Slackware Linux Essentials
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8.2 The Command -Line

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8.2.1 Running Programs

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It's hard to get much accomplished without running a program; you might be able to -prop something up with your computer or hold a door open, and some will make the most -lovely humming noise when running, but that's really about it. And I think we can all -agree that its use as a humming doorstop isn't what brought the personal computer the -popularity it now enjoys.

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So, remember how almost everything in Linux is a file? Well, that goes for programs, -too. Every command you run (that isn't built into the shell) resides as a file somewhere. -You run a program simply by specifying the full path to it.

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For instance, remember that su command from the last section? -Well, it's actually in the /bin directory: /bin/su would run it nicely.

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So why, then, does just typing su work? After all, you didn't -say it was in /bin. It could just as easily have been in /usr/local/share, right? How did it know? The answer to that lies in the PATH environment variable; most shells have either PATH or something very much like PATH. It -basically contains a list of directories to look in for programs you try to run. So when -you ran su, your shell ran through its list of directories, -checking each one for an executable file called su that it could -run; the first one it came to, it ran. This happens whenever you run a program without -specifying a full path to it; if you get a ““Command -not found”” error, that only means that the program you tried to run -isn't in your PATH. (Of course, this would be true if the program -doesn't exist at all...) We'll discuss environment variables in more depth in Section 8.3.1.

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Remember also that “.” is shorthand for the -current directory, so if you happened to be in /bin, ./su would have worked as an explicit full path.

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8.2.2 Wildcard Matching

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Nearly every shell recognizes some characters as being substitutes or abbreviations -that mean anything goes here. Such characters are aptly named wildcards; the most common -are * and ?. By convention, ? usually matches any single character. For instance, suppose -you're in a directory with three files: ex1.txt, ex2.txt, and ex3.txt. You want to copy -all of those files (using the cp command we cover in Section 10.5.1) to -another directory, say /tmp. Well, typing cp ex1.txt ex2.txt ex3.txt /tmp is entirely too much work. It's much -easier to type cp ex?.txt /tmp; the ? -will match each of the characters “1”, “2”, and “3”, -and each in turn will be substituted in.

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What's that you say? That's still too much work? You're right. It's appalling; we have -labor laws to protect us from that sort of thing. Fortunately, we also have *. As was already mentioned, * matches -“any number of characters”, including 0. So if those three files were the -only ones in the directory, we could have simply said cp * /tmp -and gotten them all in one fell swoop. Suppose, though, that there is also a file called -ex.txt and one called hejaz.txt. We -want to copy ex.txt but not hejaz.txt; cp ex* /tmp will do that for -us.

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cp ex?.txt /tmp, would, of course, only get our original -three files; there's no character in ex.txt to match that ?, so it would be left out.

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Another common wildcard is the bracket pair [ ]. Any -characters inside the brackets will be substituted in place of the [ -] to find matches. Sound confusing? It's not too bad. Suppose for instance, we have -a directory containing the following 8 files: a1, a2, a3, a4, aA, aB, aC, and -aD . We want to only find the files ending in numbers; [ ] will do this for us.

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-% ls a[1-4]
-a1 a2 a3 a4
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But what we really want is just a1, a2, and a4? In the previous example we -used - to mean all values between 1 and 4. We can also separate individual entries with -commas.

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-% ls a[1,2,4]
-a1 a2 a4
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I know what you're thinking now, “Well what about letters?” Linux is -case-sensitive, meaning that a and A are different characters and are only related in your mind. -Capitals always come before lowercase letters, so A and B come before a and b. Continuing with our earlier example, if we wanted files a1, and A1, we can find these quickly -with [ ].

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-% ls [A,a]1
-A1 a1
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Note, that if we had included a hyphen instead of a comma, we would have gotten -incorrect results.

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-% ls [A-a]1
-A1 B1 C1 D1 a1
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You can also combine hyphen and comma strings.

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-% ls [A,a-d]
-A1 a1 b1 c1 d1
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8.2.3 Input/Output Redirection and Piping

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(Here comes something cool.)

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-% ps > blargh
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Y'know what that is? That's me running ps to see which -processes are running; ps is covered in Section 11.3. That's not the cool part. The cool part -is > blargh, which means, roughly, take the output from ps and write it to a file called blargh. -But wait, it gets cooler.

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-% ps | less
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That one takes the output from ps and pipes it through less, so I can scroll through it at my leisure.

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-% ps >> blargh
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This is the third most commonly used redirector; it does the same thing as -“>”, except that “>>” will append output from ps to the file blargh, if said file -exists. If not, just like “>”, it will be created. (“>” -will obliterate the current contents of blargh.)

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There is also a “<” operator, which means take your input from the -following, but it's not used nearly so often.

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-% fromdos < dosfile.txt > unixfile.txt
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Redirection gets really fun when you start piling it up:

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-% ps | tac >> blargh
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That will run ps, reverse the lines of its output, and append -those to the file blargh. You can stack as many of these up as -you want; just be careful to remember that they get interpreted from left to right.

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See the bash(1) man page for more detailed information on -redirection.