Table of Contents
So you've installed Slackware and you're staring at a terminal prompt, what now? Now would be a good time to learn about the basic command line tools. And since you're staring at a blinking curser, you probably need a little assistance in knowing how to get around, and that is what this chapter is all about.
Your Slackware Linux system comes with lots of built-in documentation for nearly every installed application. Perhaps the most common method of reading system documentation is man(1). man (short for manual) will bring up the included man-page for any application, system call, configuration file, or library you tell it too. For example, man man will bring up the man-page for man itself.
Unfortunately, you may not always know what application you need to use for the task at hand. Thankfully, man has built-in search abilities. Using the -k switch will cause man to search for every man-page that matches your search terms.
The man-pages are organized into groups or sections by their content type. For example, section 1 is for user applications. man will search each section in order and display the first match it finds. Sometimes you will find that a man-page exists in more than one section for a given entry. In that case, you will need to specify the exact section to look in. In this book, all applications and a number of other things will have a number on their right-hand side in parenthesis. This number is the man page section where you will find information on that tool.
darkstar:~$ man -k printf printf (1) - format and print data printf (3) - formatted output conversion darkstar:~$ man 3 printf
Man Page Sections
|3||C Library Calls|
|5||File Formats / Protocols|
|7||Conventions / Macro Packages|
|9||Kernel API Descriptions|
|n||“New” - typically used to Tcl/Tk|
Dealing with Files and Directories
Listing Files and Directory Contents
ls(1) is used to list files and directories,
their permissions, size, type, inode number, owner and group, and
plenty of additional information. For example, let's list what's in
/ directory for your new Slackware Linux system.
darkstar:~$ ls / bin/ dev/ home/ lost+found/ mnt/ proc/ sbin/ sys/ usr/ boot/ etc/ lib/ media/ opt/ root/ srv/ tmp/ var/
Notice that each of the listings is a directory. These are easily distinguished from regular files due to the trailing /; standard files do not have a suffix. Additionally, executable files will have an asterisk suffix. But ls can do so much more. To get a view of the permissions of a file or directory, you'll need to do a “long list”.
darkstar:~$ ls -l /home/alan/Desktop -rw-r--r-- 1 alan users 15624161 2007-09-21 13:02 9780596510480.pdf -rw-r--r-- 1 alan users 3829534 2007-09-14 12:56 imgscan.zip drwxr-xr-x 3 alan root 168 2007-09-17 21:01 ipod_hack/ drwxr-xr-x 2 alan users 200 2007-12-03 22:11 libgpod/ drwxr-xr-x 2 alan users 136 2007-09-30 03:16 playground/
A long listing lets you view the permisions, user and group ownership, file size, last modified date, and of course, the file name itself. Notice that the first two entires are files, and the last three are directories. This is denoted by the very first character on the line. Regular files get a “-”; directories get a “d”. There are several other file types with their own denominators. Symbolic links for example will have an “l”.
Lastly, we'll show you how to list dot-files, or hidden files. Unlike other operating systems such as Microsoft Windows, there is no special property that differentiates “hidden” files from “unhidden” files. A hidden file simply begins with a dot. To display these files along with all the others, you just need to pass the -a argument to ls.
darkstar:~$ ls -a .xine/ .xinitrc-backup .xscreensaver .xsession-errors SBo/ .xinitrc .xinitrc-xfce .xsession .xwmconfig/ Shared/
You also likely noticed that your files and directories appear in different colors. Many of the enhanced features of ls such as these colors or the trailing characters indicating file-type are special features of the ls program that are turned on by passing various arguments. As a convienience, Slackware sets up ls to use many of these optional arguments by default. These are controlled by the LS_OPTIONS and LS_COLORS environment variables. We will talk more about environment variables in chapter 5.
