Ubuntu – How to check for a matching pattern in bash

bashregex

I have a file in which value starts with ABC or ABD.

After fetching the value from input file how to check it starts with ABC or ABD?

eg:

value=$(cat file | awk 'NF{print;exit}' | awk '{print $1}')

and now I want to check this $value is starting with ABC or ABD . how?

Best Answer

Using `case`

To check if a shell variable starts with ABC or ABD, a traditional (and very portable) method is to use a case statement:

case "$value" in
  AB[CD]*) echo yes;;
   *)      echo no;;
esac

Because this requires no external processes, it should be fast.

Using `[`

Alternatively, one can use a test command:

if [ "${value#AB[CD]}" != "$value" ]
then
   echo yes
else
   echo no
fi

This is also quite portable.

Using `[[`

Lastly, one can use the more modern and less portable test command:

if [[ $value == AB[CD]* ]]
then
   echo yes
else
   echo no
fi

Reading the file and testing all in one step

To read the first nonempty line of a file and test if its first field start with ABC or ABD:

awk 'NF{if ($1~/^AB[CD]/) print "yes"; else print "no";exit}' file

Related Solutions

Bash Scripts – Execute Script When Network Interface is Up

Try doing this :

#!/bin/bash

PATH+=/sbin
adapter=eth0
internal=""

until [[ $internal =~ [0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3} ]]; do
    sleep 1
    internal=$(
        ifconfig "$adapter" |
        awk -F'(inet add?r:| +|:)' '/inet add?r/{print $3}'
    )
done

# send sms

As you can see, I use only one pipe : | with awk, no need more, awk can do the whole processing.

Ubuntu – Bash always evaluate Regex as true

I was too quick on this one, your test works. I suspect you somehow have set $SCREEN_NAME either globally or previously in the script as a string containing an illegal character.

^[a-zA-Z0-9_.-]+$ matches a non-NULL string containing only the allowed characters, so if $SCREEN_NAME is a non-NULL string containing only the allowed characters, $SCREEN_NAME =~ ^[a-zA-Z0-9_.-]+$ 's value is 0.

! inverts its value, so if $SCREEN_NAME is a non-NULL string containing only the allowed characters, ! [[ $SCREEN_NAME =~ ^[a-zA-Z0-9_.-]+$ ]]'s value is 1.

So the condition is ok.

Run echo $SCREEN_NAME in both bash and your script and check whether the actual output matches what's expected.

Edit: To remove the trailing carriage return, one solution is to pipe grep's output to tr before cut:

SCREEN_NAME=$(grep -i 'server_screen_title' server.properties | tr -d '\r' | cut -f2 -d'=')

Best Answer

Using case

Using [

Using [[