is 1 and 2
Repeat either of those commands to continue to increment the value of $I.
NOTE
Although most elements of shell scripts are relatively freeform (where white space, such as spaces or tabs, is insignificant), both let and expr are particular about spacing. The let command insists on no spaces between each operand and the mathematical operator, whereas the syntax of the expr command requires white space between each operand and its operator. In contrast to those, bc isn't picky about spaces, but it can be trickier to use because it does floating-point arithmetic.
To see a complete list of the kinds of arithmetic that you can perform using the let command, type help let at the bash prompt.
Using programming constructs in shell scripts
One of the features that makes shell scripts so powerful is that their implementation of looping and conditional execution constructs is similar to those found in more complex scripting and programming languages. You can use several different types of loops, depending on your needs.
The ″if…then″ statements
The most commonly used programming construct is conditional execution, or the if statement. It is used to perform actions only under certain conditions. There are several variations of if statements for testing various types of conditions.
The first if…then example tests if VARIABLE is set to the number 1. If it is, then the echo command is used to say that it is set to 1. The fi statement then indicates that the if statement is complete, and processing can continue.
VARIABLE=1 if [ $VARIABLE -eq 1 ] ; then echo "The variable is 1" fi
Instead of using -eq, you can use the equal sign (=), as shown in the following example. The = works best for comparing string values, while -eq is often better for comparing numbers. Using the else statement, different words can be echoed if the criterion of the if statement isn't met ($STRING = ″Friday″). Keep in mind that it's good practice to put strings in double quotes.
STRING="Friday" if [ $STRING = "Friday" ] ; then echo "WhooHoo. Friday." else echo "Will Friday ever get here?" fi
You can also reverse tests with an exclamation mark (!). In the following example, if STRING is not Monday, then ″At least it's not Monday″ is echoed.
STRING="FRIDAY" if [ "$STRING" != "Monday" ] ; then echo "At least it's not Monday" fi
In the following example, elif (which stands for “else if”) is used to test for an additional condition (for example, whether filename is a file or a directory).
filename="$HOME" if [ -f "$filename" ] ; then echo "$filename is a regular file" elif [ -d "$filename" ] ; then echo "$filename is a directory" else echo "I have no idea what $filename is" fi
As you can see from the preceding examples, the condition you are testing is placed between square brackets [ ]. When a test expression is evaluated, it returns either a value of 0, meaning that it is true, or a 1, meaning that it is false. Notice that the echo lines are indented. The indentation is optional and done only to make the script more readable.
Table 7.1 lists the conditions that are testable and is quite a handy reference. (If you're in a hurry, you can type help test on the command line to get the same information.)
TABLE 7.1 Operators for Test Expressions
| Operator | What Is Being Tested? |
-a file
|
Does the file exist? (same as -e)
|
-b file
|
Is the file a block special device? |
-c file
|
Is the file character special (for example, a character device)? Used to identify serial lines and terminal devices. |
-d file
|
Is the file a directory? |
-e file
|
Does the file exist? (same as -a)
|
-f file
|
Does the file exist, and is it a regular file (for example, not a directory, socket, pipe, link, or device file)? |
-g file
|
Does the file have the set group id (SGID) bit set? |
-h file
|
Is the file a symbolic link? (same as -L)
|
-k file
|
Does the file have the sticky bit set? |
-L file
|
Is the file a symbolic link? |
-n string
|
Is the length of the string greater than 0 bytes? |
-O file
|
Do you own the file? |
-p file
|
Is the file a named pipe? |
-r file
|
Is the file readable by you? |
-s file
|
Does the file exist, and is it larger than 0 bytes? |
-S file
|
Does the file exist, and is it a socket? |
-t fd
|
Is the file descriptor connected to a terminal? |
-u file
|
Does the file have the set user id (SUID) bit set? |
-w file
|
Is the file writable by you? |
-x file
|
Is the file executable by you? |
-z string
|
Is the length of the string 0 (zero) bytes? |
expr1 -a expr2
|
Are both the first expression and the second expression true? |
|
expr1
|
