This section introduces other useful UNIX system utilities and covers:
ssh is a secure alternative for remote login and also for executing commands in a remote machine. It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections (i.e. graphics) can also be forwarded over the secure channel (another advantage over telnet, rlogin and rsh). ssh is not a standard system utility, although it is a de facto standard.
To login to a remote machine with your current user name:
To login to a remote machine with a different user name:
ssh clients are also available for Windows machines (e.g. there is a good ssh client called putty).
The ping utility is useful for checking round-trip response time between machines. e.g.
measures the reponse time delay between the current machine and the web server at Google. ping is also useful to check whether a machine is still "alive" in some sense.
traceroute shows the full path taken to reach a remote machine, including the delay to each machine along the route. This is particularly useful in tracking down the location of network problems.
ftp is an insecure way of transferring files between computers. When you connect to a machine via ftp, you will be asked for your username and password. If you have an account on the machine, you can use it, or you can can often use the user "ftp" or "anonymous". Once logged in via FTP, you can list files (dir), receive files (get and mget) and send files (put and mput). (Unusually for UNIX) help will show you a list of available commands. Particularly useful are binary (transfer files preserving all 8 bits) and prompt n (do not confirm each file on multiple file transfers). Type quit to leave ftp and return to the shell prompt.
scp is an ssh tool which provides a secure way of transferring files between computers. It works just like the UNIX cp command except that the arguments can specify a user and remote machine as well as files. For example:
will (subject to correct authentication) copy the file hello.txt from the user account jon on the remote machine srv.ucla.edu into the current directory (.) on the local machine.
wget provides a way to retrieve files from the web (using the HTTP protocol). wget is non-interactive, which means it can run in the background, while the user is not logged in (unlike most web browsers). The content retrieved by wget is stored as raw HTML text (which can be viewed later using a web browser).
finger and who show the list of users logged into a machine, the terminal they are using, and the date they logged in on.
write is used by users on the same machine who want to talk to each other. You should specify the user and (optionally) the terminal they are on:
Lines are only transmitted when you press return. To return to the shell prompt, press ctrl-d (the UNIX end of file marker).
sed allows you to perform basic text transformations on an input stream (i.e. a file or input from a pipeline). For example, you can delete lines containing particular string of text, or you can substitute one pattern for another wherever it occurs in a file. Although sed is a mini-programming language all on its own and can execute entire scripts, its full language is obscure and probably best forgotten (being based on the old and esoteric UNIX line editor ed). sed is probably at its most useful when used directly from the command line with simple parameters:
(substitutes pattern2 for pattern1 once per line)
(substitutes pattern2 for pattern1 for every pattern1 per line)
(deletes all lines containing pattern1)
(substitutes characters in string2 for those in string1)
awk is useful for manipulating files that contain columns of data on a line by line basis. Like sed, you can either pass awk statements directly on the command line, or you can write a script file and let awk read the commands from the script.
Say we have a file of cricket scores called cricket.dat containing columns for player number, name, runs and the way in which they were dismissed:
1 atherton 0 bowled 2 hussain 20 caught 3 stewart 47 stumped 4 thorpe 33 lbw 5 gough 6 run-out
To print out only the first and second columns we can say:
Here $n stands for the nth field or column of each line in the data file. $0 can be used to denote the whole line.
We can do much more with awk. For example, we can write a script cricket.awk to calculate the team's batting average and to check if Mike Atherton got another duck:
The BEGIN clause is executed once at the start of the script, the main clause once for every line, the /atherton/ clause only if the word atherton occurs in the line and the END clause once at the end of the script.
awk can do a lot more. See the manual pages for details (type man awk).
make is a utility which can determine automatically which pieces of a large program need to be recompiled, and issue the commands to recompile them. To use make, you need to create a file called Makefile or makefile that describes the relationships among files in your program, and the states the commands for updating each file.
Here is an example of a simple makefile:
Here [TAB] indicates the TAB key. The interpretation of this makefile is as follows:
awk -f cricket.awk cricket.dat > scores.out
make is invoked simply by typing:
Since scores.out did not exist, make executed the commands to create it. If we now invoke make again, nothing happens:
But if we modify cricket.dat and then run make again, scores.out will be updated:
make is mostly used when compiling large C, C++ or Java programs, but can (as we have seen) be used to automatically and intelligently produce a target file of any kind.
git (Concurrent Versioning System) is a source code control system often used on large programming projects to control the concurrent editing of source files by multiple authors. It keeps old versions of files and maintains a log of when, and why changes occurred, and who made them.
You can use this to add new files into a repository that you have checked-out. Does not actually affect the repository until a "cvs commit" is performed.
This command publishes your changes to other developers by updating the source code in the central repository.
UNIX installations usually come with a C and/or C++ compiler. The C compiler is usually called cc or gcc, and the C++ compiler is usually called CC or g++. Most large C or C++ programs will come with a makefile and will support the configure utility, so that compiling and installing a package is often as simple as:
However, there is nothing to prevent you from writing and compiling a simple C program yourself:
Here the C compiler (cc) takes as input the C source file hello.c and produces as output an executable program called hello. The program hello may then be executed (the ./ tells the shell to look in the current directory to find the hello program).
More information is available on most UNIX commands is available via the online manual pages, which are accessible through the man command. The online documentation is in fact divided into sections. Traditionally, they are
Sometimes man gives you a manual page from the wrong section. For example, say you were writing a program and you needed to use the rmdir system call. man rmdir gives you the manual page for the user-level command rmdir. To force man to look in Section 2 of the manual instead, type man 2 rmdir (orman -s2 rmdir on some systems).
man can also find manual pages which mention a particular topic. For example, man -k postscript should produce a list of utilities that can produce and manipulate postscript files.
info is an interactive, somewhat more friendly and helpful alternative to man. It may not be installed on all systems, however.