CODEWITHSUNDEEP
bash
el n00b
el n00b

Reputation: 1863

Use a variable for redirections in bash

I am writing a basic script that has something like this:

LOG_COMMAND=">/tmp/something.log 2>&1"
if [ "${VERBOSE}" == "y" ]; then
  LOG_COMMAND="2>&1 | tee /tmp/something.log"
fi

The problem is that when I use the LOG_COMMAND variable in my script it gets wrapped with single quotes. For example:

set -v
docker build -t test:test . ${LOG_COMMAND}
set +v

I get the following output:

$ /bin/bash testing.sh -v
+ docker build -t test:test . '2>&1' '|' tee /tmp/something.log
"docker build" requires exactly 1 argument.
See 'docker build --help'.

How can I prevent the single quotes from being included?

Upvotes: 0

Views: 77

Answers (2)

that other guy
that other guy

Reputation: 123470

The simple and robust solution is to put code you want to reuse in a function, rather than in a variable:

log() {
    if [ "$VERBOSE" = "y" ]
    then
        "$@" 2>&1 | tee -a /tmp/something.log
    else
        "$@" >> /tmp/something.log 2>&1
    fi
}

log docker build -t test:test .

The single quotes you see are not being added in any way, they're just bash's notation to show you that the value is a literal string and not part of the command's syntax. "Removing" them wouldn't change the fact that they are just uninterpreted strings any more than whoami() { echo root; } makes you the super user.

If it helps put things into context, Java has the same "problem" with the same preferred solution:

// Works
System.out.println("hello world".replaceAll("hello", "goodbye"));

// Fails
String replacer = ".replaceAll(\"hello\", \"goodbye\")";
System.out.println("hello world" + replacer);

Upvotes: 3

Charles Duffy
Charles Duffy

Reputation: 295403

Variables store data, not code; unless you're going to do something evil, you can't use a variable for this purpose. Instead, just perform the indirection ahead-of-time:

case $VERBOSE in
  y) exec 2>&1 > >(tee /tmp/something.log)
  *) exec >/tmp/something.log 2>&1
esac

...will redirect stdout and stderr for all code below them. If you only want to make some code subject to that, set up a different pair of conditionally-redirected file descriptors:

case $VERBOSE in
  y) exec 3> >(tee /tmp/something.log)
  *) exec 3>/tmp/something.log
esac

...and redirect the commands that you want to be subject to those logging rules to that FD:

something_that_should_be_logged >&3 2>&3

Upvotes: 2

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