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Playing around today, I was trying to get grep to consume the output of...

$(apt-cache rdepends hunspell-fr | sed -e 's/^\s*|\?//' -e 1s/^/\'/g -e \$s/$/\'/)

...as a newline delimited list of patterns nested within single quotes. When I run the above command and then simply copy/paste the resulting output into a new grep command, it behaves as expected...

nohatsatthetable@debian:~$ dpkg -l | grep 'hunspell-fr
Reverse Depends:
hunspell-fr-classical
thunderbird-l10n-fr
firefox-esr-l10n-fr
thunderbird-l10n-fr
task-french-desktop
hunspell-fr-revised
hunspell-fr-revised
hunspell-fr-comprehensive
hunspell-fr-comprehensive
hunspell-fr-classical
firefox-esr-l10n-fr'

ii  firefox-esr-l10n-fr                    102.4.0esr-1~deb11u1             all          French language package for Firefox ESR
ii  hunspell-fr                            1:7.0-1                          all          French dictionary for hunspell (dependency package)
ii  hunspell-fr-classical                  1:7.0-1                          all          French dictionary for hunspell (classical version)
ii  task-french-desktop                    3.68+deb11u1                     all          French desktop

However when I run the following one-liner, which to my eye should be functionally the same, grep only interprets the first line as a pattern, interpreting all subsequent lines (or words in the case of the second line) as the name of a directory or file to search within for the pattern...

nohatsatthetable@debian:~$ dpkg -l | grep $(apt-cache rdepends hunspell-fr | sed -e 's/^\s*|\?//' -e 1s/^/\'/g -e \$s/$/\'/)
grep: Reverse: No such file or directory
grep: Depends:: No such file or directory
grep: hunspell-fr-classical: No such file or directory
grep: thunderbird-l10n-fr: No such file or directory
grep: firefox-esr-l10n-fr: No such file or directory
grep: thunderbird-l10n-fr: No such file or directory
grep: task-french-desktop: No such file or directory
grep: hunspell-fr-revised: No such file or directory
grep: hunspell-fr-revised: No such file or directory
grep: hunspell-fr-comprehensive: No such file or directory
grep: hunspell-fr-comprehensive: No such file or directory
grep: hunspell-fr-classical: No such file or directory
grep: firefox-esr-l10n-fr': No such file or directory

Why does it behave in this way?

nohatsatthetable
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  • It seems as if you have forgotten to quote the `$(...)` expansion. This is possibly a duplicate of [When is double-quoting necessary?](https://unix.stackexchange.com/q/68694) – Kusalananda Nov 15 '22 at 17:10
  • @Kusalananda it would seem that you are right, I was labouring under the misaprehension that quoting the BASH command expansion would result in the `$()` being interpreted literally by the shell. This poses an even stranger question though; when filtered through `sed`, the output of `apt-cache` is encapsulated by single quotes, so what is the difference between that, and double quoting the command expansion (apart from the latter being infinitely more appropriate for the task at hand). Clearly there is a difference as one works and one doesn't, but I can't see what that difference is... – nohatsatthetable Nov 15 '22 at 17:59
  • @nohatsatthetable please read the post Kusalananda linked to. This has nothing to do with `sed` and everything to do with how the shell is expanding the `$()`. – terdon Nov 15 '22 at 19:17

1 Answers1

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Yes, as mentioned by @Kusalananda , the quotes are mandatory:

$ dpkg -l |
    grep "$(apt-cache rdepends hunspell-fr |
        sed -e 's/^\s*|\?//' -e 1s/^/\'/g -e \$s/$/\'/
 )"
Gilles Quénot
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