Linux Programs
Linux applications are represented by two special types of files: executables and scripts. Executable files are programs that can be run directly by the computer; they correspond to Windows .exe files. Scripts are collections of instructions for another program, an interpreter, to follow. These correspond to Windows .bat or .cmd files, or interpreted BASIC programs.
Linux doesn’t require executables or scripts to have a specific filename or any extension whatsoever. File system attributes are used to indicate that a file is a program that may be run. In Linux, we can replace scripts with compiled programs (and vice versa) without affecting other programs or the people who call them. In fact, at the user level, there is essentially no difference between the two.
When we log in to a Linux system, we interact with a shell program (often bash) that runs programs in the same way that the Windows command prompt does. It finds the programs we ask for by name by searching for a file with the same name in a given set of directories. The directories to search are stored in a shell variable, PATH, in much the same way as with Windows. The search path (to which we can add) is configured by your system administrator and will usually contain some standard places where system programs are stored. These include:
- /bin: Binaries, programs used in booting the system
- /usr/bin: User binaries, standard programs available to users
- /usr/local/bin: Local binaries, programs specific to an installation
An administrator’s login, such as root, may use a PATH variable that includes directories where system administration programs are kept, such as /sbin and /usr/sbin.
Optional operating system components and third-party applications may be installed in subdirectories of /opt, and installation programs might add to your PATH variable by way of user install scripts.
Note that Linux, like UNIX, uses the colon (:) character to separate entries in the PATH variable, rather than the semicolon (;) that MS-DOS and Windows use. (UNIX chose : first, so ask Microsoft why Windows is different, not why UNIX is different!) Here’s a sample PATH variable:
/usr/local/bin:/bin:/usr/bin:.:/home/neil/bin:/usr/X11R6/bin
Here the PATH variable contains entries for the standard program locations, the current directory (.), a user’s home directory, and the X Window System.