Manipulating the UNIX File System
You know how to find out what files are where in the file system using ls, but there are also various incredibly useful commands that let you manipulate and modify files, directories, and the file system itself. That's what we'll focus on here.
Task 6.1: Creating New Directories Using mkdir
 | One important aspect of UNIX that has been emphasized continually in this book is that the UNIX file system is hierarchical. The UNIX file system includes directories containing files and directories, each directory of which can contain yet more files and directories. Your own home directory, however, probably doesn't contain any directories (except “.” and “..” of course), which prevents you from exploiting what I call the virtual file cabinet of the file system. |
The command for creating directories is actually one of the least complex and most mnemonic (for UNIX, anyway) in this book: mkdir, called “make directory.”
 | Pronounce the mkdir command as “make dir.” |
Turn to your computer, move to your home directory, and examine the files and directories there. Here's an example:% cd % ls Archives/ OWL/ rumors.26Oct.Z InfoWorld/ PubAccessLists.Z rumors.5Nov.Z LISTS bin/ src/ Mail/ educ News/ mailing.lists.bitnet.Z
To create a directory, you need to specify what you'd like to name the directory and where you'd like to locate it in the file system (the default location is your current working directory):
% mkdir NEWDIR % ls Archives/ News/ mailing.lists.bitnet.Z InfoWorld/ OWL/ rumors.26Oct.Z LISTS PubAccessLists.Z rumors.5Nov.Z Mail/ bin/ src/ NEWDIR/ educ
 | Although UNIX is very flexible about file and directory names, as a general rule you'll want to avoid spaces, tabs, control characters, and the / character. |
That's all there is to it. You've created your first UNIX directory, and you can now list it with ls to see what it looks like:
% ls -ld NEWDIR drwxrwx--- 2 taylor 24 Nov 5 10:48 NEWDIR/ % ls -la NEWDIR total 2 drwxrwx--- 2 taylor 24 Nov 5 10:48 ./ drwx------ 11 taylor 1024 Nov 5 10:48 ../
Not surprisingly, the directory is empty other than the two default entries of “.” (the directory itself) and “..” (the parent directory, your home directory).
Look closely at the permissions of the directory. Remember that the permissions are a result of your umask setting. As you learned in the preceding hour, changing the umask setting changes the default directory permissions. Then, when you create a new directory, the new permissions will be in place:
% umask 07 % umask 0 % mkdir NEWDIR2 % ls -ld NEWDIR2 drwxrwxrwx 2 taylor 24 Nov 5 10:53 NEWDIR2/ % umask 222 % mkdir NEWDIR3 % ls -ld NEWDIR3 dr-xr-xr-x 2 taylor 24 Nov 5 10:54 NEWDIR3/
What happens if you try to create a directory with a name that has already been used?
% mkdir NEWDIR mkdir: NEWDIR: File existsTo create a directory someplace other than your current location, prefix the new directory name with a location:
% mkdir /tmp/testme % ls -l /tmp -rwx------ 1 zhongqi 22724 Nov 4 21:33 /tmp/a.out* -rw------- 1 xujia 95594 Nov 4 23:10 /tmp/active.10122 -rw-r--r-- 1 beast 572 Nov 5 05:59 /tmp/anon1 -rw-rw---- 1 root 0 Nov 5 10:30 /tmp/bar.report -rw------- 1 qsc 0 Nov 5 00:18 /tmp/lh013813 -rwx------ 1 steen 24953 Nov 5 10:40 /tmp/mbox.steen* -rwx------ 1 techman 3711 Nov 5 10:45 /tmp/mbox.techman* -rw-r--r-- 1 root 997536 Nov 5 10:58 /tmp/quotas -rw------- 1 zhongqi 163579 Nov 4 20:16 /tmp/sp500.1 drwxrwx--- 2 taylor 24 Nov 5 10:56 testme/ -rw-r--r-- 1 aru 90 Nov 5 02:55 /tmp/trouble21972
 | Like other basic UNIX utilities, mkdir has no command arguments, so it is quite easy to use. There are two things to keep in mind: You must have write permission to the current directory if you're creating a new directory, and you should ensure that the name of the directory is not the same as (or, to avoid confusion, similar to) a directory name that already exists. |
Task 6.2: Copying Files to New Locations Using cp
 | One of the most basic operations in any system is moving files, the modern-office computer equivalent of paper shuffling. On a computer, moving files is a simple matter of using one or two commands: You can move a file to a different location, or you can create a copy of the file and move the copy to a different location. |
The Macintosh has an interesting strategy for differentiating between moving and copying. If you drag a file to another location on the same device (a hard disk, for example), by default the computer moves the file to that location. If you drag the file to a location on a different device (from a floppy to a hard disk, for instance), the computer automatically copies the file, placing the new, identically named copy on the device.
