linux - What is the unix command to find out what executable file corresponds to a given command?

For example, if passed ls as input it should tell me that /bin/ls will run if run ls on the command-line.

Answer

The command to use varies from shell to shell.

Only a shell built-in will tell one correctly what the shell will do for a given command name, since only built-ins can fully know about aliases, shell functions, other built-ins, and so forth. Remember: Not all commands correspond to executable files in the first place.

• 
  

  For the Bourne Again shell, bash, the built-in is the type command:

  
  

  $ type '['
  [ is a shell builtin
  



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  For the Fish shell, fish, The type builtin works similarly to bash. To get just the path to an executable, use command -v:

  
  

  $ type cat
  cat is /bin/cat
  $ command -v cat
  /bin/cat
  



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  For the Korn Shell, ksh, the built-in is the whence command — with type initially set up as an ordinary alias for whence -v and the command built-in with the -v option equivalent to whence:

  
  

  $ whence -v ls
  ls is a tracked alias for /bin/ls
  



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  For the Z Shell, zsh, the built-in is the whence command, with the command built-in with the -v option equivalent to whence and the built-ins type, which, and where equivalent to whence with the options -v, -c, and -ca respectively.

  
  

  $ whence ls
  /bin/ls
  



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  For the T C Shell, tcsh, the built-in is the which command — not to be confused with any external command by that name:

  
  

  > which ls
  ls: aliased to ls-F
  > which \ls
  /bin/ls