| 1 | ---
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| 2 | title: Command Language (Oils Reference)
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| 3 | all_docs_url: ..
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| 4 | body_css_class: width40
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| 5 | default_highlighter: oils-sh
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| 6 | preserve_anchor_case: yes
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| 7 | ---
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| 8 |
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| 9 | <div class="doc-ref-header">
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| 10 |
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| 11 | [Oils Reference](index.html) —
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| 12 | Chapter **Command Language**
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| 13 |
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| 14 | </div>
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| 15 |
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| 16 | This chapter describes the command language for OSH, and some YSH extensions.
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| 17 |
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| 18 | <span class="in-progress">(in progress)</span>
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| 19 |
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| 20 | <div id="dense-toc">
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| 21 | </div>
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| 22 |
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| 23 | ## Quick Sketch: What's a Command?
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| 24 |
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| 25 | OSH:
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| 26 |
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| 27 | print-files() {
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| 28 | for name in *.py; do
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| 29 | if test -x "$name"; then
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| 30 | echo "$name is executable"
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| 31 | fi
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| 32 | done
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| 33 | }
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| 34 |
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| 35 | YSH:
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| 36 |
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| 37 | proc print-files {
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| 38 | for name in *.py {
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| 39 | if test -x $name { # no quotes needed
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| 40 | echo "$name is executable"
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| 41 | }
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| 42 | }
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| 43 | }
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| 44 |
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| 45 |
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| 46 | <h2 id="Commands">Commands</h2>
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| 47 |
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| 48 | ### simple-command
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| 49 |
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| 50 | Commands are composed of words. The first word may be the name of
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| 51 |
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| 52 | 1. An alias
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| 53 | 1. A builtin command
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| 54 | 1. A YSH `proc`
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| 55 | 1. A shell "function"
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| 56 | 1. An external command
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| 57 |
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| 58 | Examples:
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| 59 |
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| 60 | echo hi # a shell builtin doesn't start a process
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| 61 | ls /usr/bin ~/src # starts a new process
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| 62 | myproc "hello $name"
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| 63 | myshellfunc "hello $name"
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| 64 | myalias -l
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| 65 | <!-- TODO: document lookup order -->
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| 66 |
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| 67 | Redirects are also allowed in any part of the command:
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| 68 |
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| 69 | echo 'to stderr' >&2
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| 70 | echo >&2 'to stderr'
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| 71 |
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| 72 | echo 'to file' > out.txt
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| 73 | echo > out.txt 'to file'
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| 74 |
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| 75 | ### command-lookup-order
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| 76 |
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| 77 | The first word in a command can mean many things.
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| 78 |
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| 79 | (1) An [alias][] may expand into shell code at **parse** time. This includes
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| 80 | keywords!
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| 81 |
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| 82 | Aliases are **disabled** in YSH (`shopt --unset expand_aliases`).
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| 83 |
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| 84 | (2) Shell keywords like `if` have their own **parsing** rules:
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| 85 |
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| 86 | if test -d /; then # OSH style
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| 87 | echo yes
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| 88 | fi
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| 89 |
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| 90 | if test --dir / { # YSH style
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| 91 | echo yes
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| 92 | }
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| 93 |
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| 94 | (3) The first word of a [simple command][simple-command] is looked up in this
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| 95 | order:
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| 96 |
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| 97 | 1. Special builtins like `eval`
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| 98 | 1. YSH Procs or Shell Functions
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| 99 | 1. Normal Builtins like `cd`
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| 100 | 1. External Processes like `/bin/ls`
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| 101 |
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| 102 | So special builtins can't be redefined as functions, but normal builtins can.
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| 103 |
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| 104 | YSH also adds the concept of a **private** builtin. Private
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| 105 | builtins like [sleep][] are **not** consulted as the first word:
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| 106 |
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| 107 | sleep 1 # run external command
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| 108 | builtin sleep 1 # run private builtin with explicit prefix
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| 109 |
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| 110 | [alias]: chap-builtin-cmd.html#alias
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| 111 | [simple-command]: chap-cmd-lang.html#simple-command
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| 112 | [sleep]: chap-builtin-cmd.html#sleep
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| 113 |
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| 114 | To summarize, OSH commands are looked up in this order:
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| 115 |
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| 116 | 1. alias
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| 117 | 1. keyword
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| 118 | 1. special builtin
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| 119 | 1. shell function
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| 120 | 1. normal builtin
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| 121 | 1. external
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| 122 |
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| 123 | YSH commands are looked up in this order:
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| 124 |
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| 125 | 1. keyword
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| 126 | 1. special builtin
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| 127 | 1. YSH proc
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| 128 | 1. normal builtin
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| 129 | 1. external
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| 130 |
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| 131 | Logically separate:
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| 132 |
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| 133 | 6. private builtin (not the first word)
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| 134 |
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| 135 | Use [invoke --show][invoke] to see different meanings for a given name.
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| 136 |
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| 137 | [invoke]: chap-builtin-cmd.html#invoke
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| 138 |
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| 139 | ### prefix-binding
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| 140 |
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| 141 | Bindings are allowed before a simple command:
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| 142 |
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| 143 | PYTHONPATH=. mydir/myscript.py
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| 144 |
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| 145 | These bindings set a variable and mark it exported. This binding is usually
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| 146 | temporary, but when used with certain [special builtins][special], it persists.
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| 147 |
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| 148 | [special]: https://www.gnu.org/software/bash/manual/html_node/Special-Builtins.html
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| 149 |
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| 150 | - Related: [ysh-prefix-binding](ysh-prefix-binding)
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| 151 |
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| 152 | ### ysh-prefix-binding
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| 153 |
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| 154 | YSH prefix bindings look exactly like they do in shell:
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| 155 |
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| 156 | PYTHONPATH=. mydir/myscript.py
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| 157 |
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| 158 | However, they temporarily set `ENV.PYTHONPATH`, not `$PYTHONPATH`. This is
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| 159 | done by adding a new `Dict` to the prototype chain of the `Obj`.
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| 160 |
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| 161 | The new `ENV` then becomes the environment of the child processes for the
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| 162 | command.
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| 163 |
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| 164 | In YSH, prefix bindings are always temporary ENV bindings, and they don't
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| 165 | persist. This is enforced by `shopt --set strict_env_binding`.
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| 166 |
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| 167 | - Related: [ENV](chap-special-var.html#ENV), [prefix-binding](chap-cmd-lang.html#prefix-binding)
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| 168 |
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| 169 |
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| 170 | <h3 id="semicolon" class="osh-ysh-topic">semicolon ;</h3>
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| 171 |
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| 172 | Run two commands in sequence like this:
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| 173 |
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| 174 | echo one; echo two
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| 175 |
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| 176 | or this:
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| 177 |
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| 178 | echo one
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| 179 | echo two
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| 180 |
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| 181 | <h2 id="Conditional">Conditional</h2>
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| 182 |
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| 183 | <h3 id="case" class="osh-topic">case</h3>
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| 184 |
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| 185 | Match a string against a series of glob patterns. Execute code in the section
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| 186 | below the matching pattern.
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| 187 |
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| 188 | path='foo.py'
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| 189 | case "$path" in
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| 190 | *.py)
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| 191 | echo 'python'
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| 192 | ;;
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| 193 | *.sh)
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| 194 | echo 'shell'
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| 195 | ;;
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| 196 | esac
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| 197 |
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| 198 | For bash compatibility, the `;;` terminator can be substituted with either:
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| 199 |
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| 200 | - `;&` - fall through to next arm, ignoring the condition
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| 201 | - `;;&` - fall through to next arm, respecting the condition
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| 202 |
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| 203 | <h3 id="if" class="osh-topic">if</h3>
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| 204 |
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| 205 | Test if a command exited with status zero (true). If so, execute the
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| 206 | corresponding block of code.
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| 207 |
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| 208 | Shell:
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| 209 |
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| 210 | if test -d foo; then
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| 211 | echo 'foo is a directory'
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| 212 | elif test -f foo; then
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| 213 | echo 'foo is a file'
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| 214 | else
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| 215 | echo 'neither'
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| 216 | fi
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| 217 |
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| 218 | YSH:
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| 219 |
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| 220 | if test -d foo {
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| 221 | echo 'foo is a directory'
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| 222 | } elif test -f foo {
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| 223 | echo 'foo is a file'
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| 224 | } else {
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| 225 | echo 'neither'
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| 226 | }
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| 227 |
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| 228 | <h3 id="dbracket" class="osh-topic">dbracket [[</h3>
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| 229 |
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| 230 | Statically parsed boolean expressions, from bash and other shells:
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| 231 |
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| 232 | x=42
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| 233 | if [[ $x -eq 42 ]]; then
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| 234 | echo yes
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| 235 | fi # => yes
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| 236 |
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| 237 | Compare with the [test][] builtin, which is dynamically parsed.
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| 238 |
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| 239 | See [bool-expr][] for the expression syntax.
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| 240 |
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| 241 | [test]: chap-builtin-cmd.html#test
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| 242 | [bool-expr]: chap-mini-lang.html#bool-expr
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| 243 |
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| 244 |
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| 245 | <h3 id="bang" class="osh-ysh-topic">bang !</h3>
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| 246 |
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| 247 | Invert an exit code:
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| 248 |
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| 249 | if ! test -d /tmp; then
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| 250 | echo "No temp directory
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| 251 | fi
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| 252 |
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| 253 | <h3 id="and" class="osh-ysh-topic">and &&</h3>
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| 254 |
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| 255 | mkdir -p /tmp && cp foo /tmp
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| 256 |
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| 257 | <h3 id="or" class="osh-ysh-topic">or ||</h3>
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| 258 |
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| 259 | ls || die "failed"
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| 260 |
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| 261 | <h2 id="Iteration">Iteration</h2>
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| 262 |
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| 263 | <h3 id="while" class="osh-ysh-topic">while</h3>
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| 264 |
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| 265 | POSIX
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| 266 |
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| 267 | <h3 id="until" class="osh-topic">until</h3>
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| 268 |
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| 269 | POSIX
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| 270 |
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| 271 | <h3 id="for" class="osh-ysh-topic">for</h3>
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| 272 |
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| 273 | For loops iterate over words.
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| 274 |
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| 275 | YSH style:
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| 276 |
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| 277 | var mystr = 'one'
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| 278 | var myarray = :| two three |
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| 279 |
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| 280 | for i in $mystr @myarray *.py {
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| 281 | echo $i
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| 282 | }
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| 283 |
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| 284 |
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| 285 | Shell style:
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| 286 |
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| 287 | local mystr='one'
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| 288 | local myarray=(two three)
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| 289 |
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| 290 | for i in "mystr" "${myarray[@]}" *.py; do
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| 291 | echo $i
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| 292 | done
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| 293 |
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| 294 | Both fragments output 3 lines and then Python files on remaining lines.
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| 295 |
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| 296 | <h3 id="for-expr-sh" class="osh-topic">for-expr-sh</h3>
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| 297 |
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| 298 | A bash/ksh construct:
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| 299 |
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| 300 | for (( i = 0; i < 5; ++i )); do
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| 301 | echo $i
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| 302 | done
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| 303 |
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| 304 | <h2 id="Control Flow">Control Flow</h2>
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| 305 |
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| 306 | These are keywords in Oils, not builtins!
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| 307 |
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| 308 | ### break
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| 309 |
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| 310 | Break out of a loop. (Not used for case statements!)
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| 311 |
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| 312 | If optional argument N is specified, breaks N enclosing loops.
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| 313 |
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| 314 | ### continue
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| 315 |
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| 316 | Continue to the next iteration of a loop.
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| 317 |
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| 318 | If optional argument N is specified, resumes the Nth enclosing loop.
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| 319 |
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| 320 | ### return
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| 321 |
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| 322 | Return from a function with the given status (if value is omitted, the return
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| 323 | status is that of the last command executed).
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| 324 |
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| 325 | ### exit
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| 326 |
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| 327 | Exit the shell process with the given status:
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| 328 |
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| 329 | exit 2
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| 330 |
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| 331 | <h2 id="Grouping">Grouping</h2>
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| 332 |
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| 333 | ### sh-func
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| 334 |
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| 335 | POSIX:
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| 336 |
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| 337 | f() {
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| 338 | echo args "$@"
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| 339 | }
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| 340 | f 1 2 3
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| 341 |
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| 342 | ### sh-block
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| 343 |
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| 344 | POSIX:
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| 345 |
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| 346 | { echo one; echo two; }
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| 347 |
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| 348 | The trailing `;` is necessary in OSH, but not YSH. In YSH, `parse_brace` makes
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| 349 | `}` is more of a special word.
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| 350 |
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| 351 |
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| 352 | ### subshell
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| 353 |
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| 354 | ( echo one; echo two )
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| 355 |
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| 356 | In YSH, use [forkwait](chap-builtin-cmd.html#forkwait) instead of parentheses.
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| 357 |
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| 358 | <h2 id="Concurrency">Concurrency</h2>
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| 359 |
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| 360 | ### pipe
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| 361 |
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| 362 | Pipelines are a traditional POSIX shell construct:
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| 363 |
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| 364 | ls /tmp | grep ssh | sort
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| 365 |
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| 366 | Related:
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| 367 |
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| 368 | - [`PIPESTATUS`]() in OSH
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| 369 | - [`_pipeline_status`]() in YSH
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| 370 |
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| 371 | