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| [[Anchor(Tests_And_Conditionals)]] == Tests and Conditionals == Sequential execution of applications is one thing, but to achieve a sort of logic in your scripts or your command line one-liners, you'll need variables and conditionals. Conditionals are used to make decisions which determine the execution flow of a script. -------- [[Anchor(Exit_Status)]] === Exit Status === Every application results in an exit code whenever it terminates. This exit code is used by whatever application started it to evaluate whether everything went OK. This exit code is like a return value from functions. It's an integer between 0 and 255 (inclusive). Convention dictates that we use 0 to denote success, and any other number to denote failure of some sort. The specific number is entirely application-specific, and is used to hint as to what exactly went wrong. For example, the `ping` command sends ICMP packets over the network to a certain host. That host normally responds to this packet by sending the exact same one right back. This way, we can check whether we can communicate with a remote host. `ping` has a range of exit codes which can tell us what went wrong, if anything did: '''From the `ping` manual:''' . '''If ping does not receive any reply packets at all it will exit with code 1. If a packet count and deadline are both specified, and fewer than count packets are received by the time the deadline has arrived, it will also exit with code 1. On other error it exits with code 2. Otherwise it exits with code 0. This makes it possible to use the exit code to see if a host is alive or not.''' The parameter `?` shows us the exit code of the last foreground process that terminated. Let's play around a little with `ping` to see its exit codes: {{{ $ ping God ping: unknown host God $ echo $? 2 $ ping -c 1 -W 1 1.1.1.1 PING 1.1.1.1 (1.1.1.1) 56(84) bytes of data. --- 1.1.1.1 ping statistics --- 1 packets transmitted, 0 received, 100% packet loss, time 0ms $ echo $? 1 }}} -------- . '''Good Practice: [[BR]] You should make sure that your scripts always return a non-zero exit code if something unexpected happened in their execution. You can do this with the `exit` builtin:''' {{{ rm file || { echo "Could not delete file!"; exit 1; } }}} ---- . '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC52 Exit Status]''' ---- . ''Exit Code'' / ''Exit Status'': Whenever a command ends it notifies its parent (which in our case will always be the shell that started it) of its exit status. This is represented by a number ranging from 0 to 255. This code is a hint as to the success of the command's execution. -------- [[Anchor(Control_Operators)]] === Control Operators === Now that we know what exit codes are, and that an exit code of '0' means the command's execution was successful, we'll learn to use this information. The easiest way of performing a certain action depending on the success of a previous command is through the use of 'control operators'. These operators are `&&` and `||`, which respectively represent a logical AND and a logical OR. These operators are used between two commands, and they are used to control whether the second command should be executed depending on the success of the first. Let's put that theory in practice: {{{ $ mkdir d && cd d }}} This simple example has two commands, `mkdir d` and `cd d`. You could easily just use a semi-colon there to separate both commands and execute them sequentially; but we want something more. In the above example, ["BASH"] will execute `mkdir d`, then `&&` will check the result of the `mkdir` application as it finishes. If the `mkdir` application resulted in a success (exit code 0), then `&&` will execute the next command, `cd d`. If `mkdir d` failed, and returned a non-0 exit code, `&&` will skip the next command, and we will stay in the current directory. Another example: {{{ $ rm /etc/some_file.conf || echo "I couldn't remove the file!" rm: cannot remove `/etc/some_file.conf': No such file or directory I couldn't remove the file! }}} `||` is much like `&&`, but it does the exact opposite. It only executes the next command if the first '''failed'''. As such, the message is only echoed if the `rm` command was unsuccessful. You can make a sequence with these operators, but you have to be very careful when you do. Remember what exit code the operator is '''really''' going to be checking against! Here's an example that might cause confusion: {{{ $ false && true || echo "Riddle, riddle?" Riddle, riddle? $ true && false || echo "Riddle, riddle?" Riddle, riddle? }}} `true` is obviously always going to be successful. `false` is obviously always going to be unsuccessful. Can you guess why the `echo` statement is executed in both occasions? The key to understanding how to sequence these operators properly is by evaluating exit codes from left to right. In the first example, `false` is unsuccessful, so `&&` does not execute the next command (which is `true`), but the