Linux Commands -1

An A-Z Index of the Bash command line for Linux.

alias Create an alias apropos Search Help manual pages (man -k) apt-get Search for and install software packages (Debian) aspell Spell Checker awk Find and Replace text, database sort/validate/index b bash GNU Bourne-Again SHell bc Arbitrary precision calculator language bg Send to background break Exit from a loop builtin Run a shell builtin bzip2 Compress or decompress named file(s) c cal Display a calendar case Conditionally perform a command cat Display the contents of a file cd Change Directory cfdisk Partition table manipulator for Linux chgrp Change group ownership chmod Change access permissions chown Change file owner and group chroot Run a command with a different root directory cksum Print CRC checksum and byte counts clear Clear terminal screen cmp Compare two files comm Compare two sorted files line by line command Run a command - ignoring shell functions continue Resume the next iteration of a loop cp Copy one or more files to another location cron Daemon to execute scheduled commands crontab Schedule a command to run at a later time csplit Split a file into context-determined pieces cut Divide a file into several parts d date Display or change the date & time dc Desk Calculator dd Convert and copy a file, write disk headers, boot records ddrescue Data recovery tool declare Declare variables and give them attributes df Display free disk space diff Display the differences between two files diff3 Show differences among three files dig DNS lookup dir Briefly list directory contents dircolors Colour setup for `ls' dirname Convert a full pathname to just a path dirs Display list of remembered directories du Estimate file space usage e echo Display message on screen egrep Search file(s) for lines that match an extended expression eject Eject removable media enable Enable and disable builtin shell commands env Environment variables ethtool Ethernet card settings eval Evaluate several commands/arguments exec Execute a command exit Exit the shell expect Automate arbitrary applications accessed over a terminal expand Convert tabs to spaces export Set an environment variable expr Evaluate expressions f false Do nothing, unsuccessfully fdformat Low-level format a floppy disk fdisk Partition table manipulator for Linux fg Send job to foreground fgrep Search file(s) for lines that match a fixed string file Determine file type find Search for files that meet a desired criteria fmt Reformat paragraph text fold Wrap text to fit a specified width. for Expand words, and execute commands format Format disks or tapes free Display memory usage fsck File system consistency check and repair ftp File Transfer Protocol function Define Function Macros g gawk Find and Replace text within file(s) getopts Parse positional parameters grep Search file(s) for lines that match a given pattern groups Print group names a user is in gzip Compress or decompress named file(s) h hash Remember the full pathname of a name argument head Output the first part of file(s) history Command History hostname Print or set system name i id Print user and group id's if Conditionally perform a command ifconfig Configure a network interface ifdown Stop a network interface ifup Start a network interface up import Capture an X server screen and save the image to file install Copy files and set attributes j join Join lines on a common field k kill Stop a process from running l less Display output one screen at a time let Perform arithmetic on shell variables ln Make links between files local Create variables locate Find files logname Print current login name logout Exit a login shell look Display lines beginning with a given string lpc Line printer control program lpr Off line print lprint Print a file lprintd Abort a print job lprintq List the print queue lprm Remove jobs from the print queue ls List information about file(s) lsof List open files m make Recompile a group of programs man Help manual mkdir Create new folder(s) mkfifo Make FIFOs (named pipes) mkisofs Create an hybrid ISO9660/JOLIET/HFS filesystem mknod Make block or character special files more Display output one screen at a time mount Mount a file system mtools Manipulate MS-DOS files mv Move or rename files or directories n netstat Networking information nice Set the priority of a command or job nl Number lines and write files nohup Run a command immune to hangups nslookup Query Internet name servers interactively o open Open a file in its default application op Operator access p passwd Modify a user password paste Merge lines of files pathchk Check file name portability ping Test a network connection popd Restore the previous value of the current directory pr Prepare files for printing printcap Printer capability database printenv Print environment variables printf Format and print data ps Process status pushd Save and then change the current directory pwd Print Working Directory q quota Display disk usage and limits quotacheck Scan a file system for disk usage quotactl Set disk quotas r ram ram disk device rcp Copy files between two machines. read read a line from standard input readonly Mark variables/functions as readonly renice Alter priority of running processes remsync Synchronize remote files via email return Exit a shell function rm Remove files rmdir Remove folder(s) rsync Remote file copy (Synchronize file trees) s screen Multiplex terminal, run remote shells via ssh scp Secure copy (remote file copy) sdiff Merge two files interactively sed Stream Editor select Accept keyboard input seq Print numeric sequences set Manipulate shell variables and functions sftp Secure File Transfer Program shift Shift positional parameters shopt Shell Options shutdown Shutdown or restart linux sleep Delay for a specified time slocate Find files sort Sort text files source Run commands from a file `.' split Split a file into fixed-size pieces ssh Secure Shell client (remote login program) strace Trace system calls and signals su Substitute user identity sudo Execute a command as another user sum Print a checksum for a file symlink Make a new name for a file sync Synchronize data on disk with memory t tail Output the last part of files tar Tape ARchiver tee Redirect output to multiple files test Evaluate a conditional expression time Measure Program running time times User and system times touch Change file timestamps top List processes running on the system traceroute Trace Route to Host trap Run a command when a signal is set(bourne) tr Translate, squeeze, and/or delete characters true Do nothing, successfully tsort Topological sort tty Print filename of terminal on stdin type Describe a command u ulimit Limit user resources umask Users file creation mask umount Unmount a device unalias Remove an alias uname Print system information unexpand Convert spaces to tabs uniq Uniquify files units Convert units from one scale to another unset Remove variable or function names unshar Unpack shell archive scripts until Execute commands (until error) useradd Create new user account usermod Modify user account users List users currently logged in uuencode Encode a binary file uudecode Decode a file created by uuencode v v Verbosely list directory contents (`ls -l -b') vdir Verbosely list directory contents (`ls -l -b') vi Text Editor vmstat Report virtual memory statistics w watch Execute/display a program periodically wc Print byte, word, and line counts whereis Report all known instances of a command which Locate a program file in the user's path. while Execute commands who Print all usernames currently logged in whoami Print the current user id and name (`id -un') Wget Retrieve web pages or files via HTTP, HTTPS or FTP x xargs Execute utility, passing constructed argument list(s) yes Print a string until interrupted . Run a command script in the current shell ### Comment / Remark

