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Bash: quoting and expansions

The order of shell expansions, the three kinds of quoting, word splitting, the ${v:-d}, ${v%m} and ${v##m} forms and the here-document: predicting the arguments a command receives.

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  • Expansion: what is it?

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    A transformation the shell applies to the command line before starting the program: braces, tilde, variables, commands, arithmetic, word splitting, wildcards.

    The program never sees * or $HOME: it receives the result. It is not ls that replaces *.txt, it is the shell, before calling ls.

  • What does this script print?

    ```bash
    f="mon rapport.txt"
    set -- $f "$f"
    echo $#
    `

    • 3
    • 2
    • 1
    • 4
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    3

    $f unquoted is split into two words (mon and rapport.txt), "$f" makes just one: 2 + 1 = 3 arguments.

  • Among the expansions, the shell handles the … first, then the tilde and variables, next …, and finally the wildcards.

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    Among the expansions, the shell handles the braces first, then the tilde and variables, next word splitting, and finally the wildcards.

    echo {a,b}{1,2} gives a1 a2 b1 b2: braces are expanded before everything else, even before variables. Wildcards come last, on the words already split.

  • Quoting: what is it?

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    A mark that strips a character of its special meaning for the shell: single quotes, double quotes or a backslash.

    Single quotes prevent every expansion; double quotes let $ and \ through; the backslash protects only the next character.

  • A cleanup script runs rm $fichier with fichier="mon fichier" and prints rm: cannot remove 'mon': No such file or directory then rm: cannot remove 'fichier': No such file or directory. Which fix is right?

    • rm '$fichier'
    • rm -f $fichier
    • rm "$fichier"
    • rm ${fichier}
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    rm "$fichier"

    Double quotes keep the variable's content as a single argument. Single quotes would prevent expansion (rm would receive the text $fichier), -f only silences the errors, and braces protect nothing from splitting.

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