mirror of
https://https.git.savannah.gnu.org/git/bash.git
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Chet Ramey
137 lines
3.7 KiB
Plaintext
137 lines
3.7 KiB
Plaintext
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This is a description of the changes made to bash for increased ksh
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compatibility.
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1. $SECONDS
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"Each time this parameter is referenced, the number of seconds
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since shell invocation is returned. If this parameter is assigned
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a value, then the value returned will be the value that was
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assigned plus the number of seconds since the assignment."
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Files changed: variables.c
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variables.h
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subst.c
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general.c
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shell.c
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general.h
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2. $TMOUT
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"If set to a value greater than 0, the shell will terminate if a
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command is not entered within the prescribed number of seconds
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after issuing the PS1 prompt."
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Files changed: shell.c (the implementation is not perfect)
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3. $RANDOM
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"Each time this parameter is referenced, a random integer is
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generated. The sequence of random numbers can be initialized
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by assigning a numeric value to RANDOM."
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Files changed: subst.c
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variables.c
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4. $REPLY
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"This parameter is set by the `read' special command when no
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arguments are supplied."
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Files changed: builtins.c
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5. integer variables
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`declare -i' (also export) makes a variable an integer (turns on
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the integer attribute). When assignment is made to a variable with
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the -i attribute, arithmetic expression evaluation is done on the
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value before it is assigned to the variable.
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Files changed: variables.h
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variables.c
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builtins.c
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6. Arithmetic expression evaluation.
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Here is the comment at the beginning of the new file `expr.c':
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ksh-style expression evaluation.
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All arithmetic is done as long integers with no checking for overflow
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(though division by 0 is caught and flagged as an error).
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The following operators are handled, grouped into a set of levels in
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order of decreasing precedence.
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"-" [level 0 (unary negation)]
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"!" [level 1]
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"*", "/", "%" [level 2]
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"+", "-" [level 3]
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"<=", ">=", "<", ">" [level 4]
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"==", "!=" [level 5]
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"=" [level 6 (assignment)]
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(Note that most of these operators have special meaning to bash, and an
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entire expression should be quoted, e.g. "a=$a+1" or "a=a+1" to ensure
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that it is passed intact to the evaluator).
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Sub-expressions within parentheses have a precedence level greater than
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all of the above levels and are evaluated first. Within a single prece-
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dence group, evaluation is left-to-right, except for the arithmetic
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assignment operator (`='), which is evaluated right-to-left (as in C).
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The expression evaluator returns the value of the expression (assignment
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statements have as a value what is returned by the RHS). The `let'
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builtin, on the other hand, returns 0 if the last expression evaluates to
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a non-zero, and 1 otherwise.
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Implementation is a recursive-descent parser.
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Files added: expr.c
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7. `let' builtin
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Parameters may be assigned numeric values via the `let' builtin.
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Each of its arguments is an expression to be evaluated. `let'
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returns 0 if the value of the last expression is non-zero, and
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1 otherwise.
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Note that the "((...))" form of this command has not yet been
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implemented; it requires changes to the parsing functions.
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Files changed: builtins.c
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8. $_
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$_ is set to the last argument of the previous command line, after
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expansion. It is still used as before when checking for mail.
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Two new keybindings have been added to insert this into the current
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command line (M-_ and M-.).
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Files changed: mailcheck.c
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execute_cmd.c
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bashline.c
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9. `cd -'
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Equivalent to 'cd $OLDPWD'
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Files changed: builtins.c
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10. "ulimit -a"
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"List all of the current resource limits (BSD only)."
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Files changed: builtins.c
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11. ${#@} and ${#*}
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These expand to the number of positional parameters.
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Files changed: subst.c
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Chet Ramey
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Information Network Services, Case Western Reserve University
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chet@ins.CWRU.Edu
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