Shell Programming and Scripting

Code:

tmpf0="temp0".$$
tmpf1="temp1".$$
tmpf2="temp2".$$

Hi !

can anyone explain what does mean by the above expressions

i mean why double quotes has been used along with dot and dollar sign?

Regards,
TJ

I don't see a reason - you could try just without and will have the same output. Looks like somebody wanted to define some temporary file names. The $$ will contain the process id (PID) of the shell the script is being executed in. Every time the script is started anew, the PID will be different.

If you were paranoïd you would do such thing to be sure to affect a string (between the double quotes...) dot PID to a variable...

tmpf0=temp0.$$ tmpf1=temp1.$$ tmpf2=temp2.$$

tmpf0="temp0".$$ tmpf1="temp1".$$ tmpf2="temp2".$$

it means both will produce same results?

and why to include dot(.) between temp0 and $$ (temp0.$$)

and can you explain to me why we include the following line at top of every file?

#!/bin/sh

and kindly explain at low level because i'm new to shell scripting otherwise i would have not asked such trivial questions.

Thanks in advance

Why not try it?

Code:

echo temp1.$$
echo "temp1".$$

Code:

and why to include dot(.) between temp0 and $$ (temp0.$$)

No particular reason. It's just a randomly generated file name. So that when the script is run, it uses /bin/sh to process it. A PERL script might use #!/usr/bin/perl. If you do sh ./script it should effectively run it the same way without a #! being necessary but if you want the script to be capable of running all by itself, the #! line is a good idea.

thank you corona688 i was looking for such an answer...

Kind Regards,

TJ

---------- Post updated at 08:28 AM ---------- Previous update was at 02:42 AM ----------

well Corona i understood your point but it is not exactly as you said "courtesy"

for example if i run the code with out including #!/bin/sh it also work so why to include it? the following code is working fine whether i include #!/bin/sh at the top or not

value1="2"
value2="10"

result=`expr $value1 + $value2`
echo $value1 + $value2 = $result

i hope you have understood my point now

Without the #!, the OS will assume the default, /bin/sh.

But a UNIX system may have many languages available. If a script needs /bin/bash, /bin/ksh, /bin/csh, /bin/zsh, /usr/bin/perl, /usr/bin/awk, or anything else, it can put it after #! and run the script using that instead.

It's traditional to put #!/bin/sh up top even for scripts which don't need it. This is safer -- it's not impossible that a programmer might write a comment with #!, which would mess up the script if it didn't have #!/bin/sh first. If there's weird characters at the end of the line, having a #! will cause it to quit with 'bad interpreter' (since it can't find the filename /bin/sh^M) instead of trying to run invalid code inside a valid shell. It also lets a human instantly tell scripts apart from plain text files.