INTROSection: Linux User's Manual (1)
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NAMEintro - Introduction to user commands
DESCRIPTIONSection 1 of the manual describes user commands and tools, for example, file manipulation tools, shells, compilers, web browsers, file and image viewers and editors, and so on.
All commands yield a status value on termination. This value can be tested (e.g., in most shells the variable $? contains the status of the last executed command) to see whether the command completed successfully. A zero exit status is conventionally used to indicate success, and a non-zero status means that the command was unsuccessful. (Details of the exit status can be found in wait(2).) A non-zero exit status can be in the range 1 to 255, and some commands use different non-zero status values to indicate the reason why the command failed.
NOTESLinux is a flavor of Unix, and as a first approximation all user commands under Unix work precisely the same under Linux (and FreeBSD and lots of other Unix-like systems).
Under Linux there are GUIs (graphical user interfaces), where you can point and click and drag, and hopefully get work done without first reading lots of documentation. The traditional Unix environment is a CLI (command line interface), where you type commands to tell the computer what to do. That is faster and more powerful, but requires finding out what the commands are. Below a bare minimum, to get started.
LoginIn order to start working, you probably first have to login, that is, give your username and password. See also login(1). The program login now starts a shell (command interpreter) for you. In case of a graphical login, you get a screen with menus or icons and a mouse click will start a shell in a window. See also xterm(1).
The shellOne types commands to the shell, the command interpreter. It is not built-in, but is just a program and you can change your shell. Everybody has her own favorite one. The standard one is called sh. See also ash(1), bash(1), csh(1), zsh(1), chsh(1).
A session might go like
knuth login: aeb Password: ******** % date Tue Aug 6 23:50:44 CEST 2002 % cal August 2002 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 % ls bin tel % ls -l total 2 drwxrwxr-x 2 aeb 1024 Aug 6 23:51 bin -rw-rw-r-- 1 aeb 37 Aug 6 23:52 tel % cat tel maja 0501-1136285 peter 0136-7399214 % cp tel tel2 % ls -l total 3 drwxr-xr-x 2 aeb 1024 Aug 6 23:51 bin -rw-r--r-- 1 aeb 37 Aug 6 23:52 tel -rw-r--r-- 1 aeb 37 Aug 6 23:53 tel2 % mv tel tel1 % ls -l total 3 drwxr-xr-x 2 aeb 1024 Aug 6 23:51 bin -rw-r--r-- 1 aeb 37 Aug 6 23:52 tel1 -rw-r--r-- 1 aeb 37 Aug 6 23:53 tel2 % diff tel1 tel2 % rm tel1 % grep maja tel2 maja 0501-1136285 %
We see that there are commands date (that gives date and time), and cal (that gives a calendar).
The command ls lists the contents of the current directory --- it tells you what files you have. With a -l option it gives a long listing, that includes the owner and size and date of the file, and the permissions people have for reading and/or changing the file. For example, the file "tel" here is 37 bytes long, owned by aeb and the owner can read and write it, others can only read it. Owner and permissions can be changed by the commands chown and chmod.
The command cat will show the contents of a file. (The name is from "concatenate and print": all files given as parameters are concatenated and sent to "standard output", here the terminal screen.)
The command cp (from "copy") will copy a file. On the other hand, the command mv (from "move") only renames it.
The command diff lists the differences between two files. Here there was no output because there were no differences.
The command rm (from "remove") deletes the file, and be careful! it is gone. No wastepaper basket or anything. Deleted means lost.
Pathnames and the current directoryFiles live in a large tree, the file hierarchy. Each has a pathname describing the path from the root of the tree (which is called /) to the file. For example, such a full pathname might be /home/aeb/tel. Always using full pathnames would be inconvenient, and the name of a file in the current directory may be abbreviated by only giving the last component. That is why "/home/aeb/tel" can be abbreviated to "tel" when the current directory is "/home/aeb".
The command pwd prints the current directory.
DirectoriesThe command mkdir makes a new directory.
The command rmdir removes a directory if it is empty, and complains otherwise.
The command find (with a rather baroque syntax) will find files with given name or other properties. For example, "find . -name tel" would find the file "tel" starting in the present directory (which is called "."). And "find / -name tel" would do the same, but starting at the root of the tree. Large searches on a multi-GB disk will be time-consuming, and it may be better to use locate(1).
Disks and FilesystemsThe command mount will attach the file system found on some disk (or floppy, or CDROM or so) to the big file system hierarchy. And umount detaches it again. The command df will tell you how much of your disk is still free.
ProcessesOn a Unix system many user and system processes run simultaneously. The one you are talking to runs in the foreground, the others in the background. The command ps will show you which processes are active and what numbers these processes have. The command kill allows you to get rid of them. Without option this is a friendly request: please go away. And "kill -9" followed by the number of the process is an immediate kill. Foreground processes can often be killed by typing Control-C.
Getting informationThere are thousands of commands, each with many options. Traditionally commands are documented on man pages, (like this one), so that the command "man kill" will document the use of the command "kill" (and "man man" document the command "man"). The program man sends the text through some pager, usually less. Hit the space bar to get the next page, hit q to quit.
In documentation it is customary to refer to man pages by giving the name and section number, as in man(1). Man pages are terse, and allow you to find quickly some forgotten detail. For newcomers an introductory text with more examples and explanations is useful.
A lot of GNU/FSF software is provided with info files. Type "info info" for an introduction on the use of the program "info".
COLOPHONThis page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
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