Lecture 10 Shells

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Lecture 10Shells
CSCE 510 Systems Programming

• Topics
• Shell operations
• Shell implementation
• Redirecting I/O

September 15, 2005
2

• Last Time
• Exec Family examples
• execExamples.c // (in /class/csce510-001/web/Exam
  ples)
• Shells
• sh, csh, bash
• Csh features
• Shell variables
• Alias substitutions
• History !n, !!, !str, !!strn, substitution
• History array of pts to link lists
• Filename expansion e.g., ls .c
• Today
• Filename expansion implementation re_comp,
  re_exec
• Program 2

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cshell substitutions

• Read in the command into linked list or words

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Regular expressions

• Shells bash and csh dont really do real regular
  expressions. Why not?
• Shell patterns
• - matches any string e.g. ls a.c
• . matches any single character e.g. ls \.c
• str matches single character from str e.g.
  ls abc\.c
• ? makes the previous pattern optional
• Vi and grep use real regular expressions
• means Kleene closure (CSCE 355) ab means
• Functions for real regular Expressions
• Regcomp (build NFA/DFA), regexec (Match using the
  FA)
• man s 7 regexec

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Filename Expansion Implementation

• int match(const char string, char pattern)
• int status
• regex_t re
• if (regcomp(re, pattern, REG_EXTENDEDREG_NOSUB)
  ! 0)
• return(0) / report error /
• status regexec(re, string, (size_t) 0, NULL,
  0)
• regfree(re)
• if (status ! 0) return(0) / report error /
  
• return(1)

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Regex regular expressions

• several branches match one a bb
  c
• concatenation match all in sequence abc
• atom possibly followed by a single
• ?
• A bound e.g. x3,5 meaning from 3 to 5 xs
• A bracket expression abxyz meaning 0-7
  meaning
• Parentheses for grouping

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cshell command substitution

• cmd is replaced by the output of the command
• We will describe how to do this when we talk
  about the

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cshell filename completion

• TAB indicates try to extend (or complete) this
  filename
• aresgt gcc execEltTABgt // extends to gcc
  execExamples
• aresgt gcc execEltTABgt // Shows you the choices

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cshell command completion

• Same but for first word and the interpretation is
  different
• aresgt gcltTABgtltTABgt
• gcc gccbug
  gconf-merge-tree gcov
• gcc-3.3.6 gccmakedep gconftool-2
• aresgt gc

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Program 2
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(No Transcript)
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Shell Implementation

• write prompt
• read command line
• parse into linked list of words
• perform substitutions
• remap Standard IO
• build argv
• fork/exec