1 | ---
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2 | default_highlighter: oils-sh
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3 | in_progress: true
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4 | ---
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5 |
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6 | Strings: Quotes, Interpolation, Escaping, and Buffers
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7 | =====================================================
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8 |
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9 | Strings are the most important data structure in shell. YSH makes them easier
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10 | and safer!
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11 |
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12 | This doc addresses these questions:
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13 |
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14 | - How do you write different kinds of strings in YSH programs?
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15 | - How do they behave at runtime? What are the common operations?
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16 | - What are the recommended ways to use them?
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17 |
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18 | Shell Features:
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19 |
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20 | - Quotes (single or double quoted)
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21 | - Interpolation aka substitution (variable, command, etc.)
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22 |
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23 | YSH Features:
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24 |
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25 | - Escaping for safety: `${x|html}`, etc.
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26 | - Buffers for efficiency and readability: `${.myproc arg1}`, etc.
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27 | - (buffers are PHP-like)
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28 |
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29 | <div id="toc">
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30 | </div>
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31 |
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32 | ## Summary
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33 |
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34 | - YSH has three ways to write strings: single quoted, double quoted, and
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35 | C-style (which is also QSN-style).
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36 | - Each of the three types has a multiline variant. They are Python-style
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37 | triple-quoted, but they also strip leading space in an intelligent way.
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38 | - TODO: Tagged strings, like `"<h2>$x</h2>"html`
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39 | - TODO: For string safety, YSH adds the concept of "escapers" and interpolation
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40 | with `$[x]` (square brackets)
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41 | - TODO: For convenience and performance, YSH adds buffers and *builtin
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42 | substitution*: `${.myproc arg1}`.
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43 |
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44 | ### For Python/JS/C Users
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45 |
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46 | - Single and double quotes are different. Double quotes allow interpolation.
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47 | - Neither style of string respects backslash escapes like `\n` for newline.
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48 | You have to use the third form.
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49 |
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50 | ### For Shell Users
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51 |
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52 | - YSH replaces here docs with Python-style triple-quoted strings.
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53 |
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54 | Preferences:
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55 |
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56 | - Unquoted strings (command mode only)
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57 | - Single-quoted strings
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58 | - when you need to express special characters
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59 | - QSN
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60 | - Double-quoted strings
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61 | - with `$[]` interpolation
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62 | - with `${}` interpolation
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63 | - with fast command sub `${.myproc arg1}
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64 |
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65 | ### Quick Reference
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66 |
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67 | echo unquoted # bare words are allowed in command mode
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68 |
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69 | echo 'with spaces' # single quoted string
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70 | var s = 'with spaces'
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71 |
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72 | # Raw single quoted string, to emphasize literal backslashes
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73 | var s = r'C:\Program Files\'
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74 |
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75 | # C-escaped single quoted string
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76 | var line = $'foo\n'
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77 |
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78 | # double quoted with safe interpolation (TODO)
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79 | echo "<p>hello $[name]</p>" # default_escaper must be set
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80 | echo "<p>hello ${name|html}</p>" # explicit escaper
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81 |
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82 | # double quoted with unsafe interpolation
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83 | echo "hello $name"
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84 | echo "hello ${name}_suffix" # braces delimit variable name
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85 |
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86 | echo $(date +%x) # command sub
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87 |
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88 | Still TODO:
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89 |
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90 | echo ${.myproc arg1}
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91 |
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92 | cat <<< '''
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93 | one
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94 | two
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95 | '''
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96 |
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97 | cat <<< $'''
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98 | mu = \u{3bc}
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99 | nul = \x00
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100 | '''
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101 |
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102 | var s = """
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103 | multiline with ${vars}
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104 | $(date +%x)
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105 | ${.myproc arg1}
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106 | """
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107 |
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108 |
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109 | ## Use Unquoted Strings in Command Mode
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110 |
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111 | Shell is unique! You don't have to quote strings.
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112 |
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113 | - link: command vs. expression mode
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114 |
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115 | and quoted strings in expression mode
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116 |
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117 | ## Two Kinds of Single-Quoted Strings
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118 |
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119 | ### Raw with `r'C:\Program Files\'`
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120 |
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121 | - TODO: `parse_raw_strings`
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122 |
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123 | ### C-Escaped With `$'foo\n'`
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124 |
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125 | - Use the [QSN]($xref) subset
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126 |
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127 | ### QSN For *Data* Interchange
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128 |
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129 | TODO: explain the difference.
