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

User · Answer

if takes a command and checks its return value    is just a command   if grep -q     then        else        fi

User · Answer

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      amp  amp           amp   newline

User · Answer

just use bash  while read -r line do   case   line  in      MYSQL ROLE master          echo  do your stuff           echo  doesn t exist            esac done  lt   etc aws hosts conf

User · 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 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 mdash the same as how other return codes are interpreted by the shell  and the opposite of a language like C

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

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