Question

I am writing the top command output to a text file.
I am trying to write a simple bash script to calculate the percentage of used memory and send an
email if the memory used percentage exceeds, say 90%.

Here is the bash script I have thus far.

#!/bin/bash
top -n 1 -b | grep "Mem" > /home/modadm/top-output.txt
MAXMEM=/home/modadm/top-output.txt | grep "Mem" | cut -c 7-14 
USEDMEM=/home/modadm/top-output.txt | grep "Mem" | cut -c 25-31
$USEDPCT='echo $USEDMEM / $MAXMEM * 100 | bc'
$USEDPCT | mail -s "Test Email from MOD Server" [e-mail]

When I save and execute the script, I get the error “No such file or directory”:

-bash-3.2$ ./memcheck.sh
./memcheck.sh: line 4: =echo $USEDMEM / $MAXMEM * 100 | bc: No such file or directory
Null message body; hope that's ok
-bash-3.2$ 

Can someone assist? I am a newbie to bash scripting and this is my first script.

Thank you

Answer 1

You have a few problems here.

First, this doesn’t do what you want it to do.

USEDMEM=/home/modadm/top-output.txt | grep "Mem" | cut -c 25-31

You can’t pipe a filename into a command. You actually want to pipe the contents of the file into the command. You can do that with ‘cat’. However, grep is actually designed to search within a file so you can do

USEDMEM=$(grep "Mem" /home/modadm/top-output.txt | cut -c 25-31)

Note that $(cmd) is how you execute a command in a subshell. i.e., you can run some commands to compute the value of a variable in your script. You can also use `cmd` (backticks; usually on the tilde key) but that syntax is less clear.

Again, you probably want to calculate this result in a subshell. Also, don’t use $ when assigning to variables.

$USEDPCT='echo $USEDMEM / $MAXMEM * 100 | bc'

This can be rewritten as

USEDPCT=$(echo "scale=3; $USEDMEM / $MAXMEM * 100" | bc)

Finally, you want to pipe the contents of the variable into the mail program. The pipe is expecting a program to be on the left hand side. You do this by echo’ing the value of the variable into the pipe.

echo "$USEDPCT" | mail -s "Test Email from MOD Server" [e-mail]

To put everything back together:

#!/bin/bash
top -n 1 -b | grep "Mem" > /home/modadm/top-output.txt
MAXMEM=$(grep "Mem" /home/modadm/top-output.txt | cut -c 7-14)
USEDMEM=$(grep "Mem" /home/modadm/top-output.txt | cut -c 25-31)
USEDPCT=$(echo "$USEDMEM / $MAXMEM * 100" | bc -l)
echo "$USEDPCT" | mail -s "Test Email from MOD Server" [e-mail]