next `||` gets a shot too. `||` still sees that the last exit code was that from `false`, and `||` executes the next command when the previous was unsuccessful. As a result, the `echo` statement is executed. The same for the second statement again. `true` is successful, so the `&&` executes the next statement. That is `false`; the last exit code now becomes unsuccessful. After that, `||` is evaluated, which sees the unsuccessful exit code from `false` and executes the `echo` statement. It's all easy with `true`s and `false`s; but how about real commands? {{{ $ rm file && touch file || echo "File not found!" }}} All seems well with this piece of code, and when you test it, I'm sure you'll see that it actually does what it's supposed to. It tries to delete a file, and if it succeeds, it creates it again as a new and empty file; if something goes wrong we get the error message. What's the catch? Perhaps you guessed, perhaps not, but here's a hint: Imagine the file system fills up (or some other disaster occurs) in between `rm` and the `touch`; the `rm` will succeed in deleting our file, but `touch` will fail to create it anew. As a result, we get a strange error message saying that the file wasn't found while we were actually trying to '''create''' it. What's up with that? The next section will show a safer way to put both "AND" and "OR" conditions after a command. -------- . '''Good Practice: [[BR]] It's best not to get overcourageous when dealing with conditional operators. They can make your script hard to understand, especially for a person that's assigned to maintain it and didn't write it himself.''' ---- . '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC18 List of Commands]''' ---- . ''Control Operators'': These operators are used to link commands together. They check the exit code of the command that was previously ran to determine whether or not to execute the next command in the sequence. -------- [[Anchor(Conditional_Blocks)]] === Conditional Blocks === `if` is a shell builtin that executes a command, and checks that command's exit code to see whether its execution was successful. Depending on that exit code, `if` executes a specific block of code. {{{ $ if true > then echo "It was true." > else echo "It was false!" > fi It was true. }}} Here you see the basic outline of an ''if-statement''. We start by calling `if` with the argument `true`. `true` is a built-in command that always ends successfully. `if` runs that command, and once the command is done, `if` checks the exit code. Since `true` always exits successfully, `if` continues to the `then`-block, and executes that code. Should the `true` application have failed somehow, and returned an unsuccessful exit code, the `if` statement would have skipped the `then` code, and executed the `else` code block instead. There are commands that can help us a lot in doing conditional checks. They are `[` (also named `test`) and `[[`. `[` is a normal application that reads its arguments and does some checks with them. `[[` is much like `[`, but it's special, and it offers far more versatility. Let's get practical: {{{ $ if [ a = b ] > then echo "a is the same as b." > else echo "a is not the same as b." > fi a is not the same as b. }}} `if` executes the command `[` with the arguments '`a`', '`=`', '`b`' and '`]`'. `[` uses these arguments to determine what must be checked. In this case, it checks whether the string 'a' is identical to the string 'b', and if this is the case, it will exit successfully. However, since we know this is not the case, `[` will not exit successfully (its exit code will be 1). `if` sees that `[` terminated unsuccessfully and executes the code in the `else` block. Now, to see why `[[` is so much more interesting and trustworthy than `[`, let us highlight some possible problems with `[`: {{{ $ if [ my dad = my dog ] > then echo "I have a problem." > fi -bash: [: too many arguments }}} Can you guess what caused the problem? [[BR]] `[` was executed with the arguments '`my`', '`dad`', '`=`', '`my`', '`dog`' and '`]`'. `[` doesn't understand what test it's supposed to execute, because it expects the second argument to be the operator. In our case, the operator is the third argument. Yet another reason why '''quotes''' are so terribly important. Whenever we type whitespace in bash that belongs together with the words before or after it, '''we need to quote the whole string''': {{{ $ if [ 'my dad' = 'my dog' ] > then echo "I have a problem." > fi }}} This time, `[` sees an operator (`=`) in the second argument and it can continue with its work. Now, this may be easy to see and avoid, but it gets just a little trickier when we put the strings in variables, rather than literally in the statement: {{{ $ dad='my dad'; dog='my dog' $ if [ $dad = $dog ] > then echo "I have a problem." > fi -bash: [: too many arguments }}} How did we mess up this time? [[BR]] Here's a hint: ["BASH"] takes our ''if-statement'' and expands all the parameters in it. The result is `if [ my