Linux administration

• A file is a place to store data: a possibly-empty sequence of bytes
• A directory is a collection of files and other directories
• Directories are organized in a hierarchy, with the root directory at the top
• The root directory is referred to as /

• Files and directories are organized into a filesystem
• Refer to files in directories and sub-directories by separating their names with /, for example:

/bin/ls
/usr/share/dict/words
/home/jeff/recipe

• Paths to files either start at / (absolute) or from some 'current' directory

• It's common to put an extension, beginning with a dot, on the end of a filename
• The extension can indicate the type of the file:

• On Unix and Linux, file extensions are just a convention
  • The kernel just treats them as a normal part of the name
  • A few programs use extensions to determine the type of a file

• The pushd command takes you to another directory, like cd. But also saves the current directory, so that you can go back later
• For example, to visit Fred's home directory, and then go back to where you started from:

$ pushd ˜fred
$ cd Work
$ ls
...
$ popd

• popd takes you back to the directory where you last did pushd
• dirs will list the directories you can pop back to

• Modern shells help you type the names of files and directories by completing partial names
• Type the start of the name (enough to make it unambiguous) and press Tab
• For an ambiguous name (there are several possible completions), the shell can list the options:
  • For Bash, type Tab twice in succession
  • For C shells, type Ctrl+D
• Both of these shells will automatically escape spaces and special characters in the filenames

• Use the symbol * to match any part of a filename:

$ ls *.txt
accounts.txt
letter.txt
report.txt

• Just * produces the names of all files in the current directory
• The wildcard ? matches exactly one character:

$ rm -v data.?
removing data.1
removing data.2
removing data.3

• Note: wildcards are turned into filenames by the shell, so the program you pass them to can't tell that those names came from wildcard expansion

• Syntax: cp [options] source-file destination-file

• Copy multiple files into a directory: cp files directory

• Common options:

  • -f, force overwriting of destination files
  • -i, interactively prompt before overwriting files
  • -a, archive, copy the contents of directories recursively

• Copy /etc/smb.conf to the current directory:

$ cp /etc/smb.conf .

• Create an identical copy of a directory called work, and call it work-backup:

$ cp -a work work-backup

• Copy all the GIF and JPEG images in the current directory into images:

$ cp *.gif *.jpeg images/

• mv can rename files or directories, or move them to different directories
• It is equivalent to copying and then deleting but is usually much faster
• Options:
  • -f, force overwrite, even if target already exists
  • -i, ask user interactively before overwriting files
• For example, to rename poetry.txt to poems.txt:

$ mv poetry.txt poems.txt

• To move everything in the current directory somewhere else: $ mv * ˜/old-stuff/

• rm deletes ('removes') the specified files
• You must have write permission for the directory the file is in to remove it
• Use carefully if you are logged in as root!
• Options:
  • -f, delete write-protected files without prompting
  • -i, interactive — ask the user before deleting files
  • -r, recursively delete files and directories
• For example, clean out everything in /tmp, without prompting to delete each file:

$ rm -rf /tmp/*

• Some files have names which make them hard to delete
• Files that begin with a minus sign:

$ rm ./-filename
$ rm -- -filename

• Files that contain peculiar characters — perhaps characters that you can't actually type on your keyboard: Write a wildcard pattern that matches only the name you want to delete:

$ rm -i ./name-with-funny-characters*

• The ./ forces it to be in the current directory
• Using the -i option to rm makes sure that you won't delete anything else by accident

• Syntax: mkdir directory-names
• Options:
  • -p, create intervening parent directories if they don't already exist
  • -m mode, set the access permissions to mode
• For example, create a directory called mystuff in your home directory with permissions so that only you can write, but eveyone can read it:

$ mkdir -m 755 ˜/mystuff

• Create a directory tree in /tmp using one command with three subdirectories called one, two and three:

$ mkdir -p /tmp/one/two/three

• rmdir deletes empty directories, so the files inside must be deleted first
• For example, to delete the images directory:

$ rm images/*
$ rmdir images

• For non-empty directories, use rm -r directory, note: dangerous command
• The -p option to rmdir removes the complete path, if there are no other files and directories in it. These commands are equivalent:

$ rmdir -p a/b/c
$ rmdir a/b/c a/b

• The data in files comes in various different formats (executable programs, text files, etc.)
• The file command will try to identify the type of a file:

$ file /bin/bash
/bin/bash: ELF 32-bit LSB executable, Intel 80386, version 1,
dynamically linked (uses shared libs), stripped

• It also provides extra information about some types of file
• Useful to find out whether a program is actually a script:

$ file /usr/bin/zless
/usr/bin/zless: Bourne shell script text

• If file doesn't know about a specific format, it will guess:

$ file /etc/passwd
/etc/passwd: ASCII text

• Changes the access and modification times of files
• Creates files that didn't already exist
• Options:
  • -a, change only the access time
  • -m, change only the modification time
  • -t [YYYY]MMDDhhmm[.ss], set the timestamp of the file to the specified date and time. GNU touch has a -d option, which accepts times in a more flexible format
• For example, change the time stamp on homework to January 20 2001, 5:59p.m.

$ touch -t 200101201759 homework

• a. Use cd to go to your home directory, and create a new directory there called dog.
• b. Create another directory within that one called cat, and another within that called mouse.
• c. Remove all three directories. You can either remove them one at a time, or all at once.
• d. If you can delete directories with rm -r, what is the point of using rmdir for empty directories?
• e. Try creating the dog/cat/mouse directory structure with a single command.

• a. Copy the file /etc/passwd to your home directory, and then use cat to see what's in it.
• b. Rename it to users using the mv command.
• c. Make a directory called programs and copy everything from /bin into it.
• d. Delete all the files in the programs directory.
• e. Delete the empty programs directory and the users file.

• a. The touch command can be used to create new empty files. Try that now, picking a name for the new file:

$ touch baked-beans

b. Get details about the file using the ls command:

$ ls -l baked-beans

• c. Wait for a minute, and then try the previous two steps again, and see what changes. What happens when we don't specify a time to touch?
• d. Try setting the timestamp on the file to a value in the future.
• e. When you're finished with it, delete the file.