Sstrhsrtj

I often have code where I format by making a long AND/OR statements. For example:

  # Get wifi router gateway

  gateway=$(cat $leases 

            | grep -A 5 -m 1 $wifi 

            | grep option routers 

            | cut -d' ' -f5 

            | tr --delete ;)

Sometimes a single step in a command like above can be complex. Thus I end up wanting to comment it.

For example say the cut command was more complicated than it really is here. So I want to do something like:

  # Get wifi router gateway

  gateway=$(cat $leases 

            | grep -A 5 -m 1 $wifi 

            | grep option routers 

            # Here is a note

            | cut -d' ' -f5 

            | tr --delete ;)

I realize this is invalid syntax.

But I'm curious to see if anyone else has some strategies for commenting long command chains?

This seems to work in Bash, dash, etc:

#!/bin/sh

seq 20 |       # make a long list

grep '[234]'   # mut only take part of it

Similarly with && or || in place of the pipe, and also inside $( ... ).

The only thing that makes your command have invalid syntax is the fact that you needlessly escape the newlines on each line.

Your command,

gateway=$(cat $leases 

            | grep -A 5 -m 1 $wifi 

            | grep option routers 

            | cut -d' ' -f5 

            | tr --delete ;)

is identical to

gateway=$(cat $leases | grep -A 5 -m 1 $wifi | grep option routers  | cut -d' ' -f5 | tr --delete ;)

In other words, from the shell's point of view, it's not a multi-line command at all.

Inserting a comment into this before the cut makes the command substitution unterminated (the final ) is commended out):

gateway=$(cat $leases | grep -A 5 -m 1 $wifi | grep option routers  # Here is a note | cut -d' ' -f5 | tr --delete ;)

However, there is no need to escape the newlines. The following is totally valid code:

gateway=$(cat $leases

            | grep -A 5 -m 1 $wifi

            | grep option routers

            | cut -d' ' -f5

            | tr --delete ;)

Here, it's just fine to insert a comment:

gateway=$(cat $leases

            | grep -A 5 -m 1 $wifi

            | grep option routers

            # Here is a note

            | cut -d' ' -f5

            | tr --delete ;)

It's fine to do this because each part of a pipeline may be followed by one or several newlines, and a comment is ignored by the shell's parser.

Looking at the actual code, this is a safer and more portable variant (except for the initial grep that still requires using non-standard extensions):

gateway=$(grep -A 5 -m 1 -e "$wifi" <"$leases"

            | grep -F 'option routers'

            # Here is a note

            | cut -d ' ' -f 5

            | tr -d ';' )

Also, zsh, which can emulate bash (just call it as bash or run emulate bash), actually supports multi-line commands!

Just run bindkey -e or bindkey "^[^M" self-insert-unmeta, and then you can go to a new line by hitting M-Return.

It is saved as one history entry; you can move around with the arrow keys as you would expect. It will only switch entries when you go up/down at the top/bottom row.