Redirect stdout and/or stderr to path in variable

Question

How do I redirect stdout and/or stderr to a path I have specified in a variable? Note: I don't want to overwrite the variable itself, I want to make std[xxx] write to the file specified in the variable.

For example- a simple script that, if the scp command fails, instead of printing the failure message to stderr or stdout (I'm not sure to which it outputs when fail), it outputs it to a specified log file. The path to the log file is stored in the $LOG variable:

LOG=/path/to/file.log
scp file1 host@remote

# do 'whatever' if scp command succeeds:
if [ $? = 0 ];
then
    whatever
else
    # else log both stdout/stderr to ${LOG} file
    &>"${LOG}"
    # DEBUG - print contents of ${LOG} var for testing purposes
    printf "${LOG}"

The result of this script doesn't show anything in the /path/to/file.log file and simple prints /path/to/file.log to stdout. So it's as if nothing was written from &>.

I've already confirmed my particular scp command works, so I know that's not a potential issue.

Or is this even the most proper way to handle custom log files? Is there a better practice for configuring your own logging system than storing paths to log files in variables?

Answer 1

It looks like you were eventually satisfied with the outcome of this question but I propose something different.

#!/bin/bash
LOG=/path/to/file.log    
DEBUG=0 # 0 = True, 1 = False
OUTPUT=$(scp file1 host@remote 2>&1)

# do 'whatever' if scp command succeeds:
if [ $? -eq 0 ];
then
    echo "Success"
elif [[ DEBUG -eq 0 ]]
    # else log both stdout/stderr to ${LOG} file and user
    # DEBUG - Use tee to display ${LOG} contents efficiently
    printf "$OUTPUT" | tee $LOG
else
    # Send output to $LOG
    printf "$OUTPUT" >> $LOG
fi

Essentially capture STDIN/STDOUT in a variable no matter what, then if success do 'whatever' but if failure redirect STDIN/STDOUT to $LOG. Additionally with the $DEBUG flag you can tee the contents of $OUTPUT to the display and $LOG simultaneously.

Also for integer comparison you really should use -eq instead of = or ==

Answer 2

It looks like you're trying to log the output after you run the command, which isn't possible.

If you want to log the output of the scp command unconditionally, then you just have to include the redirection operator on the same line as the command itself, i.e.:

&>"${LOG}" scp file1 host@remote

If you want to only save the log output if the command fails (as it appears you're trying to do in your code), then how about redirecting the output to a temporary file and then moving the file to the desired location afterwards? It might look something like this:

#!/bin/bash

# Set path the real log file location
LOG=/path/to/file.log

# Create a temporary file to capture standard output and standard error
TEMPLOG="$(mktemp)"

# Run command and redirect output to temporary logfile
2>"${TEMPLOG}" scp file1 host@remote

# do 'whatever' if scp command succeeds:
if [ $? = 0 ];
then
    echo 'Command successful!'

# else log both stdout/stderr to ${LOG} file
else
    # Move the log file from the temporary location to the desired location
    mv "${TEMPLOG}" "${LOG}"

    # DEBUG - print contents of ${LOG} var for testing purposes
    printf "${LOG}"
fi