What does `exec 6>&1` or similar do?

Question

I'm migrating some software from Unix to Linux.

I have the following script; it is a trigger of a file transfer.

What do the exec commands do?

Will they work also on Linux?

#!/bin/bash
flog=/mypath/log/mylog_$8.log
pid=$$
flog_otherlog=/mypath/log/started_script_${8}_${pid}.log

exec 6>&1
exec 7>&2
exec >> $flog
exec 2>&1


exec 1>&6 
exec 2>&7

/usr/local/bin/sudo su - auser -c "/mypath/bin/started_script.sh $1 $pid $flog_otherlog $8" 

The started script is the following:

#!/bin/bash
flusso=$1
pidpadre=$2
flogcurr=$3
corrid=$4
pid=$$

exec >> $flogcurr
exec 2>&1

if  [ $1 = pippo ] || [ $1 = pluto ] || [ $1 = paperino ]
    then
        fullfile=${myetlittin}/$flusso
        filename="${flusso%.*}"
        datafile=$(ls -le $fullfile  | awk '{print $6, " ", $7, " ", $9, " ", $8 }')
        dimfile=$(ls -le $fullfile  | awk '{print $5 " " }')
        aaaammgg=$(ls -E $fullfile  | awk '{print $6}'| sed 's#-##g')
        aaaamm=$(echo $aaaammgg | cut -c1-6)
        dest_dir=${myetlwarehouse}/mypath/${aaaamm}
        dest_name=${dest_dir}/${filename}_${aaaammgg}.CSV
        mkdir -p $dest_dir
        cp $fullfile $dest_name
        rc_copia=$?
fi

I will change ls -le into ls -l --time-style="+%b %d %T %Y" and ls -E into ls -l --time-style=full-isoand in Linux.

Answer 1

exec [n]<&word will duplicate an input file descriptor in bash.

exec [n]>&word will duplicate an output file descriptor in bash.

See 3.6.8 in: https://www.gnu.org/software/bash/manual/html_node/Redirections.html

The order of arguments can be confusing, though.

In your script:

• exec 6>&1 creates a copy of file descriptor 1, i.e. STDOUT, and stores it as file descriptor 6.

• exec 1>&6 copies 6 back unto 1.

  It could also have been moved by appending a dash, i.e. 1<&6- closing descriptor 6 and leaving only 1.

In between, you'll usually find operations that write to STDOUT and STDIN, e.g. in a subshell.

Also see: Practical use for moving file descriptors

Answer 2

exec {number x}>&{number y} copies file descriptor X into Y.

File descriptors usage:

• 0 = stdin
• 1 = stdout
• 2 = stderr
• 3-9 = additional file descriptors

In your case, they should have been opened somewhere earlier with something like exec 3<> /tmp/some_file to set fd3 to to some file.

Usually, you can do exec 2>&1 in order to redirect stderr output into stdout.

Your bash example is not complete, as $8 referrs to the eight argument given to your script, so there's definitly something we're missing here like arguments 2-7...