How can I determine whether a command exists anywhere in my PATH?

POSIX specifies a shell builtin called command which can be used for this purpose:

# POSIX
if command -v qwerty >/dev/null; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

In BASH, there are a couple more builtins that may also be used: hash and type. Here's an example using hash:

# Bash
if hash qwerty 2>/dev/null; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

Or, if you prefer type:

# Bash
# type -P forces a PATH search, skipping builtins and so on
if type -P qwerty >/dev/null; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

KornShell has whence instead:

# ksh
if whence -p qwerty >/dev/null; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

The command builtin also returns true for shell builtins (unlike type -P). If you absolutely must check only PATH, the only POSIX way is to iterate over it:

# POSIX
IsInPath ()
(
  [ $# -eq 1 ] && [ "$1" ] || return 2
  set -f; IFS=:
  for dir in $PATH; do
    [ -z "$dir" ] && dir=. # Legacy behaviour
    [ -x "$dir/$1" ] && return
  done
  return 1
)

if IsInPath qwerty; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

Note that the function defined above uses parentheses around the body rather than the normal curly braces. This makes the body run in a subshell, and is the reason we don't need to undo set -f or IFS.

The iterative approach is also used in configure scripts. Here's a simplified version of such a test:

# Bourne
save_IFS=$IFS
IFS=:
found=no
for dir in $PATH; do
  if test -x "$dir/qwerty"; then
    echo "qwerty is installed (in $dir)"
    found=yes
    break
  fi
done
IFS=$save_IFS
if test $found = no; then
  echo "qwerty is not installed"
fi

Real configure scripts are generally much more complicated than this, since they may deal with systems where $PATH is not delimited by colons; or systems where executable programs may have optional extensions like .EXE; or $PATH variables that have the current working directory included in them as an empty string; etc. If you're interested in such things, I suggest reading an actual GNU autoconf-generated configure script. They're far too large and complicated to include in this FAQ.

The command which (which is often a csh script, although sometimes a compiled binary) is not reliable for this purpose. which may not set a useful exit code, and it may not even write errors to stderr. Therefore, in order to have a prayer of successfully using it, one must parse its output (wherever that output may be written).

# Bourne.  Last resort -- using which(1)
tmpval=`LC_ALL=C which qwerty 2>&1`
if test $? -ne 0; then
  # FOR NOW, we'll assume that if this machine's which(1) sets a nonzero
  # exit status, that it actually failed.  I've yet to see any case where
  # which(1) sets an erroneous failure -- just erroneous "successes".
  echo "qwerty is not installed.  Please install it."

else
    # which returned 0, but that doesn't mean it succeeded.  Look for known error strings.
    case "$tmpval" in
      *no\ *\ in\ *|*not\ found*|'')
        echo "qwerty is not installed.  Please install it."
        ;;
      *)
        echo "Congratulations -- it seems you have qwerty (in $tmpval)."
        ;;
    esac
fi

Note that which(1)'s output when a command is not found is not consistent across platforms. On HP-UX 10.20, for example, it prints no qwerty in /path /path /path ...; on OpenBSD 4.1, it prints qwerty: Command not found.; on Debian (3.1 through 5.0 at least) and SuSE, it prints nothing at all; on Red Hat 5.2, it prints which: no qwerty in (/path:/path:...); on Red Hat 6.2, it writes the same message, but on standard error instead of standard output; and on Gentoo, it writes something on stderr.

We strongly recommend not using which. Use one of the builtins or the iterative approaches instead.