Bash programming
The Bash Shell
One of the most powerful tools when using any Unix is the Bash Shell. It is available as either an optional or default shell on most Unix variants. When coding for the Bash Shell, you are writing scripts that will be executed by the shell. Bash Scripts are similar in nature to Batch scripts in Windows, but go much deeper into the operation of the OS.
What does it look like?
The Bash Shell syntax works very much like Batch, in that you use terminal commands. A sample script can be as simple as <source lang="bash">
- !/bin/bash
echo "hello world!" </source> and can be as complex as script designed to ping a DNS server to retrieve an IP. A typical Bash Script is used to automate monotonous tasks like scanning folders and deleting automatically backed up files, or setting permissions to new files.
How do you make a Bash Script?
To begin the coding of a Bash Script, all one needs is a text editor (such as Gedit or VI). Head the file off with: <source lang="bash">#!/bin/bash</source> (this varies by Unix variant, and depends on the placement of the bash binary). Then one would write the commands or arguments that need to be performed, such as: <source lang="bash">alias nmp="nmap -vv -T Insane -P0 -O"</source> That example makes a script that eases the use of the Nmap Port Scanner. It uses a function called aliasing, which allows complex commands to be simplified by using another sequence, and are commonly used in startup scripts. The finished product would look something like: <source lang="bash">
- !/bin/bash
alias nmp="nmap -vv -T Insane -P0 -O" </source>
Interactive Input
Taking user input is invaluable when writing an application. Recieving and interpreting a user's input is a simple task in Bash scripting. All that is needed is to declare a variable to read, and use it for something. An easy way to explain this would be to ask for a user's name, and display it: <source lang="bash">
- !/bin/bash
echo enter your name please: read NAME echo your name is $NAME </source>
Loops
Loops are an essential part to many programming languages, and this holds true for Bash scripting as well. A simple way to incorporate a loop into a Bash script would look like this: <source lang="bash">
- !/bin/bash
while true; do echo enter an address to scan: read ADDR nmap -vv -T Insane -P0 -O $ADDR done </source>