Redirecting Command output to file

Question

This seems so simple when doing it from command line but I'm not able to accomplish it inside a script. I am trying to put output of following command into a text file:

CMD= mysql -uroot -psecret -e 'SHOW SLAVE STATUS \G;'
FIL=~/replication-`date +%F`.txt
MAILTEXT=~/mailtext.txt

touch $FIL
$CMD > $FIL

Where FIL is a variable that contains path of the file to which to output command. I am running this command in a shell script from where I want to email contents of $FIL as attachment using mutt. But I am always getting 0 byte file. Also if I examine in directory the file is of 0 byte length.

Answer 1

The problem isn't with the redirection, it's with how you're storing the command in a variable. First, you can't put a space after the = in the assignment; with the space there, it sets CMD blank and runs the command mysql -uroot -psecret -e 'SHOW SLAVE STATUS \G;'. If you just remove the space, it'll set CMD to "mysql" and try to run the command -uroot -psecret -e 'SHOW SLAVE STATUS \G;'. So, as @Michał Šrajer suggested, you could wrap it in double-quotes so the whole thing gets assigned to CMD. But it still doesn't work, because when $CMD is expanded, it doesn't pay attention to the quotes inside it. When bash parses a command line, it parses quotes before it expands variables, so putting quotes inside a variable doesn't do anything useful.

Storing a command in a variable is tricky. BashFAQ #50 has some good discussion and options. In this case, the options that look relevant to me are:

1. Don't put the command in a variable in the first place. If there's no good reason for it, don't do it:

   FIL=~/replication-`date +%F`.txt
   MAILTEXT=~/mailtext.txt
   
   touch $FIL
   mysql -uroot -psecret -e 'SHOW SLAVE STATUS \G;' > $FIL
   

2. If you must put the command in a variable, use an array instead of a simple text variable. In this case, the quotes will be parsed when the variable in created, and if you use it as "${varname[@]}" the breaks between "words" will be preserved:

   CMD=(mysql -uroot -psecret -e 'SHOW SLAVE STATUS \G;')
   FIL=~/replication-`date +%F`.txt
   MAILTEXT=~/mailtext.txt
   
   touch $FIL
   "${CMD[@]}" > $FIL
   

Answer 2

change:

CMD= mysql -uroot -psecret -e 'SHOW SLAVE STATUS \G;'

to:

CMD="mysql -uroot -psecret -e 'SHOW SLAVE STATUS \G;'"

Answer 3

I've created a similar script which does work:

FIL=~/test.txt
CMD="hostname"
touch $FIL
$CMD > $FIL

Did you try without the redirection first, to see whether the outputs get generated at all?