Bash programming
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[edit] 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 writing scripts for the Bash Shell, you are creating a file that will be executed by the shell. Bash Scripts are similar in nature to Batch scripts in Microsoft Windows, but allow for much greater functionality.
[edit] What does it look like?
A sample bash script can be as simple as this:
#!/bin/bash echo "Hello World!"
Alternatively, it can be as complex as script designed to install and configure whole operating systems or troubleshoot problematic servers. A typical Bash Script is used to automate monotonous tasks like scanning directories and deleting automatically backed up files, or setting permissions to new files.
[edit] 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/vim, emacs, nano, jed, etc). Start off the file with:
#!/bin/bash
(This varies by Unix variant, and depends on the placement of the bash binary. To find out what the path to the bash binary is, use this command: which bash ). Then one would write the commands or arguments that need to be performed, such as:
alias nmp="nmap -vv -T Insane -P0 -O"
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. Next, you can call the alias to do work -- an alias is used like a constant from other programming languages, except it's used to execute programs. So, for a finished script to scan your own computer, it would look something like this:
#!/bin/bash alias nmp="nmap -vv -T Insane -P0 -O" nmp 127.0.0.1
[edit] Interactive Input
Taking user input is invaluable when writing an application. Receiving 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:
#!/bin/bash echo "Enter your name please:" read NAME echo "Hi there $NAME, pleased to meet you."
[edit] 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:
#!/bin/bash echo "Please enter a list of names to greet" read NAMES for i in $NAMES; do echo "Hi there $i, pleased to meet you." done
