[Bash] Check the existence of a command

#!/bin/bash # check the existence of a command [ base on the returns of which and alias ] # usage: #           chkcmd.sh command-list # e.g., #           chkcmd.sh pdfit #           http://scriptdemo.blogspot.ca source ~/.bashrc if [ $# -eq 0 ]; then    sed -n '2,6p' `which chkcmd.sh`    exit else    for mycmd in $*    do       isInPath=`which ${mycmd} 2< /dev/null`       if [ ${#isInPath} -eq 0 ]; then          isAlias=`alias ${mycmd} 2< /dev/null`          if [ ${#isAlias} -eq 0 ]; then             echo "${mycmd} is not in the path";          else             echo "${mycmd} is an aliased command";          fi       else          echo "${mycmd} is in the path";       fi    done fi