How to put if statement inside a lftp block

Question

I am writing a bash script to download files from ftp server using lftp. I wanted to delete the files based on the second input argument.

#!/bin/bash

cd $1

lftp -u found,e48RgK7s sftp://ftp.xxx.org << EOF
set xfer:clobber on
mget *.xml
if [ $2 = "prod"]; then
  echo "Production mode. Deleting files"
  mrm *.xml
else
  echo "Non-prod mode. Keeping files"
fi
EOF

However, if statement is not allowed in the lftp block before EOF.

Unknown command `if'.
Unknown command `then'.
Usage: rm [-r] [-f] files...
Unknown command `else'.

How do I embed if statement in such block?

Answer 1

A command substitution will do:

#!/bin/bash

cd "$1" || exit
mode=$2

lftp -u found,e48RgK7s sftp://ftp.xxx.org << EOF
set xfer:clobber on
mget *.xml
$(
    if [ "$mode" = "prod" ]; then
      echo "Production mode. Deleting." >&2 # this is logging (because of >&2)
      echo "mrm *.xml"                      # this is substituted into the heredoc
    else
      echo "Non-prod mode. Keeping files" >&2
    fi
)
EOF

Note that inside the substitution for the heredoc, we're routing log messages to stderr, not stdout. This is essential, because everything on stdout becomes a command substituted into the heredoc sent to lftp.

Other caveats to command substitution also apply: They run in subshells, so a assignment made inside the command substitution will not apply outside of it, and there's a performance cost to starting them.

---

A more efficient approach is to store your conditional components in a variable, and expand it inside the heredoc:

case $mode in
  prod)
    echo "Production mode. Deleting files" >&2
    post_get_command='mget *.xml'
    ;;
  *)
    echo "Non-production mode. Keeping files" >&2
    post_get_command=
    ;;
esac

lftp ... <<EOF
set xfer:clobber on
mget *.xml
$post_get_command
EOF