如何知道命令在linux中运行的位置

Question

Suppose i ran a command in unix like ls or rm and path variable is set as below

PATH=$JAVA_HOME/bin:.:$ORACLE_HOME/bin:/sbin:/usr/sbin:/usr/bin:/usr/ccs/bin:$COBDIR/bin:/sbin:/bin:/usr/local/bin:/bin

How can we know ls command ran from which path ? ls (just an example) may be there in /sbin as well /usr/bin

So i would like to know from where it ran ?

i cannot afford to search all directory and know in which all directory ls is lying. is there a direct way to search ls from where it ran?

Answer 1

When you run an external command in bash, bash hashes that command to avoid having to do the path lookup twice. The hash command can tell you which command was run. If the command has not been run in the hash's lifetime, it will give an error.

$ hash -t ls
-bash: hash: ls: not found

$ ls foo
$ hash -t ls
/bin/ls

It's advantageous to know how hash , which and the type command differ.

• hash tells you what path/command was used/hashed. If your PATH or filesystem changes during hash 's lifetime, hash can tell you about commands that happened before that change.
• which is an external command that looks up a command in the PATH environment variable.
• type is a builtin command that looks up a command in the local PATH variable, which can be (but hardly ever is) different from what's in the environment.

See help hash in your bash to read more about how it works.

Answer 2

Just use which to locate from where exactly it's going to be picked up from

$ which ls
  /bin/ls

It will list which path it picked up that executable from. So from above command, my ls command is in /bin directory.

Answer 3

In addition to other answers mentioning which , type , hash you could also use the whereis(1) command (if it is installed on your system).

^{whereis tells you the standard places where a command should be.}

If your interactive shell is zsh , you could also use the =ls word eg echo =ls or ls =ls to find out which ls is refered by the shell.

And you might also alias ls , or have a shell function ls masquarading the /bin/ls executable, etc.... Of course, programs could start /bin/ls without forking a shell (eg with plain fork + execve ...)

Read also execvp(3) & environ(7)