Data management in the Shell

Well Organized

By Heike Jurzik

Franz Pfluegl, Fotolia

Where graphical file managers like Konqueror and Nautilus use mouse clicks and convenient features such as drag and drop, you could use some simple commands in the shell. This article introduces the mkdir, rmdir, cd, touch, cp, mv, and rm commands, and demonstrates highly efficient data management at the console.

Creating and Deleting directories

The mkdir ("make directory") command lets you create a new folder. The following command:

mkdir folder1

creates a directory named folder1 in your current directory. The access privileges for the new directory are defined by the umask (see the "Umask" box on page 88). To assign different permissions, you can use the -m flag, with an octal number to define the permissions you want (Listing 1).

01 $ mkdir folder1
02 $ mkdir -m 777 folder2
03 $ ls -l
04 drwxr-xr-x  2 huhn huhn 4096 2006-12-28 14:07 folder1/
05 drwxrwxrwx  2 huhn huhn 4096 2006-12-28 14:08 folder2/

The command also understands relative and absolute path values. For example, to create a folder below the music directory, you don't actually need to change to the directory. Instead, you just specify the path:

mkdir music/Metallica

If the superordinate directory does not exist, mkdir will protest:

mkdir: cannot create directory `music/Metallica': No such file or directory

In this case, you need to set the -p option to create hierarchies of folders at a single step. Instead of entering the following list of commands:

mkdir music
mkdir music/Metallica
mkdir music/Metallica/Load

you could achieve the same effect with the -p parameter and a single command:

mkdir -p music/Metallica/Load

The command for deleting a directory is rmdir ("remove directory"). If the folder is not completely empty, rmdir will refuse to cooperate:

$ rmdir folder1
rmdir: "folder1/": Directory not empty

In this case, you can either remove the folder content first, or just call the rm command (see the "Clean Slate" section), which has an option for enforced deleting. rmdir also supports the -p parameter, and assuming that all the folders are empty, the following command:

rmdir -p music/Metallica/Load

will remove all three directories from your disk in one fell swoop.

The Right Touch

The touch command is often used to create new, empty files. The command:

touch cluck

creates an empty file called cluck in the current directory, assuming the file does not already exist. The command also evaluates the umask to automatically set permissions.

If the file exists, touch modifies the timestamp for the file, and sets the last access and last changed times for the file to the current time. This makes a lot of sense in combination with make, for example. However, this command only works if one or multiple source files have changed.

To talk make into running, even if the sources have not changed, you can run touch against the source code files to modify the timestamps, as in touch *.tex, for example.

Clever Navigation

The cd ("change directory") command lets users navigate folders. You can specify either an absolute path, or a relative path:

cd /var/log
cd ../../var/log

The command has a couple of practical shortcuts. Typing cd without any other parameters takes you to your home directory.

cd evaluates the HOME environmental variable to discover the path to the home directory:

$ echo $HOME
/home/huhn

Bash also uses the OLDPWD environmental variable to store the name of the last folder you visited, and cd $OLDPWD takes you back to this folder.

Typing cd $OLDPWD a second time beams you back to your previous directory; the shell only stores these two directories, so that you just jump back and forth each time you give the cd $OLDPWD command. You can save some typing by replacing $OLDPWD with a minus sign (Figure 1).

Figure 1: cd - takes you to the last directory you visited, and back again.

Copying Files and Directories

The cp ("copy") program duplicates files. To use the command, you need to specify the source and the target:

cp file1 file2

The copy this command creates has the current timestamp, and belongs to the current user by default. Again, permissions are controlled by umask. If you need to keep as many properties of the original file as possible, you can set the -p flag to keep permissions and the source timestamp.

You can also specify a directory as the target. Although you can copy multiple files to a directory, the program will not copy directories:

$ cp folder1 folder2
cp: omitting directory"folder1"

To copy a directory, you need to tell cp to use recursion, by setting the -r option. However, this is not all the copy artist can do; if you tell the command to copy a symlink instead of a "normal" file, cp will copy the file to which the link points. To create a new link instead of doing this, you need to set the -d flag.

Be careful if you specify a file of the same name as the target; cp will just overwrite the existing file. You can add a safety net by specifying the -i option:

$ cp -i file1 folder/file1
cp: overwrite "folder/file1"?

Alternatively, you can talk cp into creating a backup by stipulating the -b option:

cp -b file1 folder/file1

The backup copy has a tilde ("~") at the end of its filename.

Let's move!

The mv ("move") tool can either move or rename files. Again, you need to pass a source and a target to the command. The source can be a file or a directory, and the target can be a directory, or a file or directory name. To move a file called file1 from the current directory to an existing folder called directory1, simply do the following:

mv file1 directory1

If the target folder does not exist, this command will change the file's name to directory1 - in other words, you just need to specify the new name to rename the source.

Just like cp, mv supports the -i and -b options to protect data against inadvertent deletion by prompting or creating a backup.

Clean Slate

The rm file command typically deletes the specified file without so much as a "by your leave". rm ("remove") doesn't hang about, and doesn't prompt you to see if you are really sure. The only exception to this is if you have a read-only file; in this case, rm prompts you to confirm (see Listing 2).

01 $ ls -l
02 -r--r--r--  1 huhn huhn        0 2006-12-28 16:44 file
03 ...
04 $ rm file
05 rm: remove (write-protected) file "file"?

However, this is not a recommended approach. Instead, you might prefer to use the -i option, which switches the command to interactive mode (just like it does with cp and mv):

$ rm -i file
rm: remove regular file"file"?

If you have a large number of write-protected files in a folder, and don't like the idea of having to confirm removal of each file, you can use the -f ("force") parameter to tell rm not to ask.

For more thorough deletion, rm also has the -r option. If you set it, rm will happily remove subdirectories - along with their content - recursively up to the top of the path. Where rmdir hesitates to remove a folder if it is not completely empty, rm -r just blows it away. If you are interested in seeing what is going on behind the scenes, and where rm went, just set the -v flag (Figure 2).

Figure 2: If you forget the command-line options, use the man command to view the manpage.

Files that start with a non-standard characters, such as a minus sign, are problematic; rm (and other commands) will refuse to touch them. The reason for this is that the shell interprets the first character after the minus sign as a parameter, and will thus not be able to find the intended target.

A trick helps you resolve the situation. Type rm ./-file to delete -file, rather than interpreting the target as a parameter. As an alternative, you can insert two dashes in front of the filename (rm - -file); this tells the command that whatever follows the dashes is not a parameter, but rather an argument (that is the target for the current operation).