Moving Around the Filesystem
cd is the command used to change directories. Unlike most other commands, cd is actually not it's own program, but is a shell built-in. Basically, that means cd does not have its own man page. You'll have to check your shell's documentation for more details on the cd you may be using. For the most part though, they all behave the same.
darkstar:~$ cd / darkstar:/$ls bin/ dev/ home/ lost+found/ mnt/ proc/ sbin/ sys/ usr/ boot/ etc/ lib/ media/ opt/ root/ srv/ tmp/ var/ darkstar:/$cd /usr/local darkstar:/usr/local$
Notice how the prompt changed when we changed directories? The default Slackware shell does this as a quick, easy way to see your current directory, but this is not actually a function of cd. If your shell doesn't operate in this way, you can easily get your current working directory with the pwd(1) command. (Most UNIX shells have configurable prompts that can be coaxed into providing this same functionality. In fact, this is another convience setup in the default shell for you by Slackware.)
darkstar:~$ pwd /usr/local
File and Directory Creation and Deletion
While most applications can and will create their own files and directories, you'll often want to do this on your own. Thankfully, it's very easy using touch(1) and mkdir(1).
touch actually modifies the timestamp on a file, but if that file doesn't exist, it will be created.
darkstar:~/foo$ ls -l -rw-r--r-- 1 alan users 0 2012-01-18 15:01 bar1 darkstar:~/foo$ touch bar2 -rw-r--r-- 1 alan users 0 2012-01-18 15:01 bar1 -rw-r--r-- 1 alan users 0 2012-01-18 15:05 bar2 darkstar:~/foo$ touch bar1 -rw-r--r-- 1 alan users 0 2012-01-18 15:05 bar1 -rw-r--r-- 1 alan users 0 2012-01-18 15:05 bar2
bar2 was created in our second command,
and the third command simpl updated the timestamp on
mkdir is used for (obviously enough) making directories. mkdir foo will create the directory “foo” within the current working directory. Additionally, you can use the -p argument to create any missing parent directories.
darkstar:~$ mkdir foo darkstar:~$ mkdir /slack/foo/bar/ mkdir: cannot create directory `/slack/foo/bar/': No such file or directory darkstar:~$ mkdir -p /slack/foo/bar/
In the latter case, mkdir will first create “/slack”, then “/slack/foo”, and finally “/slack/foo/bar”. If you failed to use the -p argument, mkdir would fail to create “/slack/foo/bar” unless the first two already existed, as you saw in the example.
Removing a file is as easy as creating one. The rm(1) command will remove a file (assuming of course that you have permission to do this). There are a few very common arguments to rm. The first is -f and is used to force the removal of a file that you may lack explicit permission to delete. The -r argument will remove directories and their contents recursively.
There is another tool to remove directories, the humble rmdir(1). rmdir will only remove directories that are empty, and complain noisely about those that contain files or sub-directories.
darkstar:~$ ls foo_1/ foo_2/ darkstar:~$ ls foo_1 bar_1 darkstar:~$ rmdir foo_1 rmdir: foo/: Directory not empty darkstar:~$ rm foo_1/bar darkstar:~$ rmdir foo_1 darkstar:~$ ls foo_2 bar_2/ darkstar:~$ rm -fr foo_2 darkstar:~$ ls
Archive and Compression
Everyone needs to package a lot of small files together for easy storage from time to time, or perhaps you need to compress very large files into a more manageable size? Maybe you want to do both of those together? Thankfully there are several tools to do just that.
zip and unzip
You're probably familiar with .zip files. These are compressed files that contain other files and directories. While we don't normally use these files in the Linux world, they are still commonly used by other operating systems, so we occasionally have to deal with them.
In order to create a zip file, you'll (naturally) use the zip(1) command. You can compress either files or directories (or both) with zip, but you'll have to use the -r argument for recursive action in order to deal with directories.
darkstar:~$ zip -r /tmp/home.zip /home darkstar:~$ zip /tmp/large_file.zip /tmp/large_file
The order of the arguments is very important. The first filename must be the zip file to create (if the .zip extension is ommitted, zip will add it for you) and the rest are files or directories to be added to the zip file.
