I noticed this when trying to build sharutils, which has libopts.m4 that ultimately comes from autogen.

• QA Notice: command not found:
• ./configure: 1: which: not found
• ./configure: 1: which: not found

The code looks like this:

  [while :
  do
      test -x "$POSIX_SHELL" && break
      POSIX_SHELL=`which bash`
      test -x "$POSIX_SHELL" && break
      POSIX_SHELL=`which dash`
      test -x "$POSIX_SHELL" && break
      POSIX_SHELL=/usr/xpg4/bin/sh
      test -x "$POSIX_SHELL" && break
      POSIX_SHELL=`/bin/sh -c '
          exec 2>/dev/null
          if ! true ; then exit 1 ; fi
          echo /bin/sh'`
      test -x "$POSIX_SHELL" && break
      ]AC_MSG_ERROR([cannot locate a working POSIX shell])[
  done]
  AC_DEFINE_UNQUOTED([POSIX_SHELL], ["${POSIX_SHELL}"],
           [define to a working POSIX compliant shell])



Unfortunately, the use of "which bash" to check whether a bash program exists fails when the "which" command doesn't exist. See e.g.

https://unix.stackexchange.com/questions/85249/why-not-use-which-what-to-use-then/85250#85250
https://mywiki.wooledge.org/BashFAQ/081

In general, you can and should use "command -v" for this. It was part of POSIX 2008. A simple "s/which/command -v/" suffices.

However since this is a macro collection for autoconf it seems more apropos to use AC_CHECK_PROG or AC_CHECK_PROGS instead...