A note for read builtin

[Dr. Werner Fink]

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

-- 
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