dad = my dog ]`. Boom, game over. Here's how it's supposed to look like: {{{ $ if [ "$dad" = "$dog" ] > then echo "I have a problem." > fi }}} To help us out a little, ["BASH"] introduced a new style of conditional test. Original as the ["BASH"] authors are, they called it `[[`. `[[` is loaded with several very interesting features which are missing from `[`. One of them helps us in dealing with parameter expansions: {{{ $ if [[ $dad = $dog ]] > then echo "I have a problem." > fi $ if [[ I want $dad = I want $dog ]] > then echo "I want too much." > fi -bash: conditional binary operator expected -bash: syntax error near `want' }}} This time, `$dad` and `$dog` did not need quotes. Since `[[` isn't an application (while `[` is), but a ''shell keyword'', it has special magical powers. It parses its arguments before they are expanded by bash and does the expansion itself, taking the result as a single argument, even if that result contains whitespace. ''However'', be aware that simple strings still have to be quoted properly. `[[` can't know whether your literal whitespace in the statement is intentional or not; so it splits it up just like ["BASH"] normally would. Let's fix our last example: {{{ $ if [[ "I want $dad" = "I want $dog" ]] > then echo "I want too much." > fi }}} You could also combine several `if` statements into one using `elif` instead of `else`; where each test indicates another possibility: {{{ $ name=lhunath $ if [[ $name = "George" ]] > then echo "Bonjour, $name!" > elif [[ $name = "Hans" ]] > then echo "Goeie dag, $name!" > elif [[ $name = "Jack" ]] > then echo "Good day, $name!" > else > echo "You're neither George, Hans or Jack. Who the hell are you, $name?" > fi }}} Now that you've got a decent understanding of quoting issues that may arise, let's have a look at some of the other features that `[` and `[[` were blessed with: * Tests supported by `[` (also known as `test`): * '''-e FILE''': True if file exists. * '''-f FILE''': True if file is a regular file. * '''-d FILE''': True if file is a directory. * '''-h FILE''': True if file is a symbolic link. * '''-r FILE''': True if file is readable by you. * '''-s FILE''': True if file exists and is not empty. * '''-t FD ''': True if FD is opened on a terminal. * '''-w FILE''': True if the file is writable by you. * '''-x FILE''': True if the file is executable by you. * '''-O FILE''': True if the file is effectively owned by you. * '''-G FILE''': True if the file is effectively owned by your group. * '''FILE -nt FILE''': True if the first file is newer than the second. * '''FILE -ot FILE''': True if the first file is older than the second. * '''-z STRING''': True if the string is empty (it's length is zero). * '''-n STRING''': True if the string is not empty (it's length is not zero). * '''STRING = STRING''': True if the first string is identical to the second. * '''STRING != STRING''': True if the first string is not identical to the second. * '''STRING < STRING''': True if the first string sorts before the second. * '''STRING > STRING''': True if the first string sorts after the second. * '''EXPR -a EXPR''': True if both expressions are true (logical AND). * '''EXPR -o EXPR''': True if either expression is true (logical OR). * '''! EXPR''': Inverts the result of the expression (logical NOT). * '''INT -eq INT''': True if both integers are identical. * '''INT -ne INT''': True if the integers are not identical. * '''INT -lt INT''': True if the first integer is less than the second. * '''INT -gt INT''': True if the first integer is greater than the second. * '''INT -le INT''': True if the first integer is less than or equal to the second. * '''INT -ge INT''': True if the first integer is greater than or equal to the second. * Additional tests supported only by `[[`: * '''STRING = (or ==) PATTERN''': Not string comparison like with `[` (or `test`), but ''pattern matching'' is performed. True if the string matches the glob pattern. * '''STRING =~ REGEX''': True if the string matches the regex pattern. * '''( EXPR )''': Parantheses can be used to change the evaluation precedence. * '''EXPR && EXPR''': Much like the '-a' operator of `test`, but does not evaluate the second expression if the first already turns out to be false. * '''EXPR || EXPR''': Much like the '-o' operator of `test`, but does not evaluate the second expression if the first already turns out to be true. You want some examples? Sure: {{{ $ test -e /etc/X11/xorg.conf && echo "Your Xorg is configured!" Your Xorg is configured! $ test -n "$HOME" && echo "Your homedir is set!" Your homedir is set! $ [[ boar != bear ]] && echo "Boars aren't bears!" Boars aren't bears! $ [[ boar != b?ar ]] && echo "Boars don't look like bears!" $ [[ $DISPLAY ]] && echo "Your DISPLAY variable is not empty, you probably have Xorg running." Your DISPLAY variable is not empty, you probably have Xorg running. $ [[ ! $DISPLAY ]] && echo "Your DISPLAY variable is not not empty, you probably don't have Xorg running." }}} -------- . '''Good Practice: [[BR]] Whenever you're making a ["BASH"] script, you should always use `[[` (unless if for some reason you need very specific and rare functionality from `[`). [[BR]] Whenever you're making a Shell script, which may end up being used in an environment where ["BASH"] is not available, you should use `[`, because it is far more compatible. (In addition to being built in to ["BASH"] and some other shells, `[` should be available as an external application.)''' ---- . '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC20 Conditional Constructs]''' ---- . '''In the FAQ: [[BR]] [http://wooledge.org/mywiki/BashFAQ/017 How can I group expressions, e.g. (A AND B) OR C?