Shell Programming and Scripting

Hey all, so I wrote a pretty simple script. It's viewable here:

Code:

#!/bin/sh
FILE=`ls $1`
for filename in $FILE
do
echo $filename
`echo $filename | tr e 5 `
done

So, as you can see it gets the list from a command line specified directory. It then loops through, echoes the filename, and then does an echo, using the tr function.

I am just not understanding my output, just hoping for some quick clarity here. Example output is

House
./shell.sh: Hous5 not found
playground
./shell.sh: playground not found

etc etc. I suppose those are the two test case. I'm curious as to why the first echo prints fine (well not really that's expected) but the second does the exchange, but then it's looking for those files? Not really sure why this is...

Any help would be great!

EDIT: Does this for both files and directories.
EDIT2: Seems like it's doing it if I'm not assigning into a variable. Just curious as to the cause of this. Looks like echo does not have to be in the external program...doesn't matter to the program in terms of error production.
EDIT3: Looks like it's when I externally call echo, and do NOT assign it into a variable. Just as a quick question, does the assignment in shell scripting act as a piping command, putting the expected stdout into the variable?

When the subshell

Code:

`echo $filename | tr e 5 `

is evaluated, you are telling your script (shell.sh) to execute the result, i.e. Hous5, playground, etc. Since these executable files do not exist, you are getting an error.

It's not the echo, but your use of back quotes. You only need the back quotes when you want to use the output of a command such as in the assignment to a variable. For instance:

Code:

d=`date`

Assigns the output from date into the variable d.

Using them without such an assignment, or some other legitimate case, as you have causes the output to be interpreted as a command which is what is causing your error. If you remove the d= from the example above, the shell will complain that the day of week (issued as the first token from the command) is not a valid command.

the backticks (`) around a command is replaced but the output of that command. Thus
var=`echo stuff` or var=`ls $1` is replaced by the output of that.

Using `echo stuff` on a line without an assignment would first be evaluated to get the output, and then evaluated as a command. So you're trying to run these files after piped to tr.

Okay, so what I'm getting is when you backquotes a command, it evaluates it to it's entirety.

At that point it decides "Is there an assignment?" - if there was it assigns the result to the variable.

If not it will try to run the finished evaluated contents of the backquotes as an external system command?

---------- Post updated at 09:58 PM ---------- Previous update was at 09:56 PM ----------

But if the final evaluation of the backquotes is date (i.e. `echo dote | tr o a`) it will run date?

Try it and see!

Code:

[mute@geek ~]$ `echo dato | tr o e`
Wed Apr 18 02:04:19 UTC 2012

No - it will try to run the OUTPUT of date
Your example is the same as

Code:

Tue Apr 17 20:01:54 MDT 2012

Type that then hit return. Or if you understand eval, it implies an

Code:

 eval Tue Apr 17 20:01:54 MDT 2012

either is a nonsense command. The shell will complain that it cannot run the file "Tue"