File exists but cannot be executed from another folder

Question

On my Fedora 16 machine, I have an executable file named 'WarpImageMultiTransform', and I can run it when I cd into the folder that contains it:

[huangchao@localhost bin]$ ./WarpImageMultiTransform --help
Usage: 
./WarpImageMultiTransform ImageDimension moving_image output_image  -R reference_image --use-NN   SeriesOfTransformations--(See Below) 

But when I try to run it from other folder, it says the file doesn't exist:

[huangchao@localhost antsIntro]$  ls /home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform
/home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform
[huangchao@localhost antsIntro]$ ./home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform --help
-bash: ./home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform: No such file or directory

Does anyone know why?

Answer 1

. at the beginning of a path means the path is relative to the current directory. / means it's an absolute path. This command:

ls /home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform

lists absolute directory. But your second command:

./home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform --help

Means: "execute /home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform relatively to my current directory". This will work only if your current directory is /, as it will be equivalent to calling it without the period.

You need the period only to execute files from current directory, other dirs (relative or absolute) don't require a period.

Answer 2

I found the answer. Just using

[huangchao@localhost antsIntro]$ /home/huangchao/ANTs-1.9.x-Linux/bin/WarpImageMultiTransform --help

will work.

Answer 3

If you want to have easy access to an executable from any directory, you can add the location of the executable file to your $PATH. On *nix systems, if a word is typed into a terminal, the system will search through the directories in the user's path to find an executable with that name.

• To see your current $PATH do

  echo $PATH
  

• To add a directory called new_dir to your $PATH for the current session only do

  PATH=$PATH:/new_dir
  

• To add a directory called new_dir to your $PATH for every session (assuming you are using bash), add this line to your $HOME/.bashrc file

  PATH=$PATH:/new_dir
  

So, in your case, you would add this line to .bashrc:

PATH=$PATH:/home/huangchao/ANTs-1.9.x-Linux/bin/

You will then be able to execute WarpImageMultiTransform simply by typing the command name wherever you are.