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274 CHAPTER 17 COOL SHELL TRICKS and then pipe the results to grep, which is able to search through text for a user-defined string (see Chapter 15): ls l | grep i ‘flower’ In this example, the shell runs the ls l command, and then passes the output to grep. The grepcommand then searches the output for the word flower (the i option tells it to ignore uppercase and lowercase). If grep finds any results, it will show them on your screen. The key point to remember is that grep is used here as it normally is at the command prompt. The only difference is that it s being passed input from a previous command, rather than being used on its own. You can pipe more than once on a command line. Suppose you know that the filename of the picture you want involves the words flower and daffodil, yet you re unsure of where they might fall in the filename. In this case, you could type the following: ls l | grep i flower | grep -i daffodil This will pass the result of the directory listing to the first grep, which will search the output for the word flower. The second pipe causes the output from grep to be passed to the second grep command, where it s then searched for the word daffodil. Any results are then displayed on your screen. Redirecting Output Redirecting is like piping, except that the output is passed to a file rather than to another command. Redirecting can also work the other way: the contents of a file can be passed to a command. If you wanted to create a file that contained a directory listing, you could type this: ls l > directorylisting.txt The angle bracket (>) between the commands tells BASH to direct the output of the ls l command into a file called directorylisting.txt. If a file with this name exists, it s overwritten with new data. If it doesn t exist, it s created from scratch. You can add data to an already existing file using two angle brackets: ls l >> directorylisting.txt This will append the result of the directory listing to the end of the file directorylisting.txt, although, once again, if the file doesn t exist, it will be created from scratch. Redirecting output can get very sophisticated and useful. Take a look at the following: cat myfile1.txt myfile2.txt > myfile3.txt As you learned in Chapter 15, the cat command joins two or more files together. If the command were used on its own without the redirection, it would cause BASH to print myfile1.txt on the screen, immediately followed by myfile2.txt. As far as BASH is concerned, it has joined myfile1.txtto myfile2.txt, and then sent them to standard output (the screen). By specifying a redirection, you have BASH send the output to a third file. Using cat with redirection is a handy way of combining two files.

CHAPTER 17 COOL SHELL TRICKS Let s assume that you have a directory that is packed full of files. You want to do a long directory listing (ls l) to see what permissions various files have. But doing this produces reams of output that fly off the screen. Typing something like the following provides a solution: ls l | less The | symbol between the two commands is the pipe. It can be found on most US keyboards next to the square bracket keys (above the Enter key; you ll need to hold down the Shift key to get it). What happens in the example is that ls l is run by the shell, but rather than sending the output to the screen, the pipe symbol (|) tells BASH to send it to the command that follows to less. In other words, the listing is displayed within less, where you can read it at your leisure. You can use Page Up and Page Down or the arrow keys to scroll through it. Once you quit less, the listing evaporates into thin air; the piped output is never actually stored as a file. In the previous section, you saw how you can use the history command to view the command history. At around 1,000 entries, its output scrolls off the screen in seconds. However, you can pipe it to less, like so: history | less Figure 17-3 shows the result on my test PC. Figure 17-3. Piping the output of the history command into the less command lets you read the output fully. You can pipe the output of any command. One of the most common uses is when searching for a particular string in the output of a command. For example, let s say you know that, within a crowded directory, there s a file with a picture of some flowers. You know that the word flower is in the filename, but can t recall any other details. One solution is to perform a directory listing,

272 CHAPTER 17 COOL SHELL TRICKS Pressing Ctrl+R lets you search the command history from the command prompt. This particular tool can be tricky to get used to, however. As soon as you start typing, BASH will autocomplete the command based on matches found in the history file, starting with the last command in the history. What you type appears before the colon, while the autocompletion appears afterwards. Because BASH autocompletes as you type, things can get a little confusing when you re working with the command history, particularly if it initially gets the match wrong. For example, typing cd will show the last instance of the use of cd, as in the example in Figure 17-2. This might not be what you re looking for, so you must keep typing the command you do want until it autocompletes correctly. Figure 17-2. BASH history completion is very useful but can also be confusing. Piping and Directing Output It s not uncommon for a directory listing or output from another command to scroll off the screen. When using a GUI program like GNOME Terminal, you can use the scroll bars to view the output, but what if you are working at the bare command-line prompt? By pressing Shift+Page Up and Shift+Page Down, you can scroll the window up to take a look at some of the old output, but very little is cached in this way, and you won t see more than a few screens. A far better solution is to pipe the output of the directory listing into a text viewer. Another useful technique is to redirect output to a file. Piping the Output of Commands Piping was one of the original innovations provided by Unix. It simply means that you can pass the output of one command to another, which is to say the output of one command can be used as input for another. This is possible because shell commands work like machines. They usually take input from the keyboard (referred to technically as standard input) and, when they ve done their job, usually show their output on the screen (known as standard output). The commands don t need to take input from the keyboard, and they don t need to output to the screen. Piping is the process of diverting the output before it reaches the screen and passing it to another command for further processing.

