Using Arrays for Sorting in Bash - Desklib
Added on 2019-09-22
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(1) Using arrays for sortingArrays are useful "vessels" for holding values being sorted. You can load an array with values you want to sort, then sort them in place within the array using the bubble sort algorithm. This assignment is for the purpose of practicing array handling-- creating arrays, writing values into their elements, reading values back, measuring arrays' length, adding new elements to them, etc. The bubble sort algorithm is secondary, so it's spelled out below in the instructions.The exercise to perform:Write a script that will accept the name of a file on the command line, load the file's words into an array, sort them with the bubble sort algorithm, and output the sorted result. Put all the logic into a function. Have the main program do nothing but receive from the command line, as arguments, the items to be sorted; then call the function, passing those items to it. The function will load them into an array and then proceed to sort.Remember the bubble sort algorithm? In sorting a list of items it compares adjacent list items and sorts them as a pair. That is, if they are already in order (first is "lesser" than second) nothing happens, but if out of order (first is "greater" than second) it swaps or transposes them (they exchange places). It goes through the list from beginning to end, swapping wherever necessary as it goes. At that point, whatever element in the list is greatest will be at the end of the list. It performs another pass, but this time doesn't treat all the pairs in the list; it omits the last pair since it's already right. On the third pass it omits the last two pairs. On the nth pass it omits the last n-1 pairs.AlgorithmIn the main program:- call the function, giving a list of arguments on its command line which are the words found in the fileIn the function:- assign the incoming arguments to an array, so each argument individually gets its own array element- determine the number of elements (i.e., the length of the array). The number of element pairs is one less than the number of elements.- run an outer loop a fixed number of iterations, that number being the number of pairs. Run this loop's counter downward, from number-of-pairs through zero.- within it run another loop, also with a fixed number of iterations. That number should be the outer loop's counter value. Run the innerloop counter upward, from zero through one less than the current outer-loop-counter.- within the inner loop, test the array element that has current inner-loop-counter as index against its "upward" neighbor, the one whoseindex is one greater than the current inner-loop-counter. The test is whether it's greater. If it is, swap them.- after the looping is done, at the bottom of your function, output the array. It will be sorted.This worked for me. You might depart from it in some details as long as you are effecting a bubble sort.
Submit your finished program to me and I will try it on a couple of my files. Please name it "sorter-bubble" and place it in your assignments directory on the class server.------------------------Excerpts about handling arrays from bash man page:In the context where an assignment statement is assigning a value to a shell variable or array index, the += operator can be used to append to or add to the variable's previous value.... When += is applied to an array variable using compound assignment (see Arrays below), the variable's value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's maximum index (for indexed arrays) or added as additional key-value pairs in an associative array. ...Arrays Bash provides one-dimensional indexed and associative array vari- ables.Any variable may be used as an indexed array; the declare builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Indexed arrays are referenced using inte- gers (including arithmetic expressions) and are zero-based; asso- ciative arrays are referenced using arbitrary strings. An indexed array is created automatically if any variable is assigned to using the syntax name[subscript]=value. The subscript is treated as an arithmetic expression that must evaluate to a num- ber. If subscript evaluates to a number less than zero, it is used as an offset from one greater than the array's maximum index (so a subcript of -1 refers to the last element of the array). To explic- itly declare an indexedarray,use declare -a name(see SHELL BUILTIN COMMANDS below). declare -a name[subscript] is also accepted; the subscript is ignored. Associative arrays are created using declare -A name. Attributes may be specified for an array variable using the declare and readonly builtins.Each attribute applies to all members of an array. Arrays are assigned tousing compound assignments of the form name=(value1 ... valuen), where each value is of the form [sub- script]=string. Indexed array assignments do not requirethe bracketand subscript. Whenassigning to indexed arrays, if the optional brackets and subscript are supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero. When assigning to an associative array, the subscript is required. This syntax is also accepted bythe declare builtin. Individual array elementsmay be assigned to using the name[subscript]=value syntax introduced above. Any element of an array may be referenced using${name[subscript]}. The braces are required to avoid conflicts with pathname expansion. If subscript is @ or *, the word expands to all members of name. These subscripts differ onlywhen the word appears within double quotes.If the word is double-quoted, ${name[*]} expands to a sin- gle word with the value of each array member separated by the first character of the IFS special variable, and ${name[@]} expands each
elementof name to a separate word. When there are no array mem- bers, ${name[@]} expands to nothing. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. This is analogous to the expansion of the special parameters * and @ (see Special Parameters above).${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is *or @,the expansion is the number of elements in the array. Referencing an array variable without a subscript is equivalent to referencingthe array with a subscript of 0. An array variable is considered set if a subscript has been assigned a value. The null string is a valid value. The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at index subscript. Care must be taken to avoid unwanted side effects caused by pathname expansion. unset name, where name is an array, or unset name[subscript], where sub- script is * or @, removes the entire array. The declare, local, and readonly builtins each accept a -a option to specifyan indexed array and a -A option to specify an associative array. If both options are supplied, -A takes precedence. The read builtinaccepts a -a option to assign a list of words read from the standard input to an array. The set and declare builtins display array values in a way that allows them to be reused as assignments.(2) safe remote poweroffIf you run a root command shell on your linux machine and remotely administer another one, also via a root command shell, it's a matter of time before you power off the local one only to discover that, oops, no, you just shut down the remote instead by accident. At that point a remote physical presence is needed, to press the power button. Get in the car, or buy your plane ticket, or embarrass yourself with another phone call to somebody onsite asking them to again fix your mistake for you. Or, you could write some substitute prophylactic code under the same name(s) as the command(s) you habitually use for turning the machine off (probably shutdown, poweroff, or halt). Write it so it deflects (does not actually run) the poweroff attempt, instead printing a helpfully alarmist message reminding the adminstrator that OMG he appears tobe trying to turn off the remote machine, telling him how to do so if it's what he actually intended, and exiting. Never again will he do it unawares.Let's assume the command you always use to put your machines down is poweroff. There is probably more than one solution. But any of them will involve emplacing some code that goes by the name "poweroff" and gets called ahead of the usual code that actually turns out the lights, eclipsing it. The new code will not turn the machine off, but rather print a message saying so and giving the user an alternative direct command that will run the regular poweroff. Except, that's a bit of overkill if done unconditionally. You really want to take that step only in case of a remote connection. If "poweroff" is issued on a local one, you want to let it go through. It's OK, the protection is unneeded, because turning off the machine by accident is recoverable by just turning it right back on again. By contrast a remote user doesn't have that luxury; so the new code should be conditional on whether the user's connection is local or remote.The assignment to perform:Operate as root. Write a script to be run in whenever the root user types "poweroff" in place of the regular poweroff. The script logic is to be:determine if the connection is on a regular tty # (how? tty, ps, test, $, $0 might help; see Advanced Bash Scripting Guidesec 36.1) if "tty" print a message on screen saying in effect "You're local, I'm gonna turn you off now" pull the trigger by running the regular poweroff (machine turns off)else print a warning message on screen declaring which machine this is by hostname and printing an alternative command formulation the user could use that would pull trigger by running the regular poweroff exit with error (exit status value 1) (machine does not turn off)endif
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