bash: ????: command not found

Question

( I'm using bash 3.2 in the Terminal app on OS X 10.11.4. )

I have this line in my .bashrc file: alias ll='ls -alFh'

I ran echo ll > test && chmod +x test to create a test executable. Below are the results of running multiple commands, their exit codes (via echo $? ), and stdout :

1. test
   exit code 1
   produces no stdout

2. ./test
   exit code 127
   produces ./test: line 1: ll: command not found

3. . test
   exit code 127
   produces -bash: ????: command not found

4. . ./test
   exit code 0
   produces identical result to manually running ll

I understand that exit code 1 is a generic error and that exit code 127 means the shell can't find the command . Will someone please explain what is happening in each of these cases and why, including a description of the stdout? I am especially confused at #3 with the ???? .

Answer 1

First you run:

echo ll > test && chmod +x test

then these cases.

Case 3:

When you execute:

. test

it is equivalent of:

source test

source is a shell builtin that tells the shell to read the given script file and execute the commands in the current shell environment . However since current path or . is not in your path, it finds test using PATH environment variable which is /bin/test .

/bin/test is not really a script file that can be read/executed by source ; it ends up reading a binary file and errors out since that file is a binary file, not an ascii text file and errors out writing:

????: command not found

You will get same behavior when you run source date or source ls as these are all binary files.

Case 1:

You are executing shell builtin test without any arguments that makes it exits with exit value: 1

Case 2:

When you run ./test it attempts to run ll and alias is not available in spawned sub-shell hence it cannot find alias ll . Due to that fact it exits with exit value: 127 with ./test: line 1: ll: command not found error on stderr .

Case 4:

. ./test . ./test is same as source ./test that run in current shell only. Therefore it is able to find alias you've set earlier for ll hence it runs aliased command ls -alFh and exits with 0