CHAPTER 17 COOL SHELL TRICKS While it might not seem very useful for day-to-day work, Ctrl+D is vital for programs that expect you to enter data at the command line. You might run into these as you explore BASH. If ever you read in a man page that a program requires an EOF message during input, you ll know what to press. Using the Command History The original hackers who invented the tools used under Unix hated waiting around for things to happen. After all, being a hacker is all about finding the most efficient way of doing any particular task. Because of this, the BASH shell includes many features designed to optimize the user experience. The most important of these is the command history. BASH remembers every command you enter (even the ones that don t work!) and stores them as a list on your hard disk. During any BASH session, you can cycle through this history using the up and down arrow keys. Pressing the up arrow key takes you back into the command history, and pressing the down arrow key takes you forward. The potential of the command history is enormous. For example, rather than retype that long command that runs a program with command options, you can simply use the cursor keys to locate it in the history and press Enter. Tip Typing !-3 will cause BASH to move three paces back in the history file and run that command. In other words, it will run what you entered three commands ago. On my Ubuntu test system, BASH remembers 1000 commands. You can view all of the remembered commands by typing history at the command prompt. The history list will scroll off the screen because it s so large, but you can use the scroll bars of the GNOME Terminal window to read it. Each command in the history list is assigned a number. You can run any of the history commands by preceding their number with an exclamation mark (!), referred to as a bang, or sometimes a shriek. For example, you might type !923. On my test system, command number 923 in the BASH history is cd .., so this has the effect of switching me into the parent directory. Command numbering remains in place until you log out (close the GNOME Terminal window or end a virtual console session). After this, the numbering is reordered. There will still be 1000 commands, but the last command you entered before logging out will be at the end of the list, and the numbering will work back 1000 places until the first command in the history list. Tip One neat trick is to type two bangs: !!. This tells BASH to repeat the last command you entered. Rather than specifying a command number, you can type something like !cd. This will cause BASH to look in the history file, find the last instance of a command line that started with cd, and then run it.

270 CHAPTER 17 COOL SHELL TRICKS Table 17-1. Keyboard Shortcuts in BASH (Continued) Shortcut Description Editing Ctrl+U Delete everything behind cursor to start of line Ctrl+K Delete from cursor to end of line Ctrl+W Delete from cursor to beginning of word Alt+D Delete from cursor to end of word Ctrl+T Transpose characters on left and right of cursor Alt+T Transpose words on left and right of cursor Miscellaneous Ctrl+L Clear screen (everything above current line) Ctrl+U Undo everything since last command1 Alt+R Undo changes made to the line2 Ctrl+Y Undo deletion of word or line caused by using Ctrl+K, Ctrl+W, and so on3 Alt+L Lowercase current word (from the cursor to end of word) 1 In most cases, this has the effect of clearing the line. 2 This is different from Ctrl+U, because it will leave intact any command already on the line, such as one pulled from your command history. 3 This allows primitive cutting and pasting. Delete the text and then immediately undo, after which the text will remain in the buffer and can be pasted with Ctrl+Y. Shortcuts for System Control In terms of the control over your system offered by keyboard commands, as mentioned in Chapter 16, pressing Ctrl+Z has the effect of stopping the current program. It suspends the program until you switch back into it or tell it to resume in another way, or manually kill it. In the same style, pressing Ctrl+C while a program is running will quit it. This sends the program s process a termination signal, a little like killing it using the top program. Ctrl+C can prove handy if you start a program running by accident and quickly want to end it, or if a command takes longer than you expected to work and you cannot wait for it to complete. It s also a handy way of attempting to end crashed programs. Some complicated programs don t take too kindly to being quit in this way, particularly those that need to save data before they terminate. However, most should be okay. Ctrl+D is another handy keyboard shortcut. This sends the program an end-of-file (EOF) message. In effect, this tells the program that you ve finished your input. This can have a variety of effects, depending on the program you re running. For example, pressing Ctrl+D on its own at the shell prompt when no program is running will cause you to log out (if you re using a GUI terminal emulator like GNOME Terminal, the program will quit). This happens because pressing Ctrl+D informs the BASH shell program that you ve finished your input. BASH then interprets this as the cue that it should log you out. After all, what else can it do if told there will be no more input?

