Commenting in a Bash script

Question

How can I comment on each line of the following lines from a script?

   cat ${MYSQLDUMP} | \
   sed '1d' | \
   tr ",;" "\n" | \
   sed -e 's/[asbi]:[0-9]*[:]*//g' -e '/^[{}]/d' -e 's/""//g' -e '/^"{/d' | \
   sed -n -e '/^"/p' -e '/^print_value$/,/^option_id$/p' | \
   sed -e '/^option_id/d' -e '/^print_value/d' -e 's/^"\(.*\)"$/\1/' | \
   tr "\n" "," | \
   sed -e 's/,\([0-9]*-[0-9]*-[0-9]*\)/\n\1/g' -e 's/,$//' | \
   sed -e 's/^/"/g' -e 's/$/"/g' -e 's/,/","/g' >> ${CSV}

If I try and add a comment say "cat ${MYSQLDUMP} | \ #Output MYSQLDUMP File", I get:

Delete: not found

Is it possible to comment here or not because of "| \"?

Answer 1

As DigitalRoss pointed out, the trailing backslash is not necessary when the line woud end in |. And you can put comments on a line following a |:

 cat ${MYSQLDUMP} |         # Output MYSQLDUMP file
 sed '1d' |                 # skip the top line
 tr ",;" "\n" | 
 sed -e 's/[asbi]:[0-9]*[:]*//g' -e '/^[{}]/d' -e 's/""//g' -e '/^"{/d' |
 sed -n -e '/^"/p' -e '/^print_value$/,/^option_id$/p' |
 sed -e '/^option_id/d' -e '/^print_value/d' -e 's/^"\(.*\)"$/\1/' |
 tr "\n" "," |
 sed -e 's/,\([0-9]*-[0-9]*-[0-9]*\)/\n\1/g' -e 's/,$//' |   # hate phone numbers
 sed -e 's/^/"/g' -e 's/$/"/g' -e 's/,/","/g' >> ${CSV}

Answer 2

The backslash escapes the #, interpreting it as its literal character instead of a comment character.

Answer 3

Here is a bash script that combines the ideas and idioms of several previous comments to provide, with examples, inline comments having the general form ${__+ <comment text>}.

In particular

• <comment text> can be multi-line
• <comment text> is not parameter-expanded
• no subprocesses are spawned (so comments are efficient)

There is one restriction on the <comment text>, namely, unbalanced braces '}' and parentheses ')' must be protected (i.e., '\}' and '\)').

There is one requirement on the local bash environment:

• the parameter name __ must be unset

Any other syntactically valid bash parameter-name will serve in place of __, provided that the name has no set value.

An example script follows

# provide bash inline comments having the form
#     <code> ${__+ <comment>} <code> 
#     <code> ${__+ <multiline
#                   comment>} <code>

# utility routines that obviate "useless use of cat"
function bashcat { printf '%s\n' "$(</dev/stdin)"; }
function scat { 1>&2 bashcat; exit 1; }

# ensure that '__' is unset && remains unset
[[ -z ${__+x} ]] &&  # if '__' is unset
  declare -r __ ||   # then ensure that '__' remains unset 
  scat <<EOF         # else exit with an error
Error: the parameter __='${__}' is set, hence the
  comment-idiom '\${__+ <comment text>}' will fail
EOF

${__+ (example of inline comments)
------------------------------------------------
the following inline comment-idiom is supported
    <code> ${__+ <comment>} <code> 
    <code> ${__+ <multiline
                  comment>} <code> 
(advisory) the parameter '__' must NOT be set;
  even the null declaration __='' will fail
(advisory) protect unbalanced delimiters \} and \) 
(advisory) NO parameter-expansion of <comment> 
(advisory) NO subprocesses are spawned
(advisory) a functionally equivalent idiom is 
    <code> `# <comment>` <code> 
    <code> `# <multiline
               comment>` <code>
however each comment spawns a bash subprocess
that inelegantly requires ~1ms of computation 
------------------------------------------------}

Answer 4

$IFS comment hacks

This hack uses parameter expansion on $IFS, which is used to separate words in commands:

$ echo foo${IFS}bar
foo bar

Similarly:

$ echo foo${IFS#comment}bar
foo bar

Using this, you can put a comment on a command line with contination:

$ echo foo${IFS# Comment here} \
> bar
foo bar

but the comment will need to be before the \ continuation.

Note that parameter expansion is performed inside the comment:

$ ls file
ls: cannot access 'file': No such file or directory
$ echo foo${IFS# This command will create file: $(touch file)}bar
foo bar
$ ls file
file

Rare exception

The only rare case this fails is if $IFS previously started with the exact text which is removed via the expansion (ie, after the # character):

$ IFS=x
$ echo foo${IFS#y}bar
foo bar
$ echo foo${IFS#x}bar
foobar

Note the final foobar has no space, illustrating the issue.

Since $IFS contains only whitespace by default, it's extremely unlikely you'll run into this problem.

---

Credit to @pjh's comment which sparked off this answer.

Answer 5

The trailing backslash must be the last character on the line for it to be interpreted as a continuation command. No comments or even whitespace are allowed after it.

You should be able to put comment lines in between your commands

# output MYSQLDUMP file
cat ${MYSQLDUMP} | \
# simplify the line
sed '/created_at/d' | \
# create some newlines
tr ",;" "\n" | \
# use some sed magic
sed -e 's/[asbi]:[0-9]*[:]*//g' -e '/^[{}]/d' -e 's/""//g' -e '/^"{/d' | \
# more magic
sed -n -e '/^"/p' -e '/^print_value$/,/^option_id$/p' | \
# even more magic
sed -e '/^option_id/d' -e '/^print_value/d' -e 's/^"\(.*\)"$/\1/' | \
tr "\n" "," | \
# I hate phone numbers in my output
sed -e 's/,\([0-9]*-[0-9]*-[0-9]*\)/\n\1/g' -e 's/,$//' | \ 
# one more sed call and then send it to the CSV file
sed -e 's/^/"/g' -e 's/$/"/g' -e 's/,/","/g' >> ${CSV}

Answer 6

This will have some overhead, but technically it does answer your question:

echo abc `#Put your comment here` \
     def `#Another chance for a comment` \
     xyz, etc.

And for pipelines specifically, there is a clean solution with no overhead:

echo abc |        # Normal comment OK here
     tr a-z A-Z | # Another normal comment OK here
     sort |       # The pipelines are automatically continued
     uniq         # Final comment

See Stack Overflow question How to Put Line Comment for a Multi-line Command.