command line argument with usage message

Question

I am having a lot of trouble using a command line argument in a script file to check if a file is being entered in the command line. So basically I have a script file named ./file and a logfile named log. I have an if then else statement to run a check to see if the right file is entered and if not the program exits with an echo error. If the right file name is entered then it continues to run script. I either get an error directory or file doesn't exist or my echo error no matter what I do. Can someone please point out what I'm doing wrong? Thank you for any help provided. I am using putty. this is what I have

  if ["./file log"]
      then
            echo ""
  else
            echo "error"
            exit 0
  fi

Answer 1

You don't need to (self-)reference the script itself. You know it must exist. You just need to see if the log file exists. Hence, use the -f test ("FILE exists and is a regular file"). See man test for more information (test is essentially equivalent to [...]).

if [ -f "log" ]
  then
    echo ""
  else
    echo "error"
    exit 0
fi

If the log file is to provided by the argument when calling the script (e.g. $ ./file log), then you can refer to the first argument with $1. Hence,

if [ -f "$1" ]

Answer 2

You must want this too. $0 is the name of the script, and $# the number of arguments. Additional notes, by default a script returns 0 on success, and <> 0 on error

#!/bin/bash 
    display_usage() { 
        echo "This script must be run with ...." 
        echo -e "\nUsage:\n$0 [arguments] \n" 
    } 

if [ $# -eq 0 ]; then
    display_usage
    exit 1
fi
.... script ....
exit 0