You can use `expr`

utility to evaluate expressions in both of the command line or shell script.

Here are a few examples:

```
expr 6 + 4
10
```

Note the spaces. Without spaces, you get the following:

```
expr 6+4
6+4
```

If you are using “*” you will need a “” before it:

```
expr 10 \* 10
100
```

This also work for variables:

```
var1=2
var1=`expr $var1 \* 2`
echo $var1
4
```

You can get the cosine(.23):

```
var1=`echo "c(.23)" | bc -l`
echo $var1
.97366639500537483696
```

You can also do substrings:

```
expr substr "BigBear" 4 4
Bear
```

And length of strings:

```
mstr="12345"
expr length $mstr
5
```

Regular expressions:

```
expr "a3" : [a-z][1-9]
2
```

Or you can get a bit fancy:

```
myexpr="[a-z][1-9]"
expr "a3" : $myexpr
2
```

This may not be the best way to find out if it is Friday, but it seems to work. It’s more of an exercise in xargs.

```
date
Fri Dec 31 16:44:47 EST 2004
date | xargs -i expr {} : "[Fri]"
1
```