Does POSIX guarantee that the standard utilities are in PATH

pathposixstandard

In the "Command Search and Execution" section, the POSIX specification says that PATH is searched when finding a utility to execute (with some exceptions). Does the specification mention anywhere that PATH will be initialized to a value that guarantees that all standard utilities will be found?

Or do I need to do something like the following to guarantee that I'll never get a "command not found" error when trying to run a standard utility?

PATH=$(command -p sh -c 'printf %s "${PATH}"')

(see the POSIX specification of the command utility)

Best Answer

Yes and no. In a POSIX environment, the utilities must behave as described by the specification. In practice, this means that conforming versions of the utilities must be present in $PATH. However, when running your program on a POSIX-compliant system, you may be running it in a non-conforming environment. In practice, what often happens is that the OS has a legacy mode and a POSIX mode, and is in the legacy mode by default. Worse than missing some commands, the legacy mode tends to have incompatibilities in them, such as options with different meanings.

You can retrieve a good PATH with getconf. Of course, it's tricky, as getconf in the original $PATH may not be the right one. The Application Usage for command shows a way to do it:

command -p getconf PATH

As far as I understand the specification, this is not necessary if you are running your program in a POSIX-conforming environment; and if you aren't running your program in a POSIX-conforming environment, POSIX doesn't apply. However, this application usage can be taken as a recommendation: if this doesn't work, you can feel entitled to complain to your vendor that whatever they are doing regarding the letter of the POSIX spec, they are not complying with its spirit.

Related Solutions

POSIX Utilities – What are the POSIX Mandatory Utilities?

From one of the sections that you cite:

Optional utilities that are present only on systems supporting the associated option; see Codes for information on the options in this volume of IEEE Std 1003.1-2001

The mandatory utilities are the ones that are not marked as optional.

For example basename has no annotation to indicate that it's optional, so it's mandatory. alias is annotated as UP, so it's only mandatory if an implementation claims to include the User Portability Utilities option. command is mandatory, but the -v and -V options are not unless the implementation claims to include the User Portability Utilities option.

Shell – How to reliably find the full path of a program on the PATH

Just search for it yourself.

export IFS=":"
[ -z "${1}" ] && exit 1
for dir in $PATH
do if [ -x "${dir}/${1}" ]
   then echo "${dir}/${1}"
        exit 0
   fi
done
echo ${1} not found
exit 1

Tested in bash, dash, ksh, mksh, zsh

Update

The above is nice for a stand alone script however if you're planning on embedding this into a larger script you may want to use something more like the following.

function find_path() {
   IFS_SAVE="${IFS}"
   export IFS=":"
   [ -z "${1}" ] && return 1
   for dir in $PATH
   do if [ -x "${dir}/${1}" ]
      then echo "${dir}/${1}"
           export IFS="${IFS_SAVE}"
           return 0
      fi
   done
   export IFS="${IFS_SAVE}"
   echo ${1} not found
   return 1
}

This is so that IFS is restored after finding the match, also swapped exit's with return's

Best Answer

Related Solutions

POSIX Utilities – What are the POSIX Mandatory Utilities?

Shell – How to reliably find the full path of a program on the PATH

Update

Related Question