Command substitution in if statement

Question

I'm trying to compare the result of a command substitution to a string, like this:

if [$(ping $1)=="ping: unknown host localhosts"]
then
  echo "no";
else
  echo "yes";
fi

What am I doing wrong here?

Use http://www.shellcheck.net/ – Stéphane Chazelas Jul 29 '17 at 00:04 — Stéphane Chazelas, Jul 29 '17 at 00:04

xhienne · Accepted Answer · 2017-07-29T00:13:04.033

5

First, you forgot the space and the quoting. Next, it's an error, so you should catch stderr. Finally localhosts is probably $1.

if [ "$(ping -c1 "$1" 2>&1)" = "ping: unknown host $1" ]

Note that ping message is probably subject to your locale settings. If you just want to test for the name resolution of a host, ping is not exactly the right tool. Try this instead:

if getent hosts "$1" > /dev/null; then ...

edited Jul 29 '17 at 00:13

answered Jul 29 '17 at 00:04

xhienne

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Also = instead of == as == is not standard. – Stéphane Chazelas Jul 29 '17 at 00:07
On my machine it works for the 'no' case but hangs for 'yes'. – Philip Kirkbride Jul 29 '17 at 00:08
@PhilipKirkbride ping never ends unless you use an option like -c. Answer edited. – xhienne Jul 29 '17 at 00:09
1

@PhilipKirkbride I have added an alternate syntax that does not involve ping. – xhienne Jul 29 '17 at 00:14

Command substitution in if statement

1 Answers1