Calling a script with ./bla.sh vs. . bla.sh

bashscriptunix

Can anybody explain to me what the shell does in the two examples A) and B) below?
It obviously behaves differently, but I can't find out why the output is different.

Example:
Let's have a script in our current directory named bla.sh with only one command:
echo ${0##/*} hello

A)
Started as: ./bla.sh
gives: ./bla.sh hello

B)
Started as: . bla.sh
gives: -bash hello

Since I use this in a script, the second output (because of the "-" in front of the -bash) kills the command. Of course, a simple -- before the ${...} helped, but I would love to understand what causes the output in the first place.
I love bash. And vi[m]. But I digress…

Best Answer

./bla.sh

Here, the command is ./bla.sh. This makes the shell look for an executable named bla.sh in the current directory, then ask the kernel to run it as a normal program, in a separate process from the shell. (It doesn't matter if bla.sh is a bash script, a perl or python one, or a compiled binary.)

. bla.sh

Here, the command is . (aka source), a built-in command of your shell. It makes the shell look for a file named bla.sh in the system path ($PATH) and interpret the contents as if they were typed by you; all this is done in the same process as the shell itself (and therefore can affect the shell's internal state).

This of course only works when bla.sh contains commands for the bash shell (if that's the one you are currently using), it won't work for perl scripts or anything else.

(This is explained in help . and help source too.)

As . and ./ are completely different things (a command vs part of a path), they can be combined, of course – using . ./bla.sh would "source" a file bla.sh in the current directory.

Usually it is best to use the ./bla.sh method. Only ~/.bashrc, ~/.profile and such files are usually sourced, because they are supposed to modify the current environment.

Related Solutions

Linux – Cron daemon running status check

ps ax|grep cron The error you get is because of the - in the ps command is not needed

It depends a bit on which version of ps you are using, on RHEL man ps says:

   This version of ps accepts several kinds of options:
   1   UNIX options, which may be grouped and must be preceded by a dash.
   2   BSD options, which may be grouped and must not be used with a dash.
   3   GNU long options, which are preceded by two dashes.

...

EXAMPLES
   To see every process on the system using standard syntax:
      ps -e
      ps -ef
      ps -eF
      ps -ely

   To see every process on the system using BSD syntax:
      ps ax
      ps axu

Is this a bash-specific method of calling a script: VARIABLE=value bash somescript

HELLO=world echo $HELLO

In the above, bash does parameter expansion before the statement is executed. That means that $HELLO is replaced with nothing (or whatever the current value of HELLO is) before the statement is executed (which would include setting HELLO to world).

Observe:

$ HELLO=world bash -c 'echo $HELLO'
world

Notice that echo $HELLO is inside single quotes. Single quotes prevent parameter expansion. That means that HELLO is not evaluated until the bash subshell starts up and sees it as an argument. By that time, HELLO will have been set equal to world.

By contrast, consider this line in which the single quotes have been replaced with double quotes:

$ HELLO=world bash -c "echo $HELLO"

Here nothing prints because double-quotes do not suppress parameter expansion. Thus, the bash command is given two arguments -c and echo. Hence, nothing prints.

All the gory details

man bash explains, in detail, what happens when commands such as the above are processed:

When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.

   1.     The words that  the  parser  has  marked  as  variable
          assignments  (those  preceding  the  command name) and
          redirections are saved for later processing.

   2.     The words that are not variable assignments  or  redi‐
          rections  are  expanded.   If  any  words remain after
          expansion, the first word is taken to be the  name  of
          the command and the remaining words are the arguments.

   3.     Redirections  are  performed  as described above under
          REDIRECTION.

   4.     The text after  the  =  in  each  variable  assignment
          undergoes  tilde  expansion, parameter expansion, com‐
          mand substitution,  arithmetic  expansion,  and  quote
          removal before being assigned to the variable.

Consider again the command:

HELLO=world echo $HELLO

The argument to echo undergoes parameter expansion in step 2 while bash does not even begin to assign the new value to HELLO until step 4.

Best Answer

Related Solutions

Linux – Cron daemon running status check

Is this a bash-specific method of calling a script: VARIABLE=value bash somescript

All the gory details

Related Question