Introduction to Ajax

Prototype framework enables you to deal with Ajax calls in a very easy and fun way that is also safe (cross-browser). Besides simple requests, this module also deals in a smart way with JavaScript code returned from a server and provides helper classes for polling.

Ajax functionality is contained in the global Ajax object. The transport for Ajax requests is xmlHttpRequest, with browser differences safely abstracted from the user. Actual requests are made by creating instances of the Ajax.Request object.

new Ajax.Request('/some_url', { method:'get' });

The first parameter is the URL of the request; the second is the options hash. The method option refers to the HTTP method to be used; default method is POST.

Remember that for security reasons (that is preventing cross-site scripting attacks) Ajax requests can only be made to URLs of the same protocol, host and port of the page containing the Ajax request. Some browsers might allow arbitrary URLs, but you shouldn't rely on support for this.

Ajax response callbacks

Ajax requests are by default asynchronous, which means you must have callbacks that will handle the data from a response. Callback methods are passed in the options hash when making a request:

new Ajax.Request('/some_url',   {     method:'get',     onSuccess: function(transport){       var response = transport.responseText || "no response text";       alert("Success! \n\n" + response);     },     onFailure: function(){ alert('Something went wrong...') }   }); 

Here, two callbacks are passed in the hash that alert of either success or failure; onSuccess and onFailure are called accordingly based on the status of the response. The first parameter passed to both is the native xmlHttpRequest object from which you can use its responseText and responseXML properties, respectively.

You can specify both callbacks, one or none - it's up to you. Other available callbacks are:

• onUninitialized,
• onLoading,
• onLoaded,
• onInteractive,
• onComplete and
• onException.

They all match a certain state of the xmlHttpRequest transport, except for onException which fires when there was an exception while dispatching other callbacks.

Also available are onXXX callbacks, where XXX is the HTTP response status like 200 or 404. Be aware that, when using those, your onSuccess and onFailure won't fire because onXXX takes precedence, therefore using these means you know what you're doing.

The onUninitialized, onLoading, onLoaded, and onInteractive callbacks are not implemented consistently by all browsers. In general, it's best to avoid using these.

Parameters and the HTTP method

You can pass the parameters for the request as the parameters property in options:

new Ajax.Request('/some_url', {   method: 'get',   parameters: {company: 'example', limit: 12}   }); 

Parameters are passed in as a hash (preferred) or a string of key-value pairs separated by ampersands (like company=example&limit=12).

You can use parameters with both GET and POST requests. Keep in mind, however, that GET requests to your application should never cause data to be changed. Also, browsers are less likely to cache a response to a POST request, but more likely to do so with GET.

One of the primary applications for the parameters property is sending the contents of a FORM with an Ajax request, and Prototype gives you a helper method for this, called Form.serialize:

new Ajax.Request('/some_url', {   parameters: $('id_of_form_element').serialize(true)   }); 

If you need to push custom HTTP request headers, you can do so with the requestHeaders option. Just pass name-value pairs as a hash or in a flattened array, like: ['X-Custom-1', 'value', 'X-Custom-2', 'other value'].

