignment of standard input or standard output or both takes place before redirection, it can be modified by redirection. For example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the standard input of command2.

Bash scripting – read tarball from stdin

The cat > file should work no matter when you call it, as long as you didn't read from it before and "use up" the input stream, or close the file descriptor, which you don't. This is a completely legal and normal command and it works, at least on my side.

Best Answer

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