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10.2.9 Quoting Strings to Pass to the Shell

Michael Brennan offers the following programming pattern, which he uses frequently:

#! /bin/sh


input_program | awk "$awkp" | /bin/sh

For example, a program of his named flac-edit has this form:

$ flac-edit -song="Whoope! That's Great" file.flac

It generates the following output, which is to be piped to the shell (/bin/sh):

chmod +w file.flac
metaflac --remove-tag=TITLE file.flac
LANG=en_US.88591 metaflac --set-tag=TITLE='Whoope! That'"'"'s Great' file.flac
chmod -w file.flac

Note the need for shell quoting. The function shell_quote() does it. SINGLE is the one-character string "'" and QSINGLE is the three-character string "\"'\"":

# shell_quote --- quote an argument for passing to the shell

function shell_quote(s,             # parameter
    SINGLE, QSINGLE, i, X, n, ret)  # locals
    if (s == "")
        return "\"\""

    SINGLE = "\x27"  # single quote
    QSINGLE = "\"\x27\""
    n = split(s, X, SINGLE)

    ret = SINGLE X[1] SINGLE
    for (i = 2; i <= n; i++)
        ret = ret QSINGLE SINGLE X[i] SINGLE

    return ret