Bash Parameters

Making files and directories

---

Many bash commands have special parameters, sometimes referred to as flags that open up a lot more possibilities.

Let's start by going to your home directory (you choose the command)

For new users, this may not return any content (besides privatemodules if you loaded tmux at the beginning.)

Let's make a workshop directory and put a file into it.

1.) Go to your home directory

cd

cd not only changes directory, it allows you to go home by typing the command all by itself without a directory name.

2.) Create a new directory for the workshop

mkdir Oct22Workshop

Note

mkdir is a specific command that allows you to make a directory. rmdir is a command that allows you to remove a directory (but only if it is empty) When nameing files and directories, avoid spaces and special characters except underscores ("_") and hyphens ("-").

Important

Spelling and Capitalization are literal in unix. Be careful when making and using files to be consistent in your process. This will make it easier to find files later.

3.) Let's go into the directory using a very common command cd --> change directory

cd Oct22Workshop

4.) Make a new file that is empty

touch emptyfile.txt

touch is a bash command that creates an empty file.

Why would you want an empty file?

Some programs require some pre-existing file names to be created.

5.) Make a new file that contains "Hello World"

echo "Hello World" > helloworld.txt

echo is a command that prints the content to the terminal window (sometimes refered to as print to screen

The > in this command tells the command to place the output into the place it is pointing.

In this case, it creates the file helloworld.txt and puts the phrase Hello World into the file.

6.) Print out the contents of the file to the terminal

cat helloworld.txt

You should see the output

Hello World

7.) Return to your home directory and run ls

cd

ls

Tip

If you want to speed up the execution of commands, you can copy and paste multiple commands at the same time.

cd
ls

Question

Please put a green checkmark in your box if you see the new directory when you type ls from your home directory).

Be Careful with Redirect

Be careful with redirect.

When using > to redirect content into a file, if the filename already exists, it will overwrite the file. This means that the original file is gone, and there is no undo in shell.

If you want to add to a file (for example if you are running the same command on several files and extracting a piece of information that you want to put together at the end) you can use another form of redirect >>. Using the double redirect will add to the file instead of overwriting it.

Which one is used depends on your process. If you are only running a command once, or have an intermediate file in a process that does not need to be retained at the end, then > is okay to use.

Setting Parameters for Bash Commands

As you start using bash more and more, you will find a mix of files and directories/folders. If we want to know which is which, we can add a parameter (sometimes referred to as a flag)

This is an example of adding a parameter without an argument.

ls -F

Adding Arguments to Bash Commands

An argument is a file name or other data that is provided to a command.

ls -F Oct22Workshop

It is possible to list the files and see their types inside a specific directory by adding the argument of the directory name to the ls command.

What do you see when you run the two commands above?

Anything with a "/" after it is a directory.
Anything with a * after it are programs. (we will make a program later)
If there's nothing there it's an otherwise unremarkable file (e.g. a data file or picture).

Depending on which terminal you are using, some of the file types may have different colors.

In our ondemand shell:

Files are white Directories are blue Programs (also called executables) are green Compressed files are red (e.g. files that end in .zip or .gzip or .tar)

Other Useful Parameters for ls

Show hidden files

ls -a

You should see a file called .bashrc here. This may be a file we need for troubleshooting your work or where you can make shortcuts or add paths to your login.

Show the long form of the list command

ls -l

To see whether items in a directory are files or directories. ls -l gives a lot more information too, such as the size of the file.

It also shows the permissions of who can read, write or execute a file.

drwxrwx--- 2 username05 username05     4096 Jul 18 09:57 JulyWorkshop

The first 10 letters in this line indicates the permission settings.

Getting Help on the Command Line

There are an overwhelming number of possibilities with some of these shell commands, so knowing how to find help on demand is important.

For example, ls has a lot of flags that can be used.

ls --help

This outputs a list of all the ways that ls can be altered to find information about your files.

Parameters can be added together in some cases.

ls -ltr

This can replace ls -l -t -r l is for long form of the list (outputs the permission settings -- something we need to troubleshoot occasionally) t is to order the files chronologically r means to reverse the order of the files to put the newest file at the bottom

This command strings together three flags.

ls -l is list with details ls -t is sort the list by creation time ls -r is sort the list in reverse

For very full directories, this is helpful because it outputs the most recent set of files as the last in the list.

Another way to get help is to use the man command. Not every unix installation has this installed, but the Tufts cluster does.

man is short for "manual"

Navigating a man page

Use the spacebar to scroll through the document. Use q to leave the manual and go back to the command line prompt.

man ls

This opens up the manual on the ls command. It spells out the meaning of all the parameters in detail.

Most common bash commands have a man page that explains it (I wish they had this for emojis....).

Many programs have a help function built in, try adding --help or -h to see if some helpful information pops up. Sometimes just running the command without any arguments or parameters leads to some usage information or describes the correct command to get help.

For example, if I want to understand the command tr - which is used to change a word or character to a new value.

Most programs recognize when you ask for an incorrect parameter, and will tell you how to get more information, as in this example. To get help, type the command with the correct parameter.

Tip

For some programs, the help function may be -h, --help

tr -h

The shell outputs:

tr: invalid option -- 'h'
Try 'tr --help' for more information

tr --help

In this case, a man page does exist, so you can get even more direction by typing:

man tr