Ubuntu Command Line Quickstart
The command line is one of the most powerful tools available for use with Ubuntu, and indeed Linux. Knowledge of the commands associated with it and also how to string them together will make your life with Ubuntu much easier.
This chapter looks at some of the basic commands that you need to know to be productive at the command line. You will find out how to get to the command line, and also get to grips with some of the commands used to navigate around the file system. Later on in this book is the Command Line Masterclass (Chapter 30), which explores the subject in more depth. The skills you learn in this chapter will give you confidence when you're called upon to work at the command line.
What Is the Command Line?
Hang around Linux users for any length of time and it will not be long before you hear them speak in hushed tones about the command line or the terminal. Quite rightly too, as the command line offers a unique and powerful way to interact with Linux. However, for the most part you may never need to access the command line because Ubuntu offers a slew of graphical tools that enable you to configure most things on your system.
But sometimes things go wrong and you may not have the luxury of a graphical interface to work with. It is in these situations that a fundamental understanding of the command line and its uses can be a real life saver.
It is tempting to think of the command line as the product of some sort of black and arcane art, and in some ways it can appear to be extremely difficult and complicated to use. However, perseverance is key and by the end of this chapter you should at least be comfortable with using the command line and ready to move onto Chapter 30, "Command Line Masterclass."
More importantly, though, you will be able to make your way around a command line–based system, which you are likely to encounter if you work within a server environment.
This chapter introduces you to a number of commands, including commands that enable you to do the following tasks:
- Perform routine tasks— Logging in and out, using the text console, changing passwords, listing and navigating directories
- Implement basic file management— Creating files and folders, copying or moving them around the file system, renaming and ultimately deleting them (if necessary)
- Execute basic system management— Shutting down or rebooting, reading man pages, and using text-based tools to edit system configuration files
The information in this chapter is valuable for individual users or system administrators who are new to Linux and are learning to use the command line for the first time.
As with most things, Ubuntu offers you a number of ways to access the command line. You can use the Terminal entry in Applications, Accessories, but by far the simplest way is to press Ctrl + Alt + F1. Ubuntu switches to a black screen and a traditional login prompt that resembles the following:
Ubuntu 9.10 karmic karmic-dev ttyl karmic-dev login:
Ubuntu is waiting for you to log in as a user, so go ahead and enter your username and press the return key. Ubuntu then prompts you for your password, which you should enter. Note that Ubuntu does not show any characters while you are typing your password in. This is a good thing because it prevents any shoulder surfers from seeing what you've typed or the length of the password.
Hitting the Return key drops you to a shell prompt, signified by the dollar sign:
This particular prompt tells me that I am logged in as the user andrew on the system karmic-dev and I am currently in my home directory (Linux uses the tilde as shorthand for the home directory).
Another way to quickly access the terminal is to go to Applications, Accessories and choose the Terminal entry. Ubuntu opens up gnome-terminal, which allows you to access the terminal while remaining in Gnome. This time, the terminal appears as black text on a white background. Accessing the terminal this way, or by using the Ctrl + Alt + F1 method makes no difference because you are interacting directly with the terminal itself.
Navigating Through the File System
Use the cd command to navigate through the Ubuntu file system. This command is generally used with a specific directory location or pathname, like this:
$ cd /etc/apt/
Under Ubuntu, the cd command can also be used with several shortcuts. For example, to quickly move up to the parent (higher-level) directory, use the cd command like this:
$ cd ..
To return to one's home directory from anywhere in the Linux file system, use the cd command like this:
You can also use the $HOME shell environment variable to accomplish the same thing. Type this command and press Enter to return to your home directory:
$ cd $HOME
You can accomplish the same thing by using the tilde (~) like this:
$ cd ~
Don't forget the pwd command to remind you where you are within the file system!
Another important command to use is the ls command, which lists the contents of the current directory. It's commonly used by itself, but a number of options (or switches) available for ls give you more information. For instance, the following command returns a listing of all the files and directories within the current directory, including any hidden files (denoted by a . prefix) as well as a full listing, so it will include details such as the permissions, owner and group, size and last modified time and date:
$ ls -al
You can also issue the command
$ ls -R
which scans and lists all the contents of the sub-directories of the current directory. This might be a lot of information, so you may want to redirect the output to a text file so you can browse through it at your leisure by using the following:
$ ls -alR > listing.txt
We've included a table showing some of the top-level directories that are part of a standard Linux distro in Table 4.1.
