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Solution for busybox and non-bash shells

The accepted answer is certainly the best choice for bash. I'm working in a Busybox environment without access to bash, and it does not understand the exec > >(tee log.txt) syntax. It also does not do exec >$PIPE properly, trying to create an ordinary file with the same name as the named pipe, which fails and hangs.

Hopefully this would be useful to someone else who doesn't have bash.

Also, for anyone using a named pipe, it is safe to rm $PIPE, because that unlinks the pipe from the VFS, but the processes that use it still maintain a reference count on it until they are finished.

Note the use of $* is not necessarily safe.

#!/bin/sh if [ "$SELF_LOGGING" != "1" ] then # The parent process will enter this branch and set up logging # Create a named piped for logging the child's output PIPE=tmp.fifo mkfifo $PIPE # Launch the child process without redirected to the named pipe SELF_LOGGING=1 sh $0 $* >$PIPE & # Save PID of child process PID=$! # Launch tee in a separate process tee logfile <$PIPE & # Unlink $PIPE because the parent process no longer needs it rm $PIPE # Wait for child process running the rest of this script wait $PID # Return the error code from the child process exit $? fi # The rest of the script goes here 

Solution for busybox, macOS bash, and non-bash shells

The accepted answer is certainly the best choice for bash. I'm working in a Busybox environment without access to bash, and it does not understand the exec > >(tee log.txt) syntax. It also does not do exec >$PIPE properly, trying to create an ordinary file with the same name as the named pipe, which fails and hangs.

Hopefully this would be useful to someone else who doesn't have bash.

Also, for anyone using a named pipe, it is safe to rm $PIPE, because that unlinks the pipe from the VFS, but the processes that use it still maintain a reference count on it until they are finished.

Note the use of $* is not necessarily safe.

#!/bin/sh if [ "$SELF_LOGGING" != "1" ] then # The parent process will enter this branch and set up logging # Create a named piped for logging the child's output PIPE=tmp.fifo mkfifo $PIPE # Launch the child process with stdout redirected to the named pipe SELF_LOGGING=1 sh $0 $* >$PIPE & # Save PID of child process PID=$! # Launch tee in a separate process tee logfile <$PIPE & # Unlink $PIPE because the parent process no longer needs it rm $PIPE # Wait for child process, which is running the rest of this script wait $PID # Return the error code from the child process exit $? fi # The rest of the script goes here 

Solution for busybox and non-bash shells

The accepted answer is certainly the best choice for bash. I'm working in a Busybox environment without access to bash, and it does not understand the exec > >(tee log.txt) syntax. It also does not do exec >$PIPE properly, trying to create an ordinary file with the same name as the named pipe, which fails and hangs.

Hopefully this would be useful to someone else who doesn't have bash.

Also, for anyone using a named pipe, it is safe to rm $PIPE, because that unlinks the pipe from the VFS, but the processes that use it still maintain a reference count on it until they are finished.

Note the use of $* is not necessarily safe.

#!/bin/sh if [ "$SELF_LOGGING" != "1" ] then PIPE=tmp.fifo mkfifo $PIPE # Keep PID of this process SELF_LOGGING=1 sh $0 $* >$PIPE & PID=$! tee logfile <$PIPE & # Safe to rm pipe because the child processes have references to it rm $PIPE wait $PID # Return same error code as original process exit $? fi 

Solution for busybox and non-bash shells

The accepted answer is certainly the best choice for bash. I'm working in a Busybox environment without access to bash, and it does not understand the exec > >(tee log.txt) syntax. It also does not do exec >$PIPE properly, trying to create an ordinary file with the same name as the named pipe, which fails and hangs.

Hopefully this would be useful to someone else who doesn't have bash.

Also, for anyone using a named pipe, it is safe to rm $PIPE, because that unlinks the pipe from the VFS, but the processes that use it still maintain a reference count on it until they are finished.

Note the use of $* is not necessarily safe.

#!/bin/sh if [ "$SELF_LOGGING" != "1" ] then # The parent process will enter this branch and set up logging # Create a named piped for logging the child's output PIPE=tmp.fifo mkfifo $PIPE # Launch the child process without redirected to the named pipe SELF_LOGGING=1 sh $0 $* >$PIPE & # Save PID of child process PID=$!   # Launch tee in a separate process tee logfile <$PIPE & # Unlink $PIPE because the parent process no longer needs it rm $PIPE      # Wait for child process running the rest of this script wait $PID # Return the error code from the child process exit $? fi # The rest of the script goes here 

The accepted answer is certainly the best choice for bash. I'm working in a Busybox environment without access to bash, and it does not understand the exec > >(tee log.txt) syntax. It also does not do exec >$PIPE properly, trying to create an ordinary file with the same name as the named pipe, which fails and hangs.

Hopefully this would be useful to someone else who doesn't have bash.

Also, for anyone using a named pipe, it is safe to rm $PIPE, because that unlinks the pipe from the VFS, but the processes that use it still maintain a reference count on it until they are finished.

Note the use of $* is not necessarily safe.

#!/bin/sh if [ "$SELF_LOGGING" != "1" ] then PIPE=tmp.fifo mkfifo $PIPE # Keep PID of this process SELF_LOGGING=1 sh $0 $* >$PIPE & PID=$! tee logfile <$PIPE & # Safe to rm pipe because the child processes have references to it rm $PIPE wait $PID # Return same error code as original process exit $? fi 

Solution for busybox and non-bash shells

The accepted answer is certainly the best choice for bash. I'm working in a Busybox environment without access to bash, and it does not understand the exec > >(tee log.txt) syntax. It also does not do exec >$PIPE properly, trying to create an ordinary file with the same name as the named pipe, which fails and hangs.

Hopefully this would be useful to someone else who doesn't have bash.

Also, for anyone using a named pipe, it is safe to rm $PIPE, because that unlinks the pipe from the VFS, but the processes that use it still maintain a reference count on it until they are finished.

Note the use of $* is not necessarily safe.

#!/bin/sh if [ "$SELF_LOGGING" != "1" ] then PIPE=tmp.fifo mkfifo $PIPE # Keep PID of this process SELF_LOGGING=1 sh $0 $* >$PIPE & PID=$! tee logfile <$PIPE & # Safe to rm pipe because the child processes have references to it rm $PIPE wait $PID # Return same error code as original process exit $? fi