How to do an if statement from the result of an executed command

Question

I am trying to do an IF statement from the output of an executed commmand. Here is how I am trying to do it, but it doesn't work. Does anyone know the right way to do this?

if [ "`netstat -lnp | grep ':8080'`" == *java* ]; then
  echo "Found a Tomcat!"
fi

EDIT: I wonder if there is a way to do it by capturing exit code?

Regarding your recent edit: You are capturing the exit code, of the [ utility. The currently accepted answer captures the exit code of [[, and the other answer captures the exit code of the grep -q pipeline. — Kusalananda, Oct 17 '22 at 17:46

score 132 · Accepted Answer · edited Oct 17 '22 at 17:42

132

Use the bash [[ conditional construct and prefer the $(<command>) command substitution convention. Additionally, [[ prevents word splitting of variable values therefore there is no need to quote the command substitution bit..

if [[ $(netstat -lnp | grep ':8080') == *java* ]]; then
  echo "Found a Tomcat!"
fi

edited Oct 17 '22 at 17:42

Kusalananda

333,661

answered Oct 24 '12 at 16:55

iruvar

16,725

Additionally, [[ prevents word splitting of variable values therefore there is no need to quote the command substitution bit Can you elaborate on this concept? I don't see any variables in here: what would an example look like where this made a difference? – Daniel Kaplan Sep 13 '23 at 23:10

score 52 · Answer 2 · edited Oct 17 '22 at 17:45

52

Another alternative is to simply test the exit status of grep itself, which will return false (non-zero) if there was no match and true (zero) if there was one, by not using the [ command.

if netstat -lntp | grep -q ':8080.*java'; then
    echo "Found a Tomcat!"
fi

edited Oct 17 '22 at 17:45

Gilles Quénot

33,867

answered Oct 24 '12 at 18:20

Random832

10,666

1

Newb here: is this "idiomatic" bash or is it "clever code"? – Daniel Kaplan Sep 13 '23 at 23:12

score 10 · Answer 3 · answered Oct 25 '12 at 02:21

10

Even more simple,

netstat -lntp | grep ':8080.*java' > /dev/null && command

If you just want to do one thing.

answered Oct 25 '12 at 02:21

Rob Bos

4,380

score 4 · Answer 4 · answered Oct 25 '12 at 00:05

You can do more precise, yet simple, matching with awk.

if netstat -lnp | awk '$4 ~ /:8080$/ && $7 ~ /java/ {exit(0)} END {exit(1)}'; then …

To match the structure of your command more closely, the portable way of doing wildcard matching on a string in a shell is with the case construct.

case "$(netstat -lnp | grep ':8080')" in
  *java*)  echo "Found a Tomcat!";;
esac

score 1 · Answer 5 · edited Aug 29 '20 at 21:15

1

Here's how I did it in a script that determines whether my music player is open or not, and if a song is playing if it is open.

rbstatus=$(rhythmbox-client --print-playing)
pgrep -x rhythmbox >/dev/null && if [ "$rbstatus" = "-" ]; then
   echo "No music playing"
else
   rhythmbox-client --print-playing
fi || echo "Music player is Closed."

edited Aug 29 '20 at 21:15

fra-san

10,205
2
22
43

answered Aug 29 '20 at 07:19

Can221-ParOS

11

Welcome on U&L! A side note: consider that &&/|| is not equivalent to if/else. Your code will print "Music player is Closed." even if pgrep succeeds and the command in the else block fails (returns a non-zero status) for some reason (likely not a real issue in your case, of course). – fra-san Aug 29 '20 at 21:13
Nice alternative way of doing it. :-) – djangofan Oct 19 '22 at 23:02

score 1 · Answer 6 · answered Oct 17 '22 at 17:49

1

A simple and logic way:

netcat -zw3 localhost 8000 && echo 'tomcat is up'

answered Oct 17 '22 at 17:49

Gilles Quénot

33,867

How to do an if statement from the result of an executed command

6 Answers6