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How to script: A Bash crash course

An easy step-by-step guide to the Bash command-line shell and shell scripting

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Special Bash files
There are a few special files in your home directory that Bash reads when you start a new shell. These are .bash_profile, .bashrc, .bash_history, and .bash_logout. Note that the file names start with a period, which designates them as hidden files. To see them in a directory listing, you have to use the -a switch with ls, or ls -a.

The .bash_history file contains the commands you've entered into the shell. These can be viewed by running the history command. The .bash_profile and .bashrc files contain special instructions to create your environment by setting aliases, environment variables such as your path, and any other variables you wish to set or commands you want to run. For instance, you might add this line to .bashrc:

export PS1='[\u@\h \W - \d]\

It would set your prompt to show the date:

[myname@lab1 ~ - Mon Feb 25]$

You can also set aliases in your .bashrc file. Aliases are a handy way to reduce typing for common commands. For instance, if you find that you're constantly having to type something like cd ~/myfiles/documents/project1/data, you might want add an alias to your .bashrc like so:

alias cdp1=' cd ~/myfiles/documents/project1/data'

Then you would only have to type cdp1 to navigate to the directory.

The purpose of the .bash_logout file is to store commands to be run when you terminate the shell, but it's not used often.

File globbing
Globs are Bash's method of pattern matching. Bash doesn't have regular expressions, so globs are used to make it easier to work on multiple files that match a single pattern.

A simple glob would be file.*. The asterisk matches anything, so this expression would match files named file.1 and file.reallylongnamehere. Globs can get more complex and thus more useful. A glob such as file.? narrows the options. This would match file.1 or file.2 or file.a, but would not match file.11 or file.ab or file.reallylongnamehere. This is because the ? denotes a single character.

You can also use square brackets to reference multiple options, so file.[ab] would match file.a and file.b but not file.c. Further, you can use brackets to select based on ranges of the alphabet or numbers. Thus, file.[a-d] would match file.a, file.b, file.c, and file.d but not file.e. You can also negate a match with the ^ character. Thus, file.[^ab] would match neither file.a nor file.b, but it would match file.c.

File globs are very different from regular expressions, but they are very handy when working with many files or in scripting.

Standard output and standard error
The two main methods that Bash uses to pass output from programs or scripts back to your session are called stdout and stderr. These are used for normal output and error reporting, respectively. If a program is running normally, it will use stdout to communicate with the session, but if something fails, it will send errors through stderr instead. Among other things, this allows you to redirect normal or error output from any program to a file instead of the screen.

You generally don't have to worry too much about stdout and stderr when you're just getting started, but understanding what they are and what they do will be important when you get deeper into Bash.

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