UNIX lacks this subtlety. Instead, UNIX lets you choose which of the two operations you'd like to perform. The two commands are typically succinct UNIX mnemonics: mv to move files, and cp to copy files. The mv command also serves the dual duty of enabling you to rename files.
 | Pronounce cp as “sea pea.” When you talk about copying a file, however, say “copy.” Similarly, pronounce mv as “em vee,” but when you speak of moving a file, say “move.” |
I find myself using cp more than mv because it offers a slightly safer way to organize files: If I get confused and rename it such that it steps on another file (you'll see what I mean in a moment), I still have original copies of all the files.
The format of a cp command is to specify first the name of the file you want to copy and then the new filename. Both names must be either relative filenames (that is, without a leading slash or other indication of the directory) or absolute filenames. Start out by making a copy of your .login file, naming the new copy login.copy:% cp .login login.copy % ls -ld .login login.copy -rw------- 1 taylor 1858 Oct 12 21:20 .login -rw------- 1 taylor 1858 Nov 5 12:08 login.copy
You can see that the new file is identical in size and permissions but that it has a more recent creation date, which certainly makes sense.
What happens if you try to copy a directory?
% cp olddir newdir cp: olddir: Is a directory (not copied).Generally, UNIX won't permit you to use the cp command to copy directories.
 | I found that this command worked—sort of—on one machine I have used. The system's response to the cp command indicated that something peculiar was happening with the following message: cp: .: Is a directory (copying as plain file) But, the system also created newdir as a regular, executable file. You may find that your system reacts in this manner, but you probably do not have any use for it. |
The cp command is quite powerful, and it can copy many files at once if you specify a directory as the destination rather than specifying a new filename. Further, if you specify a directory destination, the program automatically will create new files and assign them the same names as the original files.
First, you need to create a second file to work with:
% cp .cshrc cshrc.copyNow try it yourself. Here is what I did:
% cp login.copy cshrc.copy NEWDIR % ls -l NEWDIR total 4 -rw------- 1 taylor 1178 Nov 5 12:18 cshrc.copy -rw------- 1 taylor 1858 Nov 5 12:18 login.copy
 | You can use the cp command to copy an original file as a new file or to a specific directory (the format being cp original-file new-file-or-directory), and you can copy a bunch of files to a directory (cp list-of-files new-directory). Experiment with creating new directories using mkdir and copying the files into the new locations. Use ls to confirm that the originals aren't removed as you go along. |
Task 6.3: Moving Files to New Locations Using mv
 | Whereas cp leaves the original file intact, making a sort of electronic equivalent of a photocopy of a paper I may pick up at my desk, mv functions like a more traditional desk: Papers are moved from one location to another. Rather than creating multiple copies of the files you're copying, mv physically relocates them from the old directory to the new. |
You use mv almost the same way that you use cp:% ls -l login.copy -rw------- 1 taylor 1858 Nov 5 12:08 login.copy % mv login.copy new.login % ls -l login.copy new.login login.copy not found -rw------- 1 taylor 1858 Nov 5 12:08 new.login
Also, you move a group of files together using mv almost the same way you do it using cp:
% cd NEWDIR % ls cshrc.copy login.copy % mv cshrc.copy login.copy .. % ls -l total 0 % ls .. Archives/ OWL/ mailing.lists.bitnet.Z InfoWorld/ PubAccessLists.Z new.login LISTS bin/ rumors.26Oct.Z Mail/ cshrc.copy rumors.5Nov.Z NEWDIR/ educ src/ News/ login.copy
Both mv and cp can be dangerous. Carefully consider the following example before trying either mv or cp on your own computer:
% ls -l login.copy cshrc.copy -rw------- 1 taylor 1178 Nov 5 12:38 cshrc.copy -rw------- 1 taylor 1858 Nov 5 12:37 login.copy % cp cshrc.copy login.copy % ls -l login.copy cshrc.copy -rw------- 1 taylor 1178 Nov 5 12:38 cshrc.copy -rw------- 1 taylor 1178 Nov 5 12:38 login.copy
Without bothering to warn me, UNIX copied the file cshrc.copy over the existing file login.copy. Notice that after the cp operation occurred, both files had the same size and modification dates.