CHAPTER 17 COOL SHELL TRICKS Figure 17-1. Autocompletion makes using BASH much easier. Using Keyboard Shortcuts Your other good friends when using BASH are the Ctrl and Alt keys. These keys provide shortcuts to vital command-line shell functions. They also let you work more efficiently when typing by providing what most programs call keyboard shortcuts. Shortcuts for Working in BASH Table 17-1 lists the most common keyboard shortcuts in BASH (there are many more; see BASH s man page for details). If you ve explored the Emacs text editor, you might find these shortcuts familiar. Such keyboard shortcuts are largely the same across many of the software packages that originate from the GNU Project. Often, you ll find an option within many Ubuntu software packages that lets you use Emacs-style navigation, in which case, these keyboard shortcuts will most likely work equally well. Table 17-1. Keyboard Shortcuts in BASH Shortcut Description Navigation Left/right cursor key Move left/right in text Ctrl+A Move to beginning of line Ctrl+E Move to end of line Ctrl+right arrow Move forward one word Ctrl+left arrow Move left one word

268 CHAPTER 17 COOL SHELL TRICKS Autocompletion with Files and Paths Tab autocompletion also works with files and paths. If you type the first few letters of a folder name, BASH will try to fill in the rest. This also obviously has limitations. There s no point in typing cd myfoland pressing Tab if there s nothing in the current directory that starts with the letters myfol. This particular autocomplete function works by looking at your current directory and seeing what s available. Alternatively, you can specify an initial path for BASH to use in order to autocomplete. Typing cd /ho and pressing Tab will cause BASH to autocomplete the path by looking in the root directory (/). In other words, it will autocomplete the command with the directory home. In a similar way, typing cd myfolder/myfowill cause BASH to attempt to autocomplete by looking for a match in myfolder. If you want to run a program that resides in the current directory, such as one you ve just downloaded for example, typing ./, followed by the first part of the program name, and then pressing Tab should be enough to have BASH autocomplete the rest. In this case, the dot and slash tell BASH to look in the current directory for any executable programs or scripts (programs with Xas part of their permissions) and use them as possible autocomplete options. BASH is clever enough to spot whether the command you re using is likely to require a file, directory, or executable, and it will autocomplete with only relevant file or directory names. Viewing Available Options The autocomplete function has a neat side effect. As I mentioned earlier, if BASH cannot find a match, pressing Tab again causes BASH to show all the available options. For example, typing baat the shell, and then pressing Tab twice will cause BASH to show all the possible commands starting with the letters ba. On my test PC, this produces the following list of commands: badblocks base-config bash batch banner basename bashbug This can be a nice way of exploring what commands are available on your system. You can then use each command with the –help command option to find out what it does, or browse the command s man page. When you apply this trick to directory and filename autocompletion, it s even more useful. For example, typing cd in a directory, and then pressing the Tab key twice will cause BASH to show the available directories, providing a handy way of retrieving a brief directory listing. Alternatively, if you ve forgotten how a directory name is spelled, you can use this technique to find out prior to switching into it. Figure 17-1 shows a few examples of using this technique with BASH.

CHAPTER 17 Cool Shell Tricks The BASH shell is the product of many years of development work by a lot of people. It comes from the old days of Unix and was an important step in computer software evolution. It s a program that retains complete simplicity, yet packs in more features than most users could ever hope to use. One of the best things about the shell is its sheer power. If you ever wonder if you can do a task differently (and more efficiently), you ll probably find that one of the many BASH developers has implemented a method to do so. Once you learn these techniques, you ll find you can whiz around the shell at blinding speed. It s just a matter of exploring the far reaches of the shell, and that s what you ll do in this chapter. Hold onto your hats, because it s an exciting ride! Using Autocompletion The Tab key is your best friend when using the shell, because it will cause BASH to automatically complete whatever you type. For example, if you want to run Ubuntu s web browser, you can enter firefox at the command line. However, to save yourself some time, you can type fir, and then press Tab. You ll then find that BASH fills in the rest for you. It does this by caching the names of the programs you might run according to the directories listed in your $PATH variable (see Chapter 13). Of course, autocompletion has some limitations. On my Ubuntu test system, typing loc didn t autocomplete locate. Instead, it caused BASH to beep. This is because on a default Ubuntu installation, there is more than one possible match. Pressing Tab again shows those matches. Depending on how much you type (how much of an initial clue you give BASH), you might find there are many possible matches. In this case, the experienced BASH user simply types another letter, which will be enough to distinguish the almost-typed word from the rest, and presses Tab again. With any luck, this should be enough for BASH to fill in the rest.