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/031 What is the difference between the old and new test commands ([ and [[)?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/041 How do I determine whether a variable contains a substring?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/054 How can I tell whether a variable contains a valid number?]''' ---- . ''If (builtin)'': This command executes the command that is given on its arguments and depending on that command's exit code executes the code in the following `then` (or optionally `else`) block. -------- [[Anchor(Conditional_Loops)]] === Conditional Loops === You've learned how to code some basic logic flow for your scripts. It's important that you understand a thing or two about keeping scripts healthy before we start introducing more complexity. ["BASH"] scripts, much like any other kind of scripts, should never be overrated. Although they have great potential once you fully understand the shell's features, they aren't always the right tool for the job. At the same time, when you make scripts, you should remember to keep them light, both in length and in complexity. Very long and/or very complex scripts are most often also very bad scripts. Those that aren't yet soon will be, because they are always very difficult to maintain and adapt/extend. A technique that we can use to try and keep code length and complexity down is the ''loop''. There are two kinds of loops. Using the correct kind of loop will help you keep your scripts readable and healthy. ["BASH"] supports `while` loops and `for` loops. The `for` loops can appear in three different forms. Here's a summary: * '''`while` ''command''''': Repeat so long as command is executed successfully (exit code: `0`). * '''`until` ''command''''': Repeat so long as command is executed unsuccessfully (exit code: `>0`). * '''`for` ''variable'' `in` ''words''''': Repeat the loop for each word, putting it into the variable. * '''`for ((` ''expression''`;` ''expression''`;` ''expression'' `))`''': Starts by evaluating the first expression, repeats the loop so long as the second expression is valid and at the end of each loop evaluates the third expression. Let's put that in practice; here are some examples to illustrate the differences but also the similarities between the loops: {{{ $ while true > do echo "Infinite loop!" > done $ (( i=10 )); while (( i > 0 )) > do echo "$i empty cans of beer." > (( i-- )) > done $ for (( i=10; i > 0; i-- )) > do echo "$i empty cans of beer." > done $ for i in {10..0} > do echo "$i empty cans of beer." > done }}} The last three loops achieve exactly the same result, using different syntax. You'll encounter this many times in your shell scripting experience. There will nearly always be multiple approaches to solving a problem. The test of your skill soon won't be about solving a problem as much as about how best to solve it. You need to learn to pick the best angle of approach for the job. Usually, the main factors to keep into account will be the simplicity and flexibility of the resulting code. My personal favorite is the last of the examples. In that example I used ''Brace Expansion'' to generate the words; but there are other ways, too. Let's take a closer look at that last example, because although it looks the easier of the two `for`s, it can often be the trickier, if you don't know exactly how it works. As I mentioned before: `for` runs through a list of words and puts each in the variable, one at a time, then loops through the body with it. The tricky part is how ["BASH"] decides what the words are. Let me explain myself by expanding the braces from that previous example: {{{ $ for i in 10 9 8 7 6 5 4 3 2 1 0 > do echo "$i empty cans of beer." > done }}} ["BASH"] takes the characters between `in` and the end of the line, and splits them up into words. You shouldn't confuse the splitting that happens here with the splitting that happens with ''Commandline Arguments'', even though they look exactly the same at first sight. Commandline arguments are split at ''spaces'', ''tabs'' and ''newlines''. The splitting in this `for` statement happens at ''spaces'' by default, but this default behaviour can be changed. The way `for` determines what delimiter to use for the splitting is by the contents of the `IFS` variable. `IFS` is