The mv command will cause the same problem:
% ls -l cshrc.copy login.copy -rw------- 1 taylor 1178 Nov 5 12:42 cshrc.copy -rw------- 1 taylor 1858 Nov 5 12:42 login.copy % mv cshrc.copy login.copy % ls -l cshrc.copy login.copy cshrc.copy not found -rw------- 1 taylor 1178 Nov 5 12:42 login.copy
 | Together, mv and cp are the dynamic duo of UNIX file organization. These commands enable you to put the information you want where you want it, leaving duplicates behind if desired. |
Task 6.4: Renaming Files with mv
 | Both Windows and Macintosh systems have easy ways to rename files. In DOS, you can use RENAME to accomplish the task. On the Mac, you can select the name under the file icon and enter a new filename. On Windows, you right-click the icon and choose 'Rename' from the pop-up menu. |
UNIX has neither option. To rename files, you use the mv command, which, in essence, moves the old name to the new name. It's a bit confusing, but it works.
Rename a directory too:
% ls -ld NEWDIR drwxrwx--- 2 taylor 512 Nov 5 12:32 NEWDIR/ % mv NEWDIR New.Sample.Directory % ls -ld New.Sample.Directory drwxrwx--- 2 taylor 512 Nov 5 12:32 New.Sample.Directory/
Be careful! Just as moving files with cp and mv can carelessly overwrite existing files, renaming files using mv can overwrite existing files:
% mv dave login.copyIf you try to use mv to rename a directory with a name that already has been assigned to a file, the command fails:
% mv New.Sample.Directory dave mv: New.Sample.Directory: rename: Not a directoryThe reverse situation works fine because the file is moved into the directory as expected. It's the subtlety of using the mv command to rename files.
If you assign a new directory a name that belongs to an existing directory, some versions of mv will happily overwrite the existing directory and name the new one as requested:
% mkdir testdir % mv New.Sample.Directory testdir
Task 6.5: Removing Directories with rmdir
 | Now that you can create directories with the mkdir command, it's time to learn how to remove directories using the rmdir command. |
With rmdir, you can remove any directory for which you have appropriate permissions:% mkdir test % ls -l test total 0 % rmdir test
Note that the output of ls shows there are no files in the test directory.
The rmdir command removes only directories that are empty:
% mkdir test % touch test/sample.file % ls -l test total 0 -rw-rw---- 1 taylor 0 Nov 5 14:00 sample.file % rmdir test rmdir: test: Directory not empty
To remove a directory, you must first remove all files therein using the rm command. In this example, test still has files in it.
Permissions are important, too. Consider what happens when I try to remove a directory that I don't have permission to touch:
% rmdir /tmp rmdir: /tmp: Permission denied % ls -l /tmp drwxrwxrwt 81 root 15872 Nov 5 14:07 /tmp/
The permissions of the parent directory, rather than the directory you're trying to remove, are the important consideration.
 | There's no way to restore a directory you've removed, so be careful and think through what you're doing. The good news is that, because with rmdir you can't remove a directory having anything in it (a second reason the attempt in the preceding example to remove /tmp would have failed), you're reasonably safe from major gaffes. You are not safe, however, with the next command, rm, because it will remove anything. |
Task 6.6: Removing Files Using rm
 | The rm command is the most dangerous command in UNIX. Lacking any sort of archival or restoration feature, the rm command removes files permanently. It's like throwing a document into a shredder instead of into a dustbin. |
Removing a file using rm is easy. Here's an example:% ls -l login.copy -rw------- 1 taylor 1178 Nov 5 13:00 login.copy % rm login.copy % ls -l login.copy login.copy not found
If you decide that you removed the wrong file and actually wanted to keep the login.copy file, it's too late. You're out of luck.
You can remove more than one file at a time by specifying each of the files to the rm command:
% ls Archives/ PubAccessLists.Z new.login InfoWorld/ bin/ rumors.26Oct.Z LISTS cshrc.copy rumors.5Nov.Z Mail/ educ src/ News/ login.copy test/ OWL/ mailing.lists.bitnet.Z testdir/ % rm cshrc.copy login.copy new.login % ls Archives/ OWL/ rumors.26Oct.Z InfoWorld/ PubAccessLists.Z rumors.5Nov.Z LISTS bin/ src/ Mail/ educ test/ News/ mailing.lists.bitnet.Z testdir/
Fortunately, rm does have a command flag that to some degree helps avoid accidental file removal. When you use the -i flag to rm (the i stands for interactive in this case), the system will ask you whether you're sure you want to remove the file:
% touch testme % rm -i testme rm: remove testme? n % ls testme testme % rm -i testme rm: remove testme? y % ls testme testme not found
Note that n is no and y is yes. Delete the file.