CHAPTER 16 TAKING CONTROL OF THE SYSTEM Tip If you ve ever tried to run a GUI program from the shell, you might have realized that the shell is inaccessible while it s running. Once you quit the GUI program, the control of the shell will be returned to you. By specifying that the program should run in the background with the & (ampersand symbol), you can run the GUI program and still be able to type away and run other commands. You can send several jobs to the background, and each one will be given a different job number. In this case, when you wish to switch into a running job, you can type its number. For example, the following command will switch you to the background job assigned the number 3: %3 You can exit a job that is currently running by pressing Ctrl+Z. It will still be there in the background, but it won t be running (officially, it s said to be sleeping). To restart it, you can switch back to it, as just described. Alternatively, you can restart it but still keep it in the background. For example, to restart job 2 in the background, leaving the shell prompt free for you to enter other commands, type the following: %2 & You can bring the command in the background into the foreground by typing the following: fg When a background job has finished, something like the following will appear at the shell: [1]+ Done unzip myfile.zip Using jobs within the shell can be a good way of managing your workload. For example, you can move programs into the background temporarily while you get on with something else. If you re editing a file in vi, you can press Ctrl+Z to stop the program. It will remain in the background, and you ll be returned to the shell, where you can type other commands. You can then resume vi later on by typing fg or typing %followed by its job number. Summary This chapter has covered taking complete control of your system. We ve looked at what processes are, how they re separate from programs, and how they can be controlled or viewed using programs such as topand ps. In addition, we explored job management under BASH. You saw that you can stop, start, and pause programs at your convenience. In the next chapter, we ll take a look at several tricks and techniques that you can use with the BASH shell to finely hone your command-line skills.

264 CHAPTER 16 TAKING CONTROL OF THE SYSTEM CLEARING UP CRASHES Sometimes, a crashed process can cause all kinds of problems. The shell you re working at may stop working, or the GUI itself might stop working properly. In cases like this, it s important to remember that you can have more that one instance of the command- line shell up and running at any one time. For example, if a process crashes and locks up GNOME Terminal, simply start a new instance of GNOME Terminal (Applications . Accessories . Terminal). Then use top within the new window to kill the process that is causing trouble for the other terminal window. If the crashed program affects the entire GUI, you can switch to a virtual console by pressing Ctrl+Alt+F1. Although the GUI disappears, you will not have killed it, and no programs will stop running. Instead, you ve simply moved the GUI to the background while a shell console takes over the screen. Then you can use the virtual console to run top and attempt to kill the process that is causing all the problems. When you re ready, you can switch back to the GUI by pressing Ctrl+Alt+F7. Controlling Jobs Whenever you start a program at the shell, it s assigned a job number. Jobs are quite separate from processes and are designed primarily for users to understand what programs are running on the system. You can see which jobs are running at any one time by typing the following at the shell prompt: jobs When you run a program, it usually takes over the shell in some way and stops you from doing anything until it s finished what it s doing. However, it doesn t have to be this way. Adding an ampersand symbol (&) after the command will cause it to run in the background. This is not much use for commands that require user input, such as vior top, but it can be very handy for commands that churn away until they re completed. For example, suppose that you want to decompress a large zip file. For this, you can use the unzipcommand. As with Windows, decompressing large zip files can take a lot of time, during which time the shell would effectively be unusable. However, you can type the following to retain use of the shell: unzip myfile.zip & When you do this, you ll see something similar to the following, although the four-digit number will be different: [1] 7483 This tells you that unzip is running in the background and has been given job number 1. It also has been given process number 7483 (although bear in mind that when some programs start, they instantly kick off other processes and terminate the one they re currently running, so this won t necessarily be accurate).