Just a remark about the sub shell usage in bash in comparision to
ksh. Let's try:
strace -f -o bash.strace bash -c 'echo a b | read a b'
and grep about execve, clone, write on stdout, read from stdin:
> grep -E 'execve|clone|write\(1|read\(0' bash.strace
17183 execve("/bin/bash", ["bash", "-c", "echo a b | read a b"], [/* 99
vars */]) = 0
17183 clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xb7d61708)
= 17184
17183 clone( <unfinished ...>
17183 <... clone resumed> child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xb7d61708)
= 17185
17184 write(1, "a b\n", 4 <unfinished ...>
17185 read(0, "a", 1) = 1
17185 read(0, " ", 1) = 1
17185 read(0, "b", 1) = 1
17185 read(0, "\n", 1) = 1
and now the same with the Korn shell
strace -f -o ksh.strace ksh -c 'echo a b | read a b'
> grep -E 'execve|clone|write\(1|read\(0' ksh.strace | sed 's/^/ /'
17198 execve("/bin/ksh", ["ksh", "-c", "echo a b | read a b"], [/* 99 vars
*/]) = 0
17198 clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xb7bb3918)
= 17199
17199 write(1, "a b\n", 4) = 4
17198 read(0, <unfinished ...>
as now is visible the last command in the pipe sequence done
in the bash is a real sub process whereas in the ksh it is not.
The question rises: Why does the bash require a sub peocess/shell
for the final command of a pipe sequence.
Werner
--
"Having a smoking section in a restaurant is like having
a peeing section in a swimming pool." -- Edward Burr
