[Help-bash] Why `<` has to be after `while`?

[Peng Yu]

Hi,

The following example shows that `<` has to be after `while`. But
there is no such restriction of a regulate command. Is it really
necessary to have this restriction for `while`?

$ cat main.sh
#!/usr/bin/env bash
# vim: set noexpandtab tabstop=2:

tmpfile=$(mktemp -u)
seq 3 > "$tmpfile"

set -v
cat < "$tmpfile"
< "$tmpfile" cat

while read -r x
do
echo "$x"
done < "$tmpfile"

< "$tmpfile" while read -r x
do
echo "$x"
done
$ ./main.sh
cat < "$tmpfile"
1
2
3
< "$tmpfile" cat
1
2
3

while read -r x
do
echo "$x"
done < "$tmpfile"
1
2
3

< "$tmpfile" while read -r x
./main.sh: line 16: while: command not found
do
./main.sh: line 17: syntax error near unexpected token `do'
./main.sh: line 17: `do'

[Eric Blake]

Post by Peng Yu
Hi,
The following example shows that `<` has to be after `while`. But
there is no such restriction of a regulate command. Is it really
necessary to have this restriction for `while`?

Yes, this matches the grammar as originally implemented by the first
'sh' more than 40 years ago, and standardized by POSIX. Shell keywords
(like 'while') are parsed differently than regular commands, and part of
that difference is that redirection operators can't precede a keyword.