bash Regular Expression in if Statement

Question

I am stumped at the error I am getting from a script I'm trying to run to suspend my machine. I am trying to use a regex in an elif statement to suspend my machine after a specific time period.

#!/bin/bash
echo "When would you like to suspend the machine?"
read "sustime"
if [ "$sustime" = "now" ]
then
    sudo pm-suspend
elif [[ "$sustime" =~ [0-9]*[smhd] ]]
then
    time=`expr "$sustime" : '\([0-9]+)\)'`
    ttype=`expr "$sustime" : '.*\([smhd]\)'`
    sudo sleep $time$ttype ; sudo pm-suspend
else
    echo "Please enter either [now] or [#s|m|h|d]"
fi

The code doesn't work on the elif line, for example if I input 5s, the output of the script is:

$ sh dbussuspend.sh 
When would you like to suspend the machine?
5s
dbussuspend.sh: 10: dbussuspend.sh: [[: not found
Please enter either [now] or [#s|m|h|d]

However, it should read that I've entered the string 5s run the code block under elif. I've actually tried any regex in place of [0-9]*[smhd], all with the same error.

Side note: you probably want [0-9]+[smhd] otherwise h will be an acceptable input. — Laurel
– Laurel, Commented May 3, 2016 at 23:53
You don't need expr; you can use capture groups in the original regular expression, then access the captured values from the array BASH_REMATCH. — chepner
– chepner, Commented May 4, 2016 at 1:05

Community · Accepted Answer · 2017-05-23 11:44:05Z

6

this problem is not caused by your script, but by how you're invoking it:

sh dbussuspend.sh

should be:

bash dbussuspend.sh

bash know how to [[, but sh doesn't...

better still, do as Gordon Davisson suggests. do this once:

chmod +x dbussuspend.sh

and thenceforth, invoke like this:

./dbussuspend.sh

additionally, Etan Reisner and chepner question your use of expr, and laurel your bash regex. GNU coreutils sleep supports e.g. sleep 30s, sleep 2m, sleep 1h. check this on your system with man sleep. if so, then this will work:

elif [[ "$sustime" =~ ^[0-9]+[smhd]$ ]]
then
    sudo sleep $sustime ; sudo pm-suspend

(^ and $ in ^[0-9]+[smhd]$ to match the start and end of the string and prevent matching e.g. "uzeifue1sziufzr".)

edited May 23, 2017 at 11:44

CommunityBot

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answered May 4, 2016 at 0:18

webb

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3 Comments

Etan Reisner Over a year ago

Also don't use expr. Especially when it looks like you are doing that to split input and then just put it immediately back together again.

Gordon Davisson Over a year ago

Even better than explicitly using bash, you should make the script executable (if it isn't already) with chmod +x dbussuspend.sh, then run it with ./dbussuspend.sh so it'll obey the shebang line (which is already /bin/bash, as it should be).

chepurko Over a year ago

Omg I love this website. Thanks a lot everyone, really clears it up for me.

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