Table 4.1. Basic Linux Directories
The root directory
Boot loader files, Linux kernel
System configuration files
User home directories
Initial RAM disk boot support (used during boot time)
Shared libraries, kernel modules
Directory for recovered files (if found after a file system check)
Mount point for removable media, such as DVDs and floppy disks
Usual mount point for local, remote file systems
Add-on software packages
Kernel information, process control
Super-user (root) home
System commands (mostly root only)
Holds information relating to services that run on your system
Real-time information on devices used by the kernel
Secondary software file hierarchy
Variable data (such as logs); spooled files
Knowing these directories can aid you in partitioning in any future systems, letting you choose to put certain directories on their own distinct partition.
Some of the important directories in Table 4.1, such as those containing user and root commands or system configuration files, are discussed in the following sections. You use and edit files under these directories when you use Ubuntu.
Linux also includes a number of GNU commands you can use to search the file system. These include the following:
- whereis command —Returns the location of the command and its man page.
- whatis command —Returns a one-line synopsis from the command's man page.
- locate file —Returns locations of all matching file(s); an extremely fast method of searching your system because locate searches a database containing an index of all files on your system. However, this database (about 4MB in size and named slocate.db, under the /var/lib/slocate directory) is built daily at 4:20 a.m. by default, and does not contain pathnames to files created during the workday or in the evening. If you do not keep your machine on constantly, you can run the updatedb command either using sudo or by using the root account to manually start the building of the database.
- apropos subject —Returns a list of commands related to subject.
Managing Files with the Shell
Managing files in your home directory involves using one or more easily remembered commands. If you have any familiarity with the now-ancient DOS, you recognize some of these commands (although their names are different from those you remember). Basic file management operations include paging (reading), moving, renaming, copying, searching, and deleting files and directories. These commands include the following:
- cat filename —Outputs contents of filename to display
- less filename —Allows scrolling while reading contents of filename
- mv file1 file2 —Renames file1 to file2
- mv file dir —Moves file to specified directory
- cp file1 file2 —Copies file1 and creates file2
- rm file —Deletes file
- rmdir dir —Deletes directory (if empty)
- grep string file(s) —Searches through files(s) and displays lines containing matching string
Note that each of these commands can be used with pattern-matching strings known as wildcards or expressions. For example, to delete all files in the current directory beginning with the letters abc, you can use an expression beginning with the first three letters of the desired filenames. An asterisk (*) is then appended to match all these files. Use a command line with the rm command like this:
$ rm abc*
Linux shells recognize many types of filenaming wildcards, but this is different from the capabilities of Linux commands supporting the use of more complex expressions. You learn more about using wildcards in Chapter 11, "Automating Tasks."
Working with Compressed Files
Another file management operation is compression and decompression of files, or the creation, listing, and expansion of file and directory archives. Linux distributions usually include several compression utilities you can use to create, compress, expand, or list the contents of compressed files and archives. These commands include
- bunzip2—Expands a compressed file
- bzip2—Compresses or expands files and directories
- gunzip—Expands a compressed file
- gzip—Compresses or expands files and directories
- tar—Creates, expands, or lists the contents of compressed or uncompressed file or directory archives known as tape archives or tarballs
Most of these commands are easy to use. The tar command, however, has a somewhat complex (although capable) set of command-line options and syntax. Even so, you can quickly learn to use tar by remembering a few simple invocations on the command line. For example, to create a compressed archive of a directory, use tar's czf options like this:
$ tar czf dirname .tgz dirname
The result is a compressed archive (a file ending in .tgz) of the specified directory (and all files and directories under it). Add the letter v to the preceding options to view the list of files added during compression and archiving. To list the contents of the compressed archive, substitute the c option with the letter t, like this:
$ tar tzf archive
Of course, if many files are in the archive, a better invocation (to easily read or scroll through the output) is
$ tar tzf archive | less
To expand the contents of a compressed archive, use tar's zxf options, like so:
$ tar zxf archive
The tar utility decompresses the specified archive and extracts the contents in the current directory.
Use Essential Commands from the /bin and /sbin Directories
The /bin directory (about 5MB if you do a full install) contains essential commands used by the system for running and booting Linux. In general, only the root operator uses the commands in the /sbin directory. Many (though not all) these commands are statically linked which means that such commands do not depend on software libraries residing under the /lib or /usr/lib directories. Nearly all the other applications on your system are dynamically linked—meaning that they require external software libraries (also known as shared libraries) to run.
Use and Edit Files in the /etc Directory
More than 10MB of system configuration files and directories reside under the /etc directory if you install all the software included with this book. Some major software packages, such as Apache, OpenSSH, and xinetd, have directories of configuration files under /etc. Other important system-related configuration files in /etc are
- fstab— The file system table is a text file listing each hard drive, CD-ROM, floppy, or other storage device attached to your PC. The table indexes each device's partition information with a place in your Linux file system (directory layout) and lists other options for each device when used with Linux (see Chapter 32, "Kernel and Module Management"). Nearly all entries in fstab can be manipulated by root using the mount command.
