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

Question

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

Handmedown

Answer 1

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.

Feola

> 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. but why the opposite....? – Theoretics

Because a zero is typically interpreted as "no error", while anything else means "some error". If assumes we want to know if the command completed successfully. – Herodias

Gets an upvote (and applause) for using the --quiet arg instead of -q in the example. Long-args are so much more readable and self document the intention really clearly. – Dingy

Answer 2

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

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

Alcorn

Answer 3

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)

Wiley

-l is redundant, grep returns immediately upon finding any match with -q – Monocoque

Answer 4

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"

Enthusiasm

Answer 5

Below code sample should work:

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

Uveitis

$? is not needed here : (echo "hello there" | grep -q "AAA") && echo "hi" || echo "bye" – Milli