learn linux and unix commands - unix shell scripting

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Arithmetic operators calculate numeric values. You can use one or more arithmetic operators to add, subtract, multiply, and divide values, and to calculate the remainder modulus of a division operation. The addition operator concatenates the input. The multiplication operator returns multiple copies of the input.

You can even mix object types in an arithmetic statement. The method that is used to evaluate the statement is determined by the type of the leftmost object in the expression. Beginning in PowerShell 3. PowerShell processes the expressions from left to right according to the precedence rules. The following examples show the effect of the precedence rules:.

The order in which PowerShell evaluates expressions might differ from other programming and scripting languages that you have used. The following example shows a complicated assignment statement.

When the quotient of a division operation is an integer, PowerShell rounds the value to the nearest integer. When the value is. You can add numbers, strings, arrays, and hash tables.

And, you can multiply numbers, strings, and arrays. However, you cannot multiply hash tables. When you add strings, arrays, or hash tables, the elements are concatenated. When you concatenate collections, such as arrays or hash tables, a new object is created that contains the objects from both collections.

If you try to concatenate hash tables that have the same key, the operation fails. You can also perform arithmetic operations on objects of different types. The operation that PowerShell performs is determined by the Microsoft. NET Framework type of the leftmost object in the operation. PowerShell tries to convert all the objects in the operation to the.

NET Framework type of the first object. If it succeeds in converting the objects, it performs the operation appropriate to the. If it fails to convert any of the objects, the arithmetic and logical binary operators in shell programming fails. The following examples demonstrate the use of the addition and multiplication operators; in operations that include different object types. Because the method that is used to evaluate statements is determined by the leftmost object, addition and multiplication in PowerShell are not strictly commutative.

Hash tables are a slightly different case. You can add hash tables to another hash table, as long as, the added hash tables don't have duplicate keys.

Also, you can add a hash table to an array; and, the entire hash table becomes an item in the array. Although the addition operators are very useful, use the assignment operators to add elements to hash tables and arrays. PowerShell automatically selects the. NET Framework numeric type that best expresses the result without losing precision.

If the result of an operation is too large for the type, the type of the result is widened to accommodate the result, as in the following example:. The type of the result will not necessarily be the same as one of the operands. In the following example, the negative value cannot be cast to an unsigned integer, and the unsigned integer is too large to be cast to Int Decimal type is an exception.

If either operand has the Decimal type, the result will be of the Decimal type. If the result is too large for the Decimal type, it will not be cast to Double. Instead, an error results. You can also use arithmetic operators with variables.

The operators act arithmetic and logical binary operators in shell programming the values of the variables. The following examples demonstrate the use of arithmetic operators with variables:. Typically, you use the arithmetic operators in expressions with numbers, strings, and arrays. However, you can also use arithmetic operators with the objects that commands return and with the properties of those objects. The following examples show how to use the arithmetic operators in expressions with PowerShell commands:.

The parenthesis operator forces the evaluation of the get-date cmdlet and the evaluation of the new-timespan -day 1 cmdlet expression, in that order. Bitwise operators act on the binary format of a value.

For example, the bit structure for the number 10 is based on 1 byteand the bit structure for the number 3 is When you use a bitwise operator to compare 10 to 3, the individual bits in each byte are compared. In a bitwise OR inclusive operation, the arithmetic and logical binary operators in shell programming bit is set to 1 when either or both input bits are 1.

The resulting bit is set to 0 only when both input bits are set to 0. Arithmetic and logical binary operators in shell programming a bitwise OR exclusive operation, the resulting bit is set to 1 only when one input bit is 1.

The bitwise NOT operator is a unary operator that produces the binary complement of the value. For example, the binary complement of 0 is -1, the maximum unsigned integer 0xffffffffand the binary complement of -1 is 0. In a bitwise shift-left operation, all bits are moved "n" places to the left, where "n" is the value of the right operand. A zero is inserted in the ones place. When the left operand is an Integer bit value, the lower 5 bits of the right operand determine how many bits of the left operand are shifted.