Naturally, unzip(1) will decompress a zip archive file.
darkstar:~$ unzip /tmp/home.zip
One of the oldest compression tools included in Slackware is gzip(1), a compression tool that is only capable or operating on a single file at a time. Whereas zip is both a compression and an archival tool, gzip is only capable of compression. At first glance this seems like a draw-back, but it is really a strength. The UNIX philosophy of making small tools that do their small jobs well allows them to be combined in myriad ways. In order to compress a file (or multiple files), simply pass them as arguments to gzip. Whenever gzip compresses a file, it adds a .gz extension and removes the original file.
darkstar:~$ gzip /tmp/large_file
Decompressing is just as straight-forward with gunzip which will create a new uncompressed file and delete the old one.
darkstar:~$ gunzip /tmp/large_file.gz darkstar:~$ ls /tmp/large_file* /tmp/large_file
But suppose we don't want to delete the old compressed file, we just want to read its contents or send them as input to another program? The zcat program will read the gzip file, decompress it in memory, and send the contents to the standard output (the terminal screen unless it is redirected, see the section called “Input and Output Redirection” for more details on output redirection).
darkstar:~$ zcat /tmp/large_file.gz Wed Aug 26 10:00:38 CDT 2009 Slackware 13.0 x86 is released as stable! Thanks to everyone who helped make this release possible -- see the RELEASE_NOTES for the credits. The ISOs are off to the replicator. This time it will be a 6 CD-ROM 32-bit set and a dual-sided 32-bit/64-bit x86/x86_64 DVD. We're taking pre-orders now at store.slackware.com. Please consider picking up a copy to help support the project. Once again, thanks to the entire Slackware community for all the help testing and fixing things and offering suggestions during this development cycle.
One alternative to gzip is the bzip2(1) compression utility which works in almost the exact same way. The advantage to bzip2 is that it boasts greater compression strength. Unfortunately, achieving that greater compression is a slow and CPU-intensive process, so bzip2 typically takes much longer to run than other alternatives.
XZ / LZMA
The latest compression utility added to Slackware is xz, which implements the LZMA compression algorithm. This is faster than bzip2 and often compresses better as well. In fact, its blend of speed and compression strength caused it to replace gzip as the compression scheme of choice for Slackware. Unfortunately, xz does not have a man page at the time of this writing, so to view available options, use the –help argument. Compressing files is accomplished with the -z argument, and decompression with -d.
darkstar:~$ xz -z /tmp/large_file
So great, we know how to compress files using all sorts of programs, but none of them can archive files in the way that zip does. That is until now. The Tape Archiver, or tar(1) is the most frequently used archival program in Slackware. Like other archival programs, tar generates a new file that contains other files and directories. It does not compress the generated file (often called a “tarball”) by default; however, the version of tar included in Slackware supports a variety of compression schemes, including the ones mentioned above.
Invoking tar can be as easy or as complicated as you like. Typically, creating a tarball is done with the -cvzf arguments. Let's look at these in depth.
|c||Create a tarball|
|x||Extract the contents of a tarball|
|t||Display the contents of a tarball|
|v||Be more verbose|
|z||Use gzip compression|
|j||Use bzip2 compression|
|J||Use LZMA compression|
tar requires a bit more precision than other applications in the order of its arguments. The -f argument must be present when reading or writing to a file for example, and the very next thing to follow must be the filename. Consider the following examples.
darkstar:~$ tar -xvzf /tmp/tarball.tar.gz darkstar:~$ tar -xvfz /tmp/tarball.tar.gz
Above, the first example works as you would expect, but the second
fails because tar has been instructed to
z file rather than the expected
Now that we've got our arguments straightened out, lets look at a few examples of how to create and extract tarballs. As we've noted, the -c argument is used to create tarballs and -x extracts their contents. If we want to create or extract a compressed tarball though, we also have to specify the proper compression to use. Naturally, if we don't want to compress the tarball at all, we can leave these options out. The following command creates a new tarball using the gzip compression alogrithm. While it's not a strict requirement, it's also good practice to add the .tar extension to all tarballs as well as whatever extension is used by the compression algorithm.
darkstar:~$ tar -czf /tmp/tarball.tar.gz /tmp/tarball/
Traditionally, UNIX and UNIX-like operating systems are filled with text files that at some point in time the system's users are going to want to read. Naturally, there are plenty of ways of reading these files, and we'll show you the most common ones.