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130 |
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131 | This is different! It's data and not code. Analogy to JSON.
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132 |
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133 | - When you want represent any string on a single line (filenames)
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134 | - To make binary data readable
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135 | - To display data in a terminal (protect against terminal codes)
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136 |
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137 | ## Use Double-Quoted Strings For Interpolation
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138 |
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139 | ### Implicit Safe Interpolation with `$[x]` (TODO)
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140 |
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141 | - Use `$[x]` for safe interpolation
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142 | - Respects `shopt --set default_escaper`
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143 |
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144 | ### Explicit Safe Interpolation With `${x|html}` (TODO)
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145 |
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146 | - Use `${x|html}` for safe interpolation
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147 |
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148 | Note you can have bugs if you use the wrong escaper!
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149 |
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150 | ### Raw Interpolation with `$x` (may be unsafe)
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151 |
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152 | - Use `$x` or `${x}`
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153 | - These are identical except for syntax
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154 | - Useful for log messages, which aren't security sensitive
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155 |
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156 | Note that you should **not** use `"${var}"` in YSH code. Use `$var` or
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157 | `${var}` because of simple word evaluation.
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158 |
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159 | ### Command Sub `$(echo hi)`
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160 |
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161 | ### Fast Command Sub `${.myproc}` (stdout capture)
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162 |
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163 | Note that only words are allowed here; not full commands. Wrap other commands
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164 | in a proc.
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165 |
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166 | - Using `write_to_buffer`
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167 |
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168 | TODO:
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169 |
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170 | echo ${.myproc foo|html} # I think this should be supported
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171 |
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172 | ## Escapers / Codecs (TODO)
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173 |
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174 | For `${x|html}` and `${.myproc|html}`
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175 |
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176 | TODO
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177 |
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178 | - how to register them
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179 | - wasm plugins?
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180 |
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181 | ## Use Triple Quoted Strings Instead of Here Docs (TODO)
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182 |
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183 | TODO
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184 |
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185 | ## Concatenate With `"$str1$str2"`
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186 |
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187 | Or `"${str1}${str2}"`
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188 |
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189 | - is `s ++ t` valid?. It isn't necessary for strings and lists
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190 | - `:| @a @b |` is the same for lists
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191 | - does this Python syntax also work? `[*a, *b]`
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192 | - Dicts: `{d, **e}` might be better
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193 |
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194 | ### Avoid Concatenation in a Loop
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195 |
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196 | setvar s = "${s}${suffix}"
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197 |
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198 | ## Append with Two Styles
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199 |
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200 | Since there is no `++` operator, there is no `++=` operator.
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201 |
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202 | ### `echo`, `printf`, `write`, and `${.myproc}` (`write_to_buffer`)
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203 |
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204 | echo, printf, and write have their output captured.
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205 |
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206 | proc p(arg) {
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207 | ### A proc that has its output captured quickly.
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208 |
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209 | echo $arg
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210 | write two
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211 |
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212 | const x = 'three'
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213 | printf '%s\n' $x
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214 |
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215 | # newline for interactive testing, but not when captured
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216 | if ! shopt -q write_to_buffer {
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217 | echo
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218 | }
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219 | }
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220 |
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221 | echo ${.p one} # $'one\ntwo\nthree\n'
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222 |
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223 | ### `append` and `join`
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224 |
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225 | var buf = :| |
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226 | append 'one ' (buf)
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227 | append $'two\n' (buf)
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228 | echo $[join(buf)]
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229 |
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230 | ## Appendix A: Deprecated Shell Constructs
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231 |
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232 | - here docs!
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233 | - Use tripled quoted strings.
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234 | - Backticks for command sub
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235 | - Use `$(echo hi)`
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236 | - Arithmetic substitution like `$((1 + 2))`
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237 | - Use YSH expressions: `$[1 + 2]`
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238 | - `${x%%prefix}` and so forth
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239 | - Use builtin YSH functions (TODO)
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240 | - Unused: bash `$""` for localization?
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241 |
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242 | ## Appendix B: Related Documents
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243 |
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244 | - Simple Word Eval: you don't need quoting as much
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245 | - Expression Language
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246 | - [QSN](qsn.html)
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