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minishell parser

Parses shell commands and returns a linked list for execution of said commands.

Syntax rules include single quotes, double quotes, semicolons, environment variables, pipes, redirections (<, >, >>)

Syntax rules

Quotes

Single-quotes ( '' ): shall preserve the literal value of each character within the single-quotes. A single-quote cannot occur within single-quotes. If uneven number of single quotes, unvalid command. Even env variables stay as strings.

Double-quotes (“”):

  • the $ shall retain its special meaning introducing parameter expansion.
  • The \ shall retain its special meaning, only when followed by one of the following characters when considered special : $ “ \

Si plusieurs quotes (double ou simple) a la suite, concatener leur resultat. (ex : echo """salut" = echo "" + echo "salut")

Quotes can be anywhere in the command (in the exec, options or arguments)

Redirections

  • Redirections can be placed anywhere in the command. Then, it only takes the next argument as the file used for the redirection. The other arguments are arguments to the command
  • If there are several redirections in a row, all files are created (if forward redirection). However, the redirection goes only to the last file (ex : echo bonjour > file1 >file2 >> file3)
  • The argument to a redirection can be delimited by whitespaces, another redirection operator, pipes or semi-colons
  • If there is an input redirection AND a pipe, the input redirection wins
  • It's ok if there is no space between an argument and a redirection (echo salut>salut)

$ (env variables)

  • Variables can only be a set of alphanumeric characters and _ . If an env variable is followed by a character other than those, the expanded variable and whatever is after are concatenated
  • If the specified environment variable doesn't exist, it is ignored (not replaced by an empty string)

Built-ins and executables

  • They are not case-sensitive

Other

  • Spaces that are escaped (\ ) are not considered argument separators, but simply characters
  • Always print a space between two arguments, even if one of the arguments is an empty string

Possible errors

Quotes

  • Quotes are unclosed (either ' or ")

Semicolons (;)

  • Before any character other than a whitespace (beginning of the command)
  • Two or more in a row (even separated by whitespaces)

Pipes (|)

  • No pipe at the beginning or end of a command (even with whitespaces)
  • Two or more in a row (even separated by whitespaces)
  • No pipe right after a semicolon

Redirections (<, >, >>)

  • Input redirection from a file that doesn't exist