Hiding output of a command
I have a script where it checks whether a package is installed or not and whether the port 8080 is being used by a particular process or not. I am not experienced at all with bash, so I did something like this:
if dpkg -s net-tools; then if netstat -tlpn | grep 8080 | grep java; then echo "Shut down server before executing this script" exit fi
else echo "If the server is running please shut it down before continuing with the execution of this script"
fi
# the rest of the script...However when the script is executed I get both the dpkg -s net-tools and the netstat -tlpn | grep 8080 | grep java outputs in the terminal, and I don't want that, how can I hide the output and just stick with the result of the ifs?
Also, is there a more elegant way to do what I'm doing? And is there a more elegant way to know what process is using the port 8080 (not just if it's being used), if any?
3 Answers
To hide the output of any command usually the stdout and stderr are redirected to /dev/null.
command > /dev/null 2>&1Explanation:
1.command > /dev/null: redirects the output of command(stdout) to /dev/null
2.2>&1: redirects stderr to stdout, so errors (if any) also goes to /dev/null
Note
&>/dev/null: redirects both stdout and stderr to /dev/null. one can use it as an alternate of /dev/null 2>&1
Silent grep: grep -q "string" match the string silently or quietly without anything to standard output. It also can be used to hide the output.
In your case, you can use it like,
if dpkg -s net-tools > /dev/null 2>&1; then if netstat -tlpn | grep 8080 | grep java > /dev/null 2>&1; then #rest thing
else echo "your message"
fiHere the if conditions will be checked as it was before but there will not be any output.
Reply to the comment:
netstat -tlpn | grep 8080 | grep java > /dev/null 2>&1: It is redirecting the output raised from grep java after the second pipe. But the message you are getting from netstat -tlpn. The solution is use second if as,
if [[ `netstat -tlpn | grep 8080 | grep java` ]] &>/dev/null; thenlsof -i :<portnumnber> should be able to do something along the lines of what you want.
While flushing the output to /dev/null is probably the easiest way, sometimes /dev/null has file permissions set so that non-root cannot flush the output there. So, another non-root way to do this is by
command | grep -m 1 -o "abc" | grep -o "123"This double-grep setup finds the matching lines with abc in them and since -o is set ONLY abc is printed and only once because of -m 1. Then the output which is either empty or abc is sent to grep to find only the parts of the string which match 123 and since the last command only outputs abc the empty string is returned. Hope that helps!
