Commenting in a Bash script inside a multiline command

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   gt  gt    CSV    If I try and add a comment like   cat   MYSQLDUMP        Output MYSQLDUMP File   I get      not found   Is it possible to comment here

User · Answer

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   gt  gt    CSV

User · Answer

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

User · Answer

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

User · Answer

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        lt comment text gt       In particular    lt comment text gt  can be multi-line   lt comment text gt  is not parameter-expanded no subprocesses are spawned  so comments are efficient    There is one restriction on the  lt comment text gt   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        lt code gt         lt comment gt    lt code gt          lt code gt         lt multiline                     comment gt    lt code gt     utility routines that obviate  useless use of cat  function bashcat   printf   s n      lt  dev stdin      function scat   1 gt  amp 2 bashcat  exit 1       ensure that      is unset  amp  amp  remains unset    -z      x      amp  amp     if      is unset   declare -r           then ensure that      remains unset    scat  lt  lt EOF           else exit with an error Error  the parameter            is set  hence the   comment-idiom          lt comment text gt    will fail EOF         example of inline comments  ------------------------------------------------ the following inline comment-idiom is supported      lt code gt         lt comment gt    lt code gt        lt code gt         lt multiline                   comment gt    lt code gt    advisory  the parameter      must NOT be set    even the null declaration       will fail  advisory  protect unbalanced delimiters    and      advisory  NO parameter-expansion of  lt comment gt    advisory  NO subprocesses are spawned  advisory  a functionally equivalent idiom is       lt code gt      lt comment gt    lt code gt        lt code gt      lt multiline                comment gt    lt code gt  however each comment spawns a bash subprocess that inelegantly requires  1ms of computation  ------------------------------------------------

User · Answer

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     gt  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

User · Answer

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   gt  gt    CSV

User · Answer

In addition to the examples by DigitalRoss  here s another form that you can use if you prefer     instead of backticks    echo abc     comment         def     comment         xyz   Of course  you can use the colon syntax with backticks as well   echo abc    comment         def    comment         xyz     Additional Notes  The reason    comment  doesn t work is because once it sees the    it treats the rest of the line as comments  including the closing parentheses  comment   So the parentheses is never closed   Backticks parse differently and will detect the closing backtick even after a

[bash] Commenting in a Bash script inside a multiline command

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