an acronym for ''Internal Field Separator''; and by default it contains a ''space'', a ''tab'' and a ''newline''. Since the ''space'' is one of the characters there, `for` uses it to split up the words in our sequence, and feeds each word to the variable `i`, one at a time. '''As a result; be VERY careful not to make the following mistake:''' {{{ $ ls The best song in the world.mp3 $ for file in $(ls *.mp3) > do rm "$file" > done rm: cannot remove `The': No such file or directory rm: cannot remove `best': No such file or directory rm: cannot remove `song': No such file or directory rm: cannot remove `in': No such file or directory rm: cannot remove `the': No such file or directory rm: cannot remove `world.mp3': No such file or directory }}} You should already know to quote the `$file` in the `rm` statement; but what's going wrong here? ["BASH"] expands the command substitution (`$(ls *.mp3)`), replaces it by its output, and as a result executes `for file in The best song in the world.mp3`. ["BASH"] splits that up into words by using ''spaces'' and tries to `rm` each word. ''Boom, you are dead''. You want to quote it, you say? Let's add another song: {{{ $ ls The best song in the world.mp3 The worst song in the world.mp3 $ for file in "$(ls *.mp3)" > do rm "$file" > done rm: cannot remove `The best song in the world.mp3 The worst song in the world.mp3': No such file or directory }}} Quotes will indeed protect the whitespace in your filenames; but they will do more than that. The quotes will protect '''all the whitespace''' from the output of `ls`. There is no way ["BASH"] can know which parts of the output of `ls` represent filenames; it's not psychic. The output of `ls` is a simple string, and ["BASH"] treats it as such. The `for` puts the whole quoted output in `i` and runs the `rm` command with it. ''Damn, dead again''. So what do we do? As suggested earlier, globs are your best friend: {{{ $ for file in *.mp3 > do rm "$file" > done }}} This time, ["BASH"] '''does''' know that it's dealing with filenames, and it '''does''' know what the filenames are, and as such it can split them up nicely. The result of expanding the glob is this: `for file in "The best song in the world.mp3" "The worst song in the world.mp3"`. Problem resolved. Let's talk about changing that delimiter. Say you've got yourself a nice cooking recipe, and you want to write a script that tells you how to use it. Sure, let's get right at it: {{{ $ recipe='2 c. all purpose flour > 6 tsp. baking powder > 2 eggs > 2 c. milk > 1/3 c. oil' $ for ingredient in $recipe > do echo "Take $ingredient; mix well." > done }}} Can you guess what the result will look like? I recommend you run the code if you can't and ponder the reason first. It will help you understand things. Yes, as explained earlier, `for` splits its stuff up into words by using the delimiters from `IFS`. To read the recipe correctly, we need to split it up by newlines alone, instead of by spaces and newlines. Here's how we do that: {{{ $ recipe='2 c. all purpose flour > 6 tsp. baking powder > 2 eggs > 2 c. milk > 1/3 c. oil' $ IFS=$'\n' $ for ingredient in $recipe > do echo "Take $ingredient; mix well." > done Take 2 c. all purpose flour; mix well. Take 6 tsp. baking powder; mix well. Take 2 eggs; mix well. Take 2 c. milk; mix well. Take 1/3 c. oil; mix well. }}} Beautiful. '''Note: This delimiter is only used when the words consist of an expansion. Not when they're literal. Literal words are always split at spaces:''' {{{ $ PATH=/bin:/usr/bin $ IFS=: $ for i in $PATH > do echo "$i" > done /bin /usr/bin $ for i in $PATH:/usr/local/bin > do echo "$i" > done /bin /usr/bin /usr/local/bin $ for i in /bin:/usr/bin:/usr/local/bin > do echo "$i" > done /bin:/usr/bin:/usr/local/bin }}} Lets focus a little more on the `while` loop. It promises even more simplicity than this `for` loop, so long as you don't need any `for` specific features. The `while` loop is very interesting for its capacity of executing commands and basing the loop's progress on the result of them. Here are a few examples of how `while` loops are very often used: {{{ $ # The sweet machine; hand out sweets for a cute price. $ while read -p $'The sweet machine.\nInsert 20c and enter your name: ' name > do echo "The machine spits out three lollipops at $name." > done $ # Check your email every five minutes. $ while sleep 5m > do kmail --check > done $ # Wait for a host to come back online. $ while ! ping -c 1 -W 1 "$host" > do echo "$host is still unavailable." > done; echo -e "$host is available again!\a" }}} The `until` loop is barely ever used, if only because it is pretty much exactly the same as the `while` loop except for the fact that its command is not checked for an exit status of zero but for an exit status of non-zero. As a result, we could rewrite our last example using an until loop as such: {{{ $ # Wait for a host to come back online. $ until ping -c 1 -W 1 "$host" > do echo "$host is still unavailable." > done; echo -e "$host is available again!