Generating a bash script from a bash script

Question

I need to generate a script from within a script but am having problems because some of the commands going into the new script are being interpreted rather than written to the new file. For example i want to create a file called start.sh in it I want to set a variable to the current IP address:

echo "localip=$(ip addr | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/')" > /start.sh

what gets written to the file is:

localip=192.168.1.78

But what i wanted was the following text in the new file:

localip=$(ip addr | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/')"

so that the IP is determined when the generated script is run.

What am i doing wrong ?

Answer 1

You're making this unnecessary hard. Use a heredoc with a quoted sigil to pass literal contents through without any kind of expansion:

cat >/start.sh <<'EOF'
localip=$(ip addr | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/')
EOF

Using <<'EOF' or <<\EOF, as opposed to just <<EOF, is essential; the latter will perform expansion just as your original code does.

---

If anything you're writing to start.sh needs to be based on current variables, by the way, be sure to use printf %q to safely escape their contents. For instance, to set your current $1, $2, etc. to be active during start.sh execution:

# open start.sh for output on FD 3
exec 3>/start.sh

# build a shell-escaped version of your argument list
printf -v argv_str '%q ' "$@"

# add to the file we previously opened a command to set the current arguments to that list
printf 'set -- %s\n' "$argv_str" >&3

# pass another variable through safely, just to be sure we demonstrate how:
printf 'foo=%q\n' "$foo" >&3

# ...go ahead and add your other contents...
cat >&3 <<'EOF'
# ...put constant parts of start.sh here, which can use $1, $2, etc.
EOF

# close the file
exec 3>&-

This is far more efficient than using >>/start.sh on every line that needs to append: Using exec 3>file and then >&3 only opens the file once, rather than opening it once per command that generates output.