- modprobe.d/— This folder holds all the instructions to load kernel modules that are required as part of the system startup, and replaces the historic modprobe.conf file.
- passwd— The list of users for the system, along with user account information. The contents of this file can be changed by various programs, such as useradd or chsh.
- shells— A list of approved shells (command-line interfaces).
Protect the Contents of User Directories—/home
The most important data on a Linux system resides in the user's directories, found under the /home directory. Segregating the system and user data can be helpful in preventing data loss and making the process of backing up easier. For example, having user data reside on a separate file system or mounted from a remote computer on the network might help shield users from data loss in the event of a system hardware failure.
Use the Contents of the /proc Directory to Interact with the Kernel
The content of the /proc directory is created from memory and exists only while Linux is running. This directory contains special "files" that either extract information from or send information to the kernel. Many Linux utilities extract information from dynamically created directories and files under this directory, also known as a virtual file system. For example, the free command obtains its information from a file named meminfo:
$ free total used free shared buffers cached Mem: 1026320 822112 204208 0 41232 481412 -/+ buffers/cache: 299468 726852 Swap: 2031608 0 2031608
This information constantly changes as the system is used. You can get the same information by using the cat command to see the contents of the meminfo file:
$ cat /proc/meminfo MemTotal: 1026320 kB MemFree: 204200 kB Buffers: 41252 kB Cached: 481412 kB SwapCached: 0 kB Active: 307232 kB Inactive: 418224 kB HighTotal: 122692 kB HighFree: 244 kB LowTotal: 903628 kB LowFree: 203956 kB SwapTotal: 2031608 kB SwapFree: 2031608 kB Dirty: 0 kB Writeback: 0 kB AnonPages: 202804 kB Mapped: 87864 kB Slab: 21736 kB SReclaimable: 12484 kB SUnreclaim: 9252 kB PageTables: 5060 kB NFS_Unstable: 0 kB Bounce: 0 kB CommitLimit: 2544768 kB Committed_AS: 712024 kB VmallocTotal: 114680 kB VmallocUsed: 6016 kB VmallocChunk: 108148 kB HugePages_Total: 0 HugePages_Free: 0 HugePages_Rsvd: 0 Hugepagesize: 4096 kB
The /proc directory can also be used to dynamically alter the behavior of a running Linux kernel by "echoing" numerical values to specific files under the /proc/sys directory. For example, to "turn on" kernel protection against one type of denial of service (DOS) attack known as SYN flooding, use the echo command to send the number 1 (one) to the following /proc path:
$ sudo echo 1 >/proc/sys/net/ipv4/tcp_syncookies
Other ways to use the /proc directory include
- Getting CPU information, such as the family, type, and speed from /proc/cpuinfo.
- Viewing important networking information under /proc/net, such as active interfaces information under /proc/net/dev, routing information in /proc/net/route, and network statistics in /proc/net/netstat.
- Retrieving file system information.
- Reporting media mount point information via USB; for example, the Linux kernel reports what device to use to access files (such as /dev/sda) if a USB camera or hard drive is detected on the system. You can use the dmesg command to see this information.
- Getting the kernel version in /proc/version, performance information such as uptime in /proc/uptime, or other statistics such as CPU load, swap file usage, and processes in /proc/stat.
Work with Shared Data in the /usr Directory
The /usr directory contains software applications, libraries, and other types of shared data for use by anyone on the system. Many Linux system administrators give /usr its own partition. A number of subdirectories under /usr contain manual pages (/usr/share/man), software package shared files (/usr/share/ name_of_package , such as /usr/share/emacs), additional application or software package documentation (/usr/share/doc), and an entire subdirectory tree of locally built and installed software, /usr/local.
Temporary File Storage in the /tmp Directory
As its name implies, the /tmp directory is used for temporary file storage; as you use Linux, various programs create files in this directory.
Access Variable Data Files in the /var Directory
The /var directory contains subdirectories used by various system services for spooling and logging. Many of these variable data files, such as print spooler queues, are temporary, whereas others, such as system and kernel logs, are renamed and rotated in use. Incoming electronic mail is usually directed to files under /var/spool/mail.
Linux also uses /var for other important system services. These include the top-most File Transfer Protocol (FTP) directory under /var/ftp (see Chapter 18, "Remote File Serving with FTP"), and the Apache web server's initial home page directory for the system, /var/www/html. (See Chapter 17, "Apache Web Server Management," for more information on using Apache.)