Another flag that is often useful for rm, but is very dangerous, is the -r flag for recursive deletion of files (a recursive command repeatedly invokes itself). When the -r flag to rm is used, UNIX will remove any specified directory along with all its contents:
% ls -ld test ; ls -lR test drwxrwxrwx 3 taylor 512 Nov 5 15:32 test/ total 1 -rw-rw---- 1 taylor 0 Nov 5 15:32 alpha drwxrwx--- 2 taylor 512 Nov 5 15:32 test2/ test/test2: total 0 -rw-rw---- 1 taylor 0 Nov 5 15:32 file1 % rm -r test % ls -ld test test not found
Without any warning or indication that it was going to do something so drastic, entering rm -r test caused not just the test directory, but all files and directories inside it as well, to be removed.
 | This latest example demonstrates that you can give several commands in a single UNIX command line. To do this, separate the commands with a semicolon. Instead of giving the commands ls -ld test and ls -lR test on separate lines, I opted for the more efficient ls -ld test; ls -lR test, which uses both commands at once. |
Task 6.7: Minimizing the Danger of the rm Command
 | At this point, you might be wondering why I am making such a big deal of the rm command and the fact that it does what it is advertised to do—ƒthat is, remove files. The answer is that learning a bit of paranoia now can save you immense grief in the future. It can prevent you from destroying a file full of information you really needed to save. |
For Windows, there are commercial programs (Norton Utilities, for instance) that can retrieve accidentally removed files. The trash can on the Macintosh can be clicked open and the files retrieved with ease. If the trash can is emptied after a file is accidentally discarded, a program such as Symantec Utilities for the Macintosh can be used to restore files.
UNIX just doesn't have that capability, though, and files that are removed are gone forever.
The only exception is if you work on a UNIX system that has an automatic, reliable backup schedule. In such a case, you might be able to retrieve from a storage tape an older version of your file (maybe).
That said, there are a few things you can do to lessen the danger of using rm and yet give yourself the ability to remove unwanted files.
You can use a shorthand, a shell alias, to attach the -i flag automatically to each use of rm. To do this, you need to ascertain what type of login shell you're running, which you can do most easily by using the following command. (Don't worry about what it all does right now. You learn about the grep command a few hours from now.)% grep taylor /etc/passwd taylor:?:19989:1412:Dave Taylor/users/taylor:/bin/cshThe last word on the line is what's important. The /etc/passwd file is one of the database files UNIX uses to track accounts. Each line in the file is called a password entry or password file entry. On my password entry, you can see that the login shell specified is /bin/csh. If you try this and you don't have an identical entry, you should have /bin/sh or /bin/ksh.
If your entry is /bin/ksh, enter exactly what is shown here, paying particular attention to the two different quotation-mark characters used in the example:
$ echo 'alias rm="/bin/rm -i"' >> ~/.profile $ . ~/.profile
Now rm includes the -i flag each time it's used.
If your entry is /bin/sh, you cannot program your system to include the -i flag each time rm is used. The Bourne shell, as sh is known, is the original command shell of UNIX. The Bourne shell lacks an alias feature, a feature that both the Korn shell (ksh) and the C shell (csh) include. As a result, I recommend that you change your login shell to one of these alternatives, if available.
To see what's available, look in the /bin directory on your machine for the specific shells:
% ls -l /bin/sh /bin/ksh /bin/csh -rwxr-xr-x 1 root 102400 Apr 8 1991 /bin/csh* -rwxr-xr-x 1 root 139264 Jul 26 14:35 /bin/ksh* -rwxr-xr-x 1 root 28672 Oct 10 1991 /bin/sh*Most of the examples in this book focus on the C Shell because I think it's the easiest of the three shells to use. To change your login shell to csh, you can use the chsh—change login shell—command:
% chsh Changing login shell for taylor. Old shell: /bin/sh New shell: /bin/cshNow you can go back to instruction 2 and set up a C shell alias. This will help you avoid mischief with the rm command.
 | The best way to avoid trouble with any of these commands is to learn to be just a bit paranoid about them. Before you remove a file, make sure it's the one you want. Before you remove a directory, make doubly sure that it doesn't contain any files you might want. Before you rename a file or directory, double-check to see whether renaming it is going to cause any trouble. |
Take your time with the commands you learned in this hour, and you should be fine. Even in the worst case, you might have the safety net of a system backup performed by a system administrator, but don't rely on it.