\a" }}} -------- . '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC19 Looping Constructs]''' ---- . '''In the FAQ: [[BR]] [http://wooledge.org/mywiki/BashFAQ/015 How can I run a command on all files with the extention .gz?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/018 How can I use numbers with leading zeros in a loop, e.g. 01, 02?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/020 How can I find and deal with file names containing newlines, spaces or both?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/030 How can I rename all my *.foo files to *.bar, or convert spaces to underscores, or convert upper-case file names to lower case?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/034 Can I do a spinner in Bash?] [[BR]] [http://wooledge.org/mywiki/BashFAQ/046 I want to check to see whether a word is in a list (or an element is a member of a set).]''' ---- . ''Loop'': A loop is a structure that is designed to repeat the code within until a certain condition has been fulfilled. At that point, the loop stops and the code beyond it is executed. [[BR]] ''For (builtin)'': A `for`-loop is a type of loop that sets a variable to each of a list of values in turn, and repeats until the list is exhausted. [[BR]] ''While (builtin)'': A `while`-loop is a type of loop that continues to run its code so long as a certain command (run before each iteration) executes successfully. [[BR]] ''Until (builtin)'': An `until`-loop is a type of loop that continues to run its code so long as a certain command (run before each iteration) executes unsuccessfully. -------- [[Anchor(Choices)]] === Choices === Sometimes you just want to built application logic depending on the content of a variable but you already know all of the possible choices of content the variable can contain. You no longer need `if`'s two ''yes or no'' blocks, but you want a block of code for every possible choice of content a variable can have. You could obviously attempt to emulate this behaviour using several `if`-statements: {{{ $ if [[ $LANG = en* ]] > then echo "Hello!"; fi $ if [[ $LANG = fr* ]] > then echo "Salut!"; fi $ if [[ $LANG = de* ]] > then echo "Guten Tag!"; fi $ if [[ $LANG = nl* ]] > then echo "Hallo!"; fi $ if [[ $LANG = it* ]] > then echo "Ciao!"; fi }}} But let's say we like to actually make clean code, shall we? ["BASH"] provides a construct called `case` exactly for this kind of situation. A `case` construct basically enumerates several possible ''Glob Patterns'' and checks the content of your parameter against these: {{{ $ case $LANG in > en*) echo "Hello!" ;; > fr*) echo "Salut!" ;; > de*) echo "Guten Tag!" ;; > nl*) echo "Hallo!" ;; > it*) echo "Ciao!" ;; > *) echo "I do not speak your language." ;; > esac }}} Each choice in a `case` statement starts with a pattern followed by a block of code that is to be executed if the parameter's content matches that pattern, which in turn is followed by two semi-colons. `case` stops matching patterns as soon as one was successful. Therefore, we can use the `*` pattern in the end to match any case that has not been caught by the other choices. Another construct of choice is the `select` construct. This statement smells like a loop and is a convenience statement for generating a menu of choices that the user can choose from. The user is presented by choices and asked to enter a number reflecting his choice. The code in the `select` block is then executed with a variable set to the choice the user made. If the user's choice was invalid, the variable is made empty: {{{ $ echo "Which of these does not belong in the group?"; \ > select choice in Apples Pears Crisps Lemons Kiwis; do > if [[ $choice = Crisps ]] > then echo "Correct! Crisps are not fruit."; break; fi > echo "Errr... no. Try again." > done }}} The menu reappears so long as the `break` statement is not executed. In the example the `break` statement is only executed when the user makes the correct choice. We can also use the `PS3` variable to define the prompt the user replies on. Instead of showing the question before executing the `select` statement, we could choose to set the question as our prompt: {{{ $ PS3="Which of these does not belong in the group (#)? " \ > select choice in Apples Pears Crisps Lemons Kiwis; do > if [[ $choice = Crisps ]] > then echo "Correct! Crisps are not fruit."; break; fi > echo "Errr... no. Try again." > done }}} -------- . '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC20 Conditional Constructs]''' ---- . '''In the FAQ: [[BR]] [http://wooledge.org/mywiki/BashFAQ/066 I want to check if [[ $var == foo || $var == bar || $var = more ... without repeating $var n times.]''' ---- . ''Case (builtin)'': This statement evaluates a parameter's value based on several given patterns (choices). [[BR]] ''Select (builtin)'': The `select` statement offers the user the choice between several options and executes a block of code for the user's choice. -------- |
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