CODEWITHSUNDEEP
bash
user77413
user77413

Reputation: 31205

How do I use a file grep comparison inside a bash if/else statement?

When our server comes up we need to check a file to see how the server is configured.

We want to search for the following string inside our /etc/aws/hosts.conf file: 

MYSQL_ROLE=master

Then, we want to test whether that string exists and use an if/else statement to run one of two options depending on whether the string exists or not.

What is the BASH syntax for the if statement? 

if [ ????? ]; then
  #do one thing
else
  #do another thing
fi

Upvotes: 155

Views: 248317

Answers (5)

Ashish Sharma
Ashish Sharma

Reputation: 672

Below code sample should work:

(echo "hello there" | grep -q "AAA") && [ $? -eq 0 ] && echo "hi" || echo "bye"

Upvotes: 1

vladr
vladr

Reputation: 66661

Note that, for PIPE being any command or sequence of commands, then:

if PIPE ; then
  # do one thing if PIPE returned with zero status ($?=0)
else 
  # do another thing if PIPE returned with non-zero status ($?!=0), e.g. error
fi

For the record, [ expr ] is a shell builtin† shorthand for test expr.

Since grep returns with status 0 in case of a match, and non-zero status in case of no matches, you can use:

if grep -lq '^MYSQL_ROLE=master' ; then 
  # do one thing 
else 
  # do another thing
fi

Note the use of -l which only cares about the file having at least one match (so that grep returns as soon as it finds one match, without needlessly continuing to parse the input file.)

†on some platforms [ expr ] is not a builtin, but an actual executable /bin/[ (whose last argument will be ]), which is why [ expr ] should contain blanks around the square brackets, and why it must be followed by one of the command list separators (;, &&, ||, |, &, newline)

Upvotes: 27

ghostdog74
ghostdog74

Reputation: 342333

just use bash

while read -r line
do
  case "$line" in
    *MYSQL_ROLE=master*)
       echo "do your stuff";;
    *) echo "doesn't exist";;      
  esac
done <"/etc/aws/hosts.conf"

Upvotes: 4

Roger Pate
Roger Pate

Reputation:

From grep --help, but also see man grep:

Exit status is 0 if any line was selected, 1 otherwise; if any error occurs and -q was not given, the exit status is 2.

if grep --quiet MYSQL_ROLE=master /etc/aws/hosts.conf; then
  echo exists
else
  echo not found
fi

You may want to use a more specific regex, such as ^MYSQL_ROLE=master$, to avoid that string in comments, names that merely start with "master", etc.

This works because the if takes a command and runs it, and uses the return value of that command to decide how to proceed, with zero meaning true and non-zero meaning false—the same as how other return codes are interpreted by the shell, and the opposite of a language like C.

Upvotes: 226

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798566

if takes a command and checks its return value. [ is just a command.

if grep -q ...
then
  ....
else
  ....
fi

Upvotes: 38

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