When the arithmetic and logical binary operators in shell programming operand is a Long bit value, the lower 6 bits of the right operand arithmetic and logical binary operators in shell programming how many bits of the left operand are shifted. In a bitwise shift-right operation, all bits are moved "n" places to the right, where "n" is specified by arithmetic and logical binary operators in shell programming right operand.

The shift-right operator -shr inserts a zero in the left-most place when shifting a positive or unsigned value to the right. The feedback system for this content will be changing soon. Old comments will not be carried over.

If content within a comment thread is important to you, please save a copy. For more information on the upcoming change, we invite you to read our blog post. Beginning in PowerShell 2. PowerShell supports the following arithmetic operators: The following examples show the effect of the precedence rules: The above code arithmetic and logical binary operators in shell programming equivalent to: The following example shows the effect of rounding to the nearest even integer.

For example, the following commands create two arrays and then add them: The following examples demonstrate this principle: The following example show how to add hash tables to each other.

ArgumentException Also, you can add a hash table to an array; and, the entire hash table becomes an item in the array. Double If the result of an operation is too large for the type, the type of the result is widened to accommodate the result, as in the following example: Double The type of the result will not necessarily be the same as one of the operands. Int64 In this example, Int64 can accommodate both types.

The following examples demonstrate the use of arithmetic operators with variables: The following examples show how to use the arithmetic operators in expressions with PowerShell commands: PowerShell supports the following bitwise operators. In a bitwise AND operation, the resulting bit is set to 1 only when both input bits are 1.

Expression Result Binary Result 21 -shl 0 21 21 -shl 1 42 21 -shl 2 84 In a bitwise shift-right operation, all bits are moved "n" places to the right, where "n" is specified by the right operand.

Expression Result Binary Hex 21 -shr 0 21 0x15 21 -shr 1 10 0x0A 21 -shr 2 5 0x05 21 -shr 31 0 0x00 21 -shr 32 21 0x15 21 -shr 64 21 0x15 21 -shr 65 10 0x0A 21 -shr 66 5 0x05 [int]::

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There are many ways to make mistakes in script programming. Here are some warnings about common errors. Shells are not particularly good about giving helpful error messages when shell scripts contain errors. For example, having a missing or non-integer argument to the test command may produce a vague error message:. A common mistake when writing a new shell script is to write too many lines of code, run the new script, and then get too many error messages.

Create shell scripts a few lines at a time, testing the script after you add each line or two so you know where the errors lie. Running the script using a shell with the debug options -x or -v set may also be helpful:. They find and run commands, and it is commands that you must use in if statements. The error message bash: The shell if keyword must always be followed by a Unix command name. Always remember to code some Unix command name after if:.

The command name after the if , above, is test. If you want to test the return status of grep , grep must be the command name that immediately follows the if keyword:. The if keyword must always be followed by a command name, and that command name is exactly one square bracket [ in the syntactic-sugar form of the test helper command.

The following code does not work; because, blanks are missing around the first square bracket, making it into an unknown command named [ The shell sees [1 and [! The following incorrect statement fails for the same reason:. The arguments to the test command must always be separate command line arguments. This next line fails because of the missing blank before the required closing square bracket:. The corrected line uses blanks:. The test helper command behaves differently depending on the number of arguments you pass to it:.

If the test command has only one single command line argument, it defaults to using -n as the implied operator test for non-empty string on the one argument. The following one-argument tests are always TRUE, though they may not appear that way at first to human eyes:.

In all the above lines, the test command has only one command line argument not counting the trailing ] that is always ignored. Since the single argument to test is not the empty string, test returns a good status and the if succeeds.

The test command is defaulting to use an implied -n operator on the left. The shell is actually executing these tests:. If you want to perform equality tests, you must separate each argument by blanks so that test sees three separate arguments, not just one:. Here are two identical mistakes:.

The test command produces no output; the file remains empty. The test command itself is left with only one single command line argument, the digit 1. With one argument and no operators, the test command returns success if the argument is not the empty string test -n 1. The string 1 is never empty, so the above test, and the if , always succeeds.

