Control Mechanisms of the UNIX Shell - The expr command
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When we evaluate variables, sometimes we would like to perform some basic arithmetic operations. From this, we will learn in the end whether or not a condition is active. The simplest command what deals with this is the expr. The expr can also perform some basic string operations.
The syntax of the expr is quite simple:
expr expression
The expression will print the result on the standard output. What can the expression be? The table below shows a list of these:
Expression | What does it do? |
ARG1 | ARG2 | ARG1 if that is not zero, otherwise ARG2 |
ARG1 & ARG2 | ARG1 if neither is zero or zero length string, otherwise 0 |
ARG1 < ARG2 | ARG1 is smaller than ARG2 |
ARG1 <= ARG2 | ARG1 is smaller or equal than ARG2 |
ARG1 = ARG2 | ARG1 is equal with ARG2 |
ARG1 != ARG2 | ARG1 is not equal with ARG2 |
ARG1 >= ARG2 | ARG1 is greater or equal than ARG2 |
ARG1 > ARG2 | ARG1 is greater than ARG2 |
ARG1 + ARG2 | Arithmetic sum |
ARG1 - ARG2 | Arithmetic subtraction |
ARG1 * ARG2 | Arithmetic multiplication |
ARG1 / ARG2 | Arithmetic divide |
ARG1 % ARG2 | Arithmetic modulo of ARG1/ARG2 |
STRING : REGEXP | string match from the first character of the STRING |
match STRING REGEXP | The same as above |
substr STRING POS LENGTH | returns from STRING a substring that start from POS and has a length of LENGTH |
index STRING CHARS | Returns the index from STRING where any characters from CHARS can be found |
length STRING | The length of the STRING |
You have to bypass the meta-characters with the help of the backslash. You have to do so with the <,>, &, |, * ones.
A couple of examples:
expr 1 + 2
a= $( expr 2 > 3)
b= `expr 1 * 2`
You need to leave the spaces between the arguments and the operators. Otherwise, you will wake up with a beautiful error message. Here is how you can use the expr with the “if structure:”
if expr 2 < 3
then
echo Two is smaller than three
fi
This will be all for today. Next week we will cover data streams, and then we will learn a more advanced and complicated command to test conditions. This is the test command. Until then, start up your console, make sure you try out what you learned today, and remember that repetition is the mother of knowledge. Thank you for reading my article. Please rate it. This will help further readers to get a more realistic picture of it. If you are unclear on anything, do not hesitate to ask your questions on the blog. Live With Passion!
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