MS-DOS and Linux Commands: Internal, External and Shell

The correct answer is C, DIR. An internal command is written into the command interpreter COMMAND.COM and is loaded into memory along with the operating system, so it can be used even when no separate program file is present on the disk. DIR, which lists the files and sub-directories of a directory, belongs to this group along with CD, MD, RD, COPY, DEL, REN, TYPE, CLS, DATE, TIME, VER, VOL, PATH and PROMPT. Option A is wrong because FORMAT, which prepares a disk by creating tracks and sectors and writing the file allocation table, exists as its own executable file and is therefore external. Option B is wrong because CHKDSK, which checks a disk for errors and reports free space, is also a separate executable. Option D is wrong because XCOPY, the extended copy command that can copy whole directory trees, is likewise an external command. Papers usually ask this question by listing three external commands and one internal one.

The correct answer is B, pwd. The name stands for print working directory, and the command prints the complete path of the directory in which the shell is placed at that moment, counted from the root directory written as a single forward slash. Option A is wrong because ls lists the names of the files and sub-directories inside a directory but does not say where that directory sits in the tree. Option C is wrong because cd changes the directory; in Linux, typing cd with no argument takes the user to the home directory rather than printing the current path, which is a point of difference from MS-DOS, where CD alone displays the current directory. Option D is wrong because path is not a Linux command of this kind; PATH in MS-DOS and the PATH variable in Linux hold the list of directories searched for programs, which is a different idea altogether.

The correct answer is A, cat. The cat command, short for concatenate, writes the contents of one or more files to the standard output, which is normally the screen, and so does the work of TYPE in MS-DOS. It can also join files together when more than one name is given. Option B is wrong because touch creates an empty file if the name does not exist and otherwise only updates the time stamp of the file; it never shows the contents. Option C is wrong because echo prints the text or the value of a variable written after it, not the contents of a file. Option D is wrong because grep searches for a pattern and prints only the lines that match it, which makes it the equivalent of the DOS command FIND rather than of TYPE. For long files, less and more are used instead of cat so that the text can be read one screen at a time.

The correct answer is C, Linus Torvalds, 1991. Torvalds, then a student at the University of Helsinki in Finland, wrote a free kernel for personal computers and released it in 1991 under the GNU General Public Licence, which allowed anyone to study, change and redistribute it. Option A is wrong because Dennis Ritchie is remembered for the C language, which he created at Bell Laboratories in the early nineteen seventies, and for his work on Unix. Option B is wrong because Richard Stallman began the GNU project in 1983 and wrote the General Public Licence; the GNU tools wrapped around the Linux kernel make the complete system, which is why it is also called GNU/Linux. Option D is wrong because Ken Thompson, with Dennis Ritchie, developed Unix at Bell Laboratories in 1969, the ancestor that Linux was written to resemble.

The correct answer is B. In the numeric form of chmod each of the three digits stands for one set of users, in the order owner, group and others, and each digit is the sum of read as 4, write as 2 and execute as 1. Since 4 plus 2 plus 1 is 7, the number 777 grants all three rights to all three sets, which is the widest permission a file can carry and is therefore avoided on shared systems. Option A is wrong because read and write for the owner alone is 600. Option C is wrong because read and execute for everyone with write for nobody is 555. Option D is wrong because such a mixture would be written as a different combination altogether, for example 140, and is not what 777 means. Two other numbers are worth remembering: 755 for programs and directories and 644 for ordinary documents.

The correct answer is B, the question mark. A question mark in a file specification matches any single character in that position, so the command DIR FILE?.TXT lists FILE1.TXT and FILEA.TXT but not FILE12.TXT. Option A is wrong because the asterisk matches any group of characters, including none, which is why the pattern made of an asterisk, a dot and TXT selects every text file in a directory. Option C is wrong because the backslash is the separator between directory names in a DOS path and has nothing to do with matching. Option D is wrong because the colon follows a drive letter, as in the C drive written with a colon after it. The same two wildcards continue to work in the Windows command prompt and in Linux, where the shell expands them before the command runs.

The correct answer is A, ps. The command ps, short for process status, prints a list of processes with their process identification numbers, the terminal they belong to, the processor time used and the command that started each one. The related command top shows the same information refreshed continuously, and kill followed by a process identification number ends a process. Option B is wrong because df, for disk free, reports how much space is used and available on each mounted file system. Option C is wrong because du, for disk usage, reports how much space a particular directory and its contents occupy. Option D is wrong because ln creates a link to a file, either a hard link, which is another directory entry for the same data, or a symbolic link made with the option s, which merely points to the original path.

The correct answer is B, the 8.3 convention. The name is read as eight dot three: up to eight characters for the name, a dot, and up to three for the extension, as in LETTER.TXT or AUTOEXEC.BAT, with spaces and certain punctuation marks not allowed. Option A is wrong because it simply reverses the two numbers; the name part is the longer one. Option C is wrong because long file names, which allow many more characters and spaces, arrived with Windows 95 and later systems, not with MS-DOS. Option D is wrong because FAT32 is a file system introduced later to handle larger disks and more clusters; a file system decides how data is laid out on the disk, while the 8.3 rule is about the form of the name. Questions on this point often pair the rule with the names of the two startup files, CONFIG.SYS and AUTOEXEC.BAT.

The correct answer is B, rmdir. The command rmdir deletes a directory only if nothing is left inside it and prints an error such as directory not empty otherwise, which makes it a safe way to clear away empty folders. Option A is wrong because rm deletes files, and with the recursive option it deletes a directory together with everything it contains, which is why it is treated with care on a live system. Option C is wrong because mv moves a file or a directory to another place and is also the way a file is renamed in Linux, the work done by both MOVE and REN in MS-DOS. Option D is wrong because mkdir creates a directory rather than removing one. The MS-DOS counterpart of rmdir is RD, and of rm is DEL, while DELTREE in MS-DOS does the work of a recursive rm.

The correct answer is B, two greater-than signs. Writing a command followed by two greater-than signs and a file name appends the output to the end of that file and keeps whatever was already in it, which is how log files are built up. Option A is wrong because a single greater-than sign also redirects output to a file but truncates it first, so the earlier contents are lost; confusing the two is a common mistake and a favourite question. Option C is wrong because the vertical bar is the pipe, which hands the output of one command to another as its input, as when a long listing is passed to less for reading page by page. Option D is wrong because a single less-than sign redirects input, that is, it makes a command read from a file instead of from the keyboard. These three ideas, redirection, appending and piping, are the heart of working at the shell.

The correct answer is C. Linux treats upper and lower case as different, so a file named Report is not the same as report, and it is built as a multi-user, multitasking system in which several accounts can work at once with separate permissions and a login. MS-DOS ignores case and is a single-user, single-tasking system with no login at all in its plain form. Option A is wrong because MS-DOS is not case sensitive; DIR and dir do the same thing. Option B is wrong because it reverses the fact, since it is Linux that supports many users. Option D is wrong because the backslash is the DOS separator while Linux uses the forward slash, and in Linux the backslash instead marks an escape character. Two further differences are worth holding: MS-DOS is proprietary and discontinued, whereas Linux is free and open source under the GNU General Public Licence.