If, for some reason, you have to POST a request with a custom post body (not parameters from the parameters option), there is a postBody option exactly for that. Be aware that when using postBody, parameters passed will never be posted because postBody takes precedence as a body - using the option means you know what you're doing.

Evaluating a JavaScript response

Sometimes the application is designed to send JavaScript code as a response. If the content type of the response matches the MIME type of JavaScript then this is true and Prototype will automatically eval() returned code. You don't need to handle the response explicitly if you don't need to.

Alternatively, if the response holds a X-JSON header, its content will be parsed, saved as an object and sent to the callbacks as the second argument:

new Ajax.Request('/some_url', { method:'get',   onSuccess: function(transport, json){       alert(json ? Object.inspect(json) : "no JSON object");     }   }); 

Use this functionality when you want to fetch non-trivial data with Ajax but want to avoid the overhead of parsing XML responses. JSON is much faster (and lighter) than XML.

Global responders

There is an object that is informed about every Ajax request: Ajax.Responders. With it, you can register callbacks that will fire on a certain state of any Ajax.Request issued:

Ajax.Responders.register({   onCreate: function(){     alert('a request has been initialized!');   },    onComplete: function(){     alert('a request completed');   } }); 

Every callback matching an xmlHttpRequest transport state is allowed here, with an addition of onCreate. Globally tracking requests like this can be useful in many ways: you can log them for debugging purposes using a JavaScript logger of your choice or make a global exception handler that informs the users of a possible connection problem.

Updating your page dynamically with Ajax.Updater

Developers often want to make Ajax requests to receive HTML fragments that update parts of the document. With Ajax.Request with an onComplete callback this is fairly easy, but with Ajax.Updater it's even easier!

Suppose you have this code in your HTML document:

<h2>Our fantastic productsh2> <div id="products">(fetching product list ...)div> 

The 'products' container is empty and you want to fill it with HTML returned from an Ajax response. No problem:

new Ajax.Updater('products', '/some_url', { method: 'get' });

That's all, no more work. The arguments are the same of Ajax.Request, except there is the receiver element in the first place. Prototype will automagically update the container with the response using the Element.update() method.

If your HTML comes with inline scripts, they will be stripped by default. You'll have to pass true as the evalScripts option in order to see your scripts being executed.

But what if an error occurs, and the server returns an error message instead of HTML? Often you don't want insert errors in places where users expected content. Fortunately, Prototype provides a convenient solution: instead of the actual container as the first argument you can pass in a hash of 2 different containers in this form: { success:'products', failure:'errors' }. Content will be injected in the success container if all went well, but errors will be written to the failure container. By using this feature your interfaces can become much more user-friendly.

You might also choose not to overwrite the current container contents, but insert new HTML on top or bottom like you would do with Insertion.Top or Insertion.Bottom. Well, you can. Just pass the insertion object as the insertion parameter to Ajax:

new Ajax.Updater('products', '/some_url', {   method: 'get',   insertion: Insertion.Top   }); 

Ajax.Updater will use the given object to make the insertion of returned HTML in the container ('products') element. Nifty.

Automate requests with the Ajax.PeriodicalUpdater

You find the Ajax.Updater cool, but want to run it in periodical intervals to repeatedly fetch content from the server? Prototype framework has that, too - it's called Ajax.PeriodicalUpdater, and basically it's running Ajax.Updater at regular intervals.

new Ajax.PeriodicalUpdater('products', '/some_url',   {     method: 'get',     insertion: Insertion.Top,     frequency: 1,     decay: 2   }); 

Two new options here are frequency and decay. Frequency is the interval in seconds at which the requests are to be made. Here, it's 1 second, which means we have an Ajax request every second. The default frequency is 2 seconds. Our users might be happy because the responsiveness of the application, but our servers might be taking quite a load if enough people leave their browsers open on the page for quite some time. That's why we have the decay option - it is the factor by which the frequency is multiplied every time when current response body is the same as previous one. First Ajax request will then be made in 1 second, second in 2, third in 4 seconds, fourth in 8 and so on. Of course, if the server always returns different content, decay will never take effect; this factor only makes sense when your content doesn't change so rapidly and your application tends to return the same content over and over.

Having frequency falloff can take the load off the servers considerably because the overall number of requests is reduced. You can experiment with this factor while monitoring server load, or you can turn it off completely by passing 1 (which is default) or simply omitting it.

Move along

Learn more about Ajax.Request, Ajax.Updater and Ajax options.

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