In the early days, if you just wanted to see the contents of a file (any file, whether it was a text file or some binary program) you would use cat(1) to view them. cat is a very simple program, which takes one or more files, concatenates them (hence the name) and sends them to the standard output, which is usually your terminal screen. This was fine when the file was small and wouldn't scroll off the screen, but inadequate for larger files as it had no built-in way of moving within a document and reading it a paragraph at a time. Today, cat is still used quite extensively, but predominately in scripts or for joining two or more files into one.
darkstar:~$ cat /etc/slackware-version Slackware 14.0
Given the limitations of cat some very intelligent people sat down and began to work on an application to let them read documents one page at a time. Naturally, such applications began to be known as “pagers”. One of the earliest of these was more(1), named because it would let you see “more” of the file whenever you wanted.
more will display the first few lines of a text file until your screen is full, then pause. Once you've read through that screen, you can proceed down one line by pressing ENTER, or an entire screen by pressing SPACE, or by a specified number of lines by typing a number and then the SPACE bar. more is also capable of searching through a text file for keywords; once you've displayed a file in more, press the / key and enter a keyword. Upon pressing ENTER, the text will scroll until it finds the next match.
This is clearly a big improvement over cat, but still suffers from some annoying flaws; more is not able to scroll back up through a piped file to allow you to read something you might have missed, the search function does not highlight its results, there is no horizontal scrolling, and so on. Clearly a better solution is possible.
In fact, modern versions of more, such as the one shipped with Slackware, do feature a back function via the b key. However, the function is only available when opening files directly in more; not when a file is piped to more.
In order to address the short-comings of more, a new pager was developed and ironically dubbed less(1). less is a very powerful pager that supports all of the functions of more while adding lots of additional features. To begin with, less allows you to use your arrow keys to control movement within the document.
Due to its popularity, many Linux distributions have begun to
exclude more in favor of
less. Slackware includes both.
Moreover, Slackware also includes a handy little pre-processor for
lesspipe.sh. This allows a user to execute
less on a number of non-text files.
lesspipe.sh will generate text output from
running a command on these files, and display it in
Less provides nearly as much functionality as one might expect from a text editor without actually being a text editor. Movement line-by-line can be done vi-style with j and k, or with the arrow keys, or ENTER. In the event that a file is too wide to fit on one screen, you can even scroll horizontally with the left and right arrow keys. The g key takes you to the top of the file, while G takes you to the end.
Searching is done as with more, by typing the / key and then your search string, but notice how the search results are highlighted for you, and typing n will take you to the next occurrence of the result while N takes you to the previous occurrence.
Also as with more, files may be opened directly in less or piped to it:
darkstar:~$ less /usr/doc/less:/README darkstar:~$ cat /usr/doc/less:/README /usr/doc/util-linux:/README | less
There is much more to less; from within the application, type h for a full list of commands.
Links are a method of referring to one file by more than one name. By using the ln(1) application, a user can reference one file with more than one name. The two files are not carbon-copies of one another, but rather are the exact same file, just with a different name. To remove the file entirely, all of its names must be deleted. (This is actually the result of the way that rm and other tools like it work. Rather than remove the contents of the file, they simply remove the reference to the file, freeing that space to be re-used. ln will create a second reference or “link” to that file.)
darkstar:~$ ln /etc/slackware-version foo darkstar:~$ cat foo Slackware 14.0 darkstar:~$ ls -l /etc/slackware-version foo -rw-r--r-- 1 root root 17 2007-06-10 02:23 /etc/slackware-version -rw-r--r-- 1 root root 17 2007-06-10 02:23 foo
Another type of link exists, the symlink. Symlinks, rather than being another reference to the same file, are actually a special kind of file in their own right. These symlinks point to another file or directory. The primary advantage of symlinks is that they can refer to directories as well as files, and they can span multiple filesystems. These are created with the -s argument.
darkstar:~$ ln -s /etc/slackware-version foo darkstar:~$ cat foo Slackware 140 darkstar:~$ ls -l /etc/slackware-version foo -rw-r--r-- 1 root root 17 2007-06-10 02:23 /etc/slackware-version lrwxrwxrwx 1 root root 22 2008-01-25 04:16 foo -> /etc/slackware-version
When using symlinks, remember that if the original file is deleted, your symlink is useless; it simply points at a file that doesn't exist anymore.
- Original source: http://www.slackbook.org/beta
- Originally written by Alan Hicks, Chris Lumens, David Cantrell, Logan Johnson