Shell Programming and Scripting

Why not run it

./script_name .....

otherwise adding the "#!/bin/bash" as the first line was a waste of time.

I am running it but dont seem to function ryte.

Can you explain how you think it should be behaving, talk us through it.

well from my understanding the script (firstline) needs to take on 2 arguments which can be done through $. So $1 could be argument 1 and $2 can be argument 2. The script needs to take the first argument and insert it into the very top - teh first line of my second file. I believe this can done by using the echo function by getting the line from $1 and then inserting it into $2 bu cant quite put my finger on it.

P.S. Sorry for all the bother matey......

That is the essential nub of the problem. For instance, lastline would be a piece of cake

Code:

#!/bin/sh
echo "$1" >>"$2"

because the >> operator appends the data. But this problem says we have to put the data as a new first line.

So my solution says, okay I know what the contents of the new file should look like

Code:

echo "$1" | cat - "$2"

but that output just dissappears, so we need to put it somewhere, like a temporary file, here $$ will be used as a temporary file as it's a unique number, ie the pid of the shell.

Code:

echo "$1" | cat - "$2" >$$

but the problem says that this new output should be in the original file, so what we do is rename this temporary file to the original file name, which as a byproduct is (a) atomic (b) unlinks the original file

Code:

mv $$ $2

Does that help?

Now tell me any down sides of my solution...

I have the following in my script


#!/bin/sh

echo "$1" | cat - "$2" >$$
mv $$ "$2"

Once i have done this and i exit the editor i have done chmod +x firstline.

and then try to see if it wor by doing - sh firstline.

but i get no such file or directory message.

Look at how I ran it

Code:

./firstline TEXT FILENAME

hence

$1 is TEXT

and

$2 is FILENAME