Integer Math

First way to do math with integer (and only integer) is to use the command “expr — evaluate expression“.

Mac-n-Cheese:~ nicolas$ expr 1 + 1
2
Mac-n-Cheese:~ nicolas$ myvar=$(expr 1 + 1)
Mac-n-Cheese:~ nicolas$ echo $myvar
2
Mac-n-Cheese:~ nicolas$ expr $myvar + 1
3
Mac-n-Cheese:~ nicolas$ expr $myvar / 3
1
Mac-n-Cheese:~ nicolas$ expr $myvar \* 3
9

When doing a “multiply by” make sure to backslash the “asterisk”  as it’s a wildcard in Bash used for expansion.

Another alternative to expr, is to use the bash builtin command let.

Mac-n-Cheese:~ nicolas$ echo $myvar
6
Mac-n-Cheese:~ nicolas$ let myvar+=1
Mac-n-Cheese:~ nicolas$ echo $myvar
7
Mac-n-Cheese:~ nicolas$ let myvar+1
Mac-n-Cheese:~ nicolas$ echo $myvar
7
Mac-n-Cheese:~ nicolas$ let myvar2=myvar+1
Mac-n-Cheese:~ nicolas$ echo $myvar2
8

Also, you can simply use the parentheses or square brackets :

Mac-n-Cheese:~ nicolas$ echo $myvar
3
Mac-n-Cheese:~ nicolas$ echo $((myvar+2))
5
Mac-n-Cheese:~ nicolas$ echo $[myvar+2]
5
Mac-n-Cheese:~ nicolas$ myvar=$((myvar+3))

This allow you to use C-style programming :

Mac-n-Cheese:~ nicolas$ echo $myvar
3
Mac-n-Cheese:~ nicolas$ echo $((myvar++))
3
Mac-n-Cheese:~ nicolas$ echo $myvar
4
Mac-n-Cheese:~ nicolas$ echo $((++myvar))
5
Mac-n-Cheese:~ nicolas$ echo $myvar
5

Floating point arithmetic

If you need to do floating point arithmetic, you will have to use a command line tool, the most common one is “bc – An arbitrary precision calculator language“.

Mac-n-Cheese:~ nicolas$ bc
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
3*5.2+7/8
15.6
15.6+299.33*2.3/7.4
108.6

Of course you can use the STDIN to send your formula to “bc” then get the output on STDOUT.

Mac-n-Cheese:~ nicolas$ echo "3.4+7/8-(5.94*3.14)" | bc
-15.25

I encourage you too take a look at the man pages to get more detail on how it works (man bc).

There are four special variables, scale, ibase, obase, and last.  scale defines how some operations use digits after the decimal point.  The default value of scale is 0. ibase and obase define the conver-
sion base for input and output numbers.  The default for both input and output is base 10.  last (an extension) is a variable that has the value of the last printed number.

The “scale” variable is really important for the precision of your results, especially when using integers only (Note: you can also use “bc -l” to use mathlib and see the result at max scale) .

Mac-n-Cheese:~ nicolas$ echo "2/3" | bc
0
Mac-n-Cheese:~ nicolas$ echo "scale=2; 2/3" | bc
.66
Mac-n-Cheese:~ nicolas$ echo "(2/3)+(7/8)" | bc
0
Mac-n-Cheese:~ nicolas$ echo "scale=2;(2/3)+(7/8)" | bc
1.53
Mac-n-Cheese:~ nicolas$ echo "scale=4;(2/3)+(7/8)" | bc
1.5416
Mac-n-Cheese:~ nicolas$ echo "scale=6;(2/3)+(7/8)" | bc
1.541666
Mac-n-Cheese:~ nicolas$ echo "(2/3)+(7/8)" | bc -l
1.54166666666666666666

You can also use the here-doc notation to pass your formula to bc :

Mac-n-Cheese:~ nicolas$ bc -l <<< "(2/3)+(7/8)"
1.54166666666666666666

                          

apachectl stop

apachectl start

apachectl graceful

You can also check your configuration files.

apachectl configtest

A common error is probably this one :

(98)Address already in use: make_sock: could not bind to address 0.0.0.0:443

no listening sockets available, shutting down

This is caused by one or more processes running on the 443 (secure socket) port. You can get all the process ID’s that are running on this port with the command fuser or a more classical “ps auxww” with a less readable format.

nicolas@grimm:~$ fuser 443/tcp

443/tcp: 7977 6815 9819 35217

Now you just have to do a kill on those process.

nicolas@grimm:~$ kill -9 7977 6815 9819 35217

Or in a more sys admin useful way :

nicolas@grimm:~$ kill -9 $(fuse 443/tcp 2>/dev/null)

Other typical error would be those following :

(28)No space left on device: Couldn’t create accept lock

or

(28)No space left on device: mod_rewrite: could not create rewrite_log_lock Configuration Failed

Brain dead things is that you have plenty of space on your disk. Problem comes from apache that was not shutdown properly and lets lot of semaphore-arrays left. So, here comes ipcs command. ipcs provides information on the ipc facilities for which the calling process has read acccess.

nicolas@grimm:~$ ipcs -s | grep www-data

0×00000000 163840 www-data 600 1
0×00000000 196609 www-data 600 1
0×00000000 229378 www-data 600 1

Before restart your apache you have to remove all this semaphore arrays by using ipcrm. ipcrm remove a message queue, semaphore set or shared memory id.

nicolas@grimm:~$ ipcs -s | grep www-data | perl -e ‘while () { @a=split(/\s+/); print `ipcrm sem $a[1]`}’

Now you should be able to start your apache. Lot of other error can happens. first thing to do is to take a look at your error log file then googleize ! ;-)