Using Commands without Options

Look at the structure of commands using a simple example: the ls command, used to list the contents of a directory. The command can be used with or without options. Entering the plain ls command shows the contents of the current directory:

Figure 14-2 The ls Command

Unlike in other operating systems, files in Linux may have a file extension, such as .txt, but do not need to have one. This makes it difficult to differentiate between files and folders in this output of the ls. By default, the colors can give you a hint: directories are usually shown in blue, files in black.

Using Commands with Options

A better way to get more details about the contents of a directory is using the ls command with a string of options. Options modify the way a command works so that you can get it to do specific tasks. Options are separated from the command with a blank and are prefixed with a hyphen. The ls -l command shows the contents of the same directory in full detail (long listing format):

Figure 14-3 The ls -l Command

On the left of each object name, information about the object is shown in several columns. The most important are the following: The first column shows the file type of the object (in this example, d for directory or - for normal files). The next nine columns show the user permissions for the object. Columns 11 and 12 show the name of the file owner and the group (in this case, tux and users). Find information about user permissions and the user concept of Linux in Section 14.2, Users and Access Permissions. The next column shows the file size in bytes. Then date and time of the last change are displayed. The last column shows the object name.

If you want to see even more, you can combine two options for the ls command and enter ls -la. The shell now also shows hidden files in the directory, indicated by a dot in front (for example, .hiddenfile).

Getting Help

Nobody is expected to know all options of all commands by heart. If you remember the command name but are not sure about the options, you can enter the command followed by a blank and --help. This --help option exists for many commands. Entering ls --help displays all the options for the ls command.

Examples of Addressing a File

The cd commands in Section 14.1.3, Working with Directories and Files used relative paths. You can use also absolute paths. For example, suppose you want to copy a file from your home directory to a subdirectory of /tmp:

To list the contents of home directories of other users, enter ls ~username . In the example directory tree in Figure 14-4, one of the sample users is tux. In this case, ls ~tux would list the contents of the home directory of tux.

History and Completion

By default, Bash remembers commands you have entered. This feature is called history. To repeat a command that has been entered before, press until the desired command appears at the prompt. Press to move forward through the list of previously entered commands. Use Ctrl + R to search in the history.

You can edit the selected command, for example, changing the name of a file, before you execute the command by pressing . To edit the command line, just move the cursor to the desired position using the arrow keys and start typing.

Completing a filename or directory name to its full length after typing its first letters is another helpful feature of Bash. To do so, type the first letters then press . If the filename or path can be uniquely identified, it is completed at once and the cursor moves to the end of the filename. You can then enter the next option of the command, if necessary. If the filename or path cannot be uniquely identified (because there are several filenames starting with the same letters), the filename or path is only completed up to the point where again several options are possible. You can then obtain a list of them by pressing a second time. After this, you can enter the next letters of the file or path then try completion again by pressing . When completing filenames and paths with the help of , you can simultaneously check whether the file or path you want to enter really exists (and you can be sure of getting the spelling right).

Wild Cards

Another convenience offered by the shell is wild cards for pathname expansion. Wild cards are characters that can stand for other characters. There are three different types of these in Bash:

?

Matches exactly one arbitrary character

*

Matches any number of characters

[set]

Matches one of the characters from the group specified inside the square brackets, which is represented here by the string set. As part of set you can also specify character classes using the syntax [:class:], where a class is one of alnum, alpha, ascii, etc.

Using ! or ^ at the beginning of the group ([!set]) matches one character other than those identified by set.

Assuming that your test directory contains the files Testfile, Testfile1, Testfile2, and datafile.

Of the four types of wild cards, the most inclusive one is the asterisk. It could be used to copy all files contained in one directory to another one or to delete all files with one command. The command rm *fil*, for instance, would delete all files in the current directory whose name includes the string fil.

Viewing Files with Less and More

Linux includes two small programs for viewing text files directly in the shell: less and more. Rather than starting an editor to read a file like Readme.txt, simply enter less Readme.txt to display the text in the console window. Use Space to scroll down one page. Use Page Up and Page Down to move forward or backward in the text. To exit less, press Q.

Instead of less, you can also use the older program more. However, it is less convenient because it does not allow you to scroll backwards.

The program less got its name from the the precept that less is more and can also be used to view the output of commands in a convenient way. To see how this works, read Redirection and Pipes.

Redirection and Pipes

Normally, the standard output in the shell is your screen or the console window and the standard input is the keyboard. However, the shell provides functions by which you can redirect the input or the output to another object, such as a file or another command. With the help of the symbols > and <, for example, you can forward the output of a command to a file (output redirection) or use a file as input for a command (input redirection). For example, if you want to write the output of a command such as ls to a file, enter ls -l > file.txt. This creates a file named file.txt that contains the list of contents of your current directory as generated by the ls command. However, if a file named file.txt already exists, this command overwrites the existing file. To prevent this, use >>. Entering ls -l >> file.txt simply appends the output of the ls command to an already existing file named file.txt. If the file does not exist, it is created.

Sometimes it is also useful to use a file as the input for a command. For example, with the tr command, you can replace characters redirected from a file and write the result to the standard output, your screen. Suppose you want to replace all characters t of your file.txt from the example above with x and print this to your screen. Do so by entering tr t x < file.txt.

Just like the standard output, the standard error output is sent to the console. To redirect the standard error output to a file named errors, append 2> errors to the corresponding command. Both standard output and standard error are saved to one file named alloutput if you append >& alloutput.

Using pipelines or pipes is also a sort redirection, although the use of the pipe is not constrained to files. With a pipe (|), you can combine several commands, using the output of one command as input for the next command. For example, to view the contents or your current directory in less, enter ls | less. This only makes sense if the normal output with ls would be too lengthy. For instance, if you view the contents of the dev directory with ls /dev, you only see a small portion in the window. View the entire list with ls /dev | less.