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124 lines
4.7 KiB
Markdown
124 lines
4.7 KiB
Markdown
# Print a random string or select random elements
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---- dataentry snipplet ---- snipplet_tags: terminal, line
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LastUpdate_dt: 2013-04-30 Contributors: Dan Douglas (ormaaj) type:
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snipplet
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------------------------------------------------------------------------
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First off, here is a fast / reliable random string function for scripts
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or libraries which can optionally assign directly to a variable.
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``` bash
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# Print or assign a random alphanumeric string of a given length.
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# rndstr len [ var ]
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function rndstr {
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if [[ $FUNCNAME == "${FUNCNAME[1]}" ]]; then
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unset -v a
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printf "$@"
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elif [[ $1 != +([[:digit:]]) ]]; then
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return 1
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elif (( $1 )); then
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typeset -a a=({a..z} {A..Z} {0..9})
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eval '${2:+"$FUNCNAME" -v} "${2:-printf}" -- %s "${a[RANDOM%'"${#a[@]}"']"{1..'"$1"'}"}"'
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fi
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}
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```
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This example prints 10 random positional parameters and operates on
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basically the same principle as the `rndstr` function above.
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~ $ ( set -- foo bar baz bork; printf '%s ' "${!_[_=RANDOM%$#+1,0]"{0..10}"}"; echo )
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bork bar baz baz foo baz baz baz baz baz bork
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<div hide> This has some interesting option parsing concepts, but is
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overly complex. This is a good example of working too hard to avoid an
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eval for no benefit and some performance penalty. :/
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``` bash
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# Print or assign a random alphanumeric string of a given length.
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# rndstr [ -v var ] len
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# Bash-only
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rndstr()
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if [[ $FUNCNAME == "${FUNCNAME[1]}" ]]; then
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# On recursion, this branch unsets the outer scope's locals and assigns the result.
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unset -v a b
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printf -v "$1" %s "${@:2}"
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elif ! { [[ $1 == -v ]] && shift; }; [[ $?+1 -ne $# || ${!#} != +([[:digit:]]) || ( $? -gt 0 && -z $1 ) ]]; then
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# This branch does input validation, strips -v, and guarantees we're left with either 1 or 2 args.
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return 1
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elif (( ! ${!#} )); then
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# If a zero-length string is requested, return success.
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return
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else
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# This line generates the string and assigns it to "b".
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local -a a=({a..z} {A..Z} {0..9}) 'b=("${a[RANDOM%'"${#a[@]}"']"{1..'"${!#}"'}"}")'
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if (( $# == 2 )); then
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# If -v, then pass a variable name and value to assign and recurse once.
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"$FUNCNAME" "$1" "${b[@]}"
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else
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# If no -v, write to stdout.
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printf %s "${b[@]}"
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fi
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fi
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```
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</div>
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The remaining examples don't use quite the same tricks, which will
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hopefully be explained elsewhere eventually. See
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[unset](../commands/builtin/unset.md#scope) for why doing assignments in this
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way works well.
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This next example is a variation on
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[print_horizontal_line](../snipplets/print_horizontal_line.md). We\'re using
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the printf field width specifier to truncate the values of a
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`sequence expansion` to one character.
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``` bash
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a=({a..z} {A..Z} {0..9})
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printf '%.1s' "${a[RANDOM%${#a[@]}]}"{0..9} $'\n'
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```
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The extra detail that makes this work is to notice that in Bash, [brace
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expansion](../syntax/expansion/brace.md) is usually the very first type of
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expansion to be processed, always before parameter expansion. Bash is
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unique in this respect -- all other shells with a brace expansion
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feature perform it almost last, just before pathname expansion. First
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the sequence expansion generates ten parameters, then the parameters are
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expanded left-to-right causing the [arithmetic](../syntax/arith_expr.md) for
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each to be evaluated individually, resulting in independent selection of
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random element of `a`. To get ten of the same element, put the array
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selection inside the format string where it will only be evaluated once,
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just like the dashed-line trick:
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``` bash
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printf "%.s${a[RANDOM%${#a[@]}]}" {0..9}
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```
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Selecting random elements whose lengths are not fixed is harder.
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``` bash
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a=(one two three four five six seven eight nine ten)
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printf '%.*s ' $(printf '%s ' "${#a[x=RANDOM%${#a[@]}]} ${a[x]}"{1..10})
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```
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This generates each parameter and it's length in pairs. The \'\*\'
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modifier instructs printf to use the value preceding each parameter as
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the field width. Note the space between the parameters. This example
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unfortunately relies upon the unquoted command substitution to perform
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unsafe wordsplitting so that the outer printf gets each argument. Values
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in the array can't contain characters in IFS, or anything that might be
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interpreted as a pattern without using `set -f`.
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Lastly, empty brace expansions can be used which don't generate any
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output that would need to be filtered. The disadvantage of course is
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that you must construct the brace expansion syntax to add up to the
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number of arguments to be generated, where the most optimal solution is
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its set of prime factors.
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``` bash
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a=(one two three)
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echo "${a[RANDOM%${#a[@]}]}"{,}{,,,,}
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```
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