Greg Wooledge wrote: > Juanma wrote: > > > [ is an ordinary command (a "shell builtin") > > > > Here is another point I find confusing: I thought a "shell builtin" didn't > > have a separate binary executable file, like 'cd' (which cd => fail), but > > some of them do have such form (which [ => /usr/bin/[ ; which pwd => > > /bin/pwd). I also fail to see how 'test' modifies the state of the shell > > itself (like 'cd' does), or why it is "impossible or inconvenient to obtain > > [its functionality] with separate utilities". > > Don't use which(1). Which is an external program, so it has no knowledge > of the shell's builtins, aliases, functions and keywords. Instead, use > type.
Another problem with 'which' is that it was originally designed and written for csh users specifically. It was a csh script and would source the users ~/.cshrc file so as to acquire their aliases from there so as to be able to report user aliases. I think it is still that way on HP-UX (and probably others) for example. However that obviously won't work well for bash, ksh, zsh, and other shell users. And IMNHO csh is a terrible shell regardless of the tcsh users using it who think otherwise. Therefore some distributions such as Debian have rewritten 'which' as a /bin/sh script meaning that 'which' behaves differently on different systems. It is non-portable. > imadev:~$ type cd > cd is a shell builtin > imadev:~$ type [[ > [[ is a shell keyword > imadev:~$ type -a test > test is a shell builtin > test is /usr/bin/test > test is /bin/test > > Bash implements test as a builtin not because it's necessary, but simply > for efficiency. Forking a whole process to test whether two strings are > equal would be horribly wasteful. An important mental concept is that test and [ must *behave* the same as if they were an external program. They are internal builtins for efficiency now but everything behaves the same if they are external programs. This is why quoting as if they were external programs is required. On the other hand [[ has always been a builtin and therefore the shell can avoid one layer of quoting and does. Bob
