• Uncategorized

About bash : How-do-I-use-a-file-grep-comparison-inside-a-bash-ifelse-statement

Question Detail

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:


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
  #do another thing

Question Answer

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
  echo not found

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.

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

if grep -q ...

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

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

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 
  # do another thing

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)

just use bash

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

Below code sample should work:

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

You may also like...

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.