Shell Script - Linux

Question

Hey,

I want to write a very simple script , which takes a process name , and return the tail of the last file name which contains the process name.

I wrote something like that :

#!/bin/sh
tail $(ls -t *"$1"*| head -1) -f

My question:

1. Do I need the first line?

2. Why isn't ls -t *"$1"*| head -1 | tail -f working?

3. Is there a better way to do it?

Answer 1

1: The first line is a so called she-bang, read the description here:

In computing, a shebang (also called a hashbang, hashpling, pound bang, or crunchbang) refers to the characters "#!" when they are the first two characters in an interpreter directive as the first line of a text file. In a Unix-like operating system, the program loader takes the presence of these two characters as an indication that the file is a script, and tries to execute that script using the interpreter specified by the rest of the first line in the file

2: tail can't take the filename from the stdin: It can either take the text on the stdin or a file as parameter. See the man page for this.

3: No better solution comes to my mind: Pay attention to filenames containing spaces: This does not work with your current solution, you need to add quotes around the $() block.

Answer 2

$1 contains the first argument, the process name is actually in $0. This however can contain the path, so you should use:

#!/bin/sh
tail $(ls -rt *"`basename $0`"*| head -1) -f

You also have to use ls -rt to get the oldest file first.

Answer 3

Following on from @theomega's answer and @Idan's question in the comments, the she-bang is needed, among other things, because some UNIX / Linux systems have more than one command shell.

Each command shell has a different syntax, so the she-bang provides a way to specify which shell should be used to execute the script, even if you don't specify it in your run command by typing (for example)

./myscript.sh

instead of

/bin/sh ./myscript.sh

Note that the she-bang can also be used in scripts written in non-shell languages such as Perl; in the case you'd put

#!/usr/bin/perl

at the top of your script.

Answer 4

Maybe you can find a solution on www.commandlinefu.com there is a lot of Unix commands

Answer 5

You can omit the shebang if you run the script from a shell, in that case the contents will be executed by your current shell instance. In many cases this will cause no problems, but it is still a bad practice.