Java™ Application Development on Linux - Dator

10Chapter 1An Embarrassment of Riches: The Linux EnvironmentFilenames are case sensitive—upper- and lowercase names are different.The files ReadMe.txt and readme.txt could both be in the same directory;they are distinct files.Avoid using spaces in filenames, as the shell uses whitespace to delineatebetween arguments on a command line. You can put a blank in a name, butthen you always have to put the name in quotes to refer to it in the shell.To give a filename more visual clues, use a period or an underscore.You can combine several in one filename, too. The filenamesread_me_before_you_begin or test.data.for_my_program may beannoyingly long to type, but they are legal filenames.NOTEThe period, or “dot,” in Linux filenames has no special meaning. If you comefrom the MS-DOS world, you may think of the period as separating the filenamefrom the extension, as in myprogrm.bas where the filename is limited to eightcharacters and the extension to three characters. Not so in Linux. There is no“extension,” it’s all just part of the filename.You will still see names like delim.c or Account.java, but the .c or.java are simply the last two characters or the last five characters, respectively,of the filenames. That said, certain programs will insist on those endings fortheir files. The Java compiler will insist that its source files end in .java andwill produce files that end in .class—but there is no special part of the filenameto hold this. This will prove to be very handy, both when you name yourfiles and when you use patterns to search for files (see below).1.3.4 PermissionsPermissions in Linux are divided into three categories: the owner of a file(usually the user who created it), the group (a collection of users), and others,meaning everyone who is not the owner and not in the group. Any file belongsto a single owner and, simultaneously, to a single group. It has separateread/write/execute permissions for its owner, its group, and all others. If youare the owner of a file, but also a member of the group that owns the file, thenthe owner permissions are what counts. If you’re not the owner, but a memberof the group, then the group permissions will control your access to the file. Allothers get the “other” permissions.If you think of the three permissions, read/write/execute, as three bits ofa binary number, then a permission can be expressed as an octal digit—wherethe most significant bit represents read permission, the middle bit is write