The exit status negation operator! The test command uses the exclamation point operator! If you combine the negation operator of test with the shell return code negation operator mentioned earlier that also uses! Rework the expression to use only a single! To test if a file is non-existent or exits but is not readable:. The shell return code negation operator! You cannot use them inside test expressions. Use -a and -o to separate Boolean clauses to the test command:. Above, the separates two different and complete test command executions.

Rather than using the test command twice, you can simply join them into one using the correct test Boolean operator:. The test helper command has six ways to compare numbers and two ways to compare strings. Boolan logic has some subtle consequences when applied to the operations performed by the test helper command. These are all correct opposites. The logical opposite of the test operator -lt less than is not -gt greater than , it is -ge greater than or equal to.

If you are not younger than your sister, you are either older or the same age. The opposite of the test operator -gt is not -lt , it is -le. The test operators -f and -d are not opposites.

If a pathname is not a file, it may or may not be a directory. It could be a directory or any number of other special file types under Unix. The test pathname operators all return success zero only if the pathname is accessible all the directories can be traversed AND the pathname exists AND if it has the given pathname property.

You need to apply more programming logic if you want to know that a pathname actually exists but is not, for example, readable:. Remember that inverting a pathname test may mean the inverted test succeeds because the pathname is not accessible or does not exist! If a test pathname operator e. Without first testing if you can access the pathname and that it actually exists, the following error message is misleading:. While it is true that the pathname is not readable, the above error message is incomplete.

You might not have permission to traverse all the directories in its pathname, or, the pathname might not even exist.

Saying the overall pathname is not readable is true, but it is only part of the truth. A more accurate error message would be:. If you want to be more specific in your error message about why the pathname is not readable, you need code to test for existence first:.

The test for readability is now done only if the pathname exists and is accessible; if the test for readability fails, you know the existing, accessible pathname item is truly not readable. The error message is more accurate now.

Any time one of the test pathname operator tests fails, be accurate in your error message. State whether the failure is due to a missing or inaccessible pathname, or due to a failure of the actual test being performed on the existing, accessible pathname. Be careful in if statements when testing multiple conditions at the same time that you do not make the failure error message unhelpful:. The error would have to read like this: Which failure was it? Such a complex error message is not helpful to the users of your scripts!

The programmer forgot that the if statement can directly test the return code of the command it executes. Calling up the test command to examine the shell variable for the return code of the previous command is superfluous.

Plain Text - plain text version of this page in Pandoc Markdown format. Shell Script Problems — arithmetic, syntax, test, boolean, etc. Allen — idallen idallen.

For example, having a missing or non-integer argument to the test command may produce a vague error message: Running the script using a shell with the debug options -x or -v set may also be helpful: This line below is wrong thinking; it forgets to use the test helper program: Always remember to code some Unix command name after if: The above syntactic-sugar line is equivalent to this also incorrect line: If you want to test the return status of grep , grep must be the command name that immediately follows the if keyword: The following code does not work; because, blanks are missing around the first square bracket, making it into an unknown command named [1: The following incorrect statement fails for the same reason: Always use blanks around [ and ]: This next line fails because of the missing blank before the required closing square bracket: The corrected line uses blanks: The following one-argument tests are always TRUE, though they may not appear that way at first to human eyes: The shell is actually executing these tests: If you want to perform equality tests, you must separate each argument by blanks so that test sees three separate arguments, not just one: Here are two identical mistakes: The correct shell scripting form does not use the redirection syntax: These script command lines below have a single empty or null string argument: To test if a file is non-existent or exits but is not readable: Use -a and -o to separate Boolean clauses to the test command: Rather than using the test command twice, you can simply join them into one using the correct test Boolean operator: You cannot replace the test!

You need to apply more programming logic if you want to know that a pathname actually exists but is not, for example, readable: Without first testing if you can access the pathname and that it actually exists, the following error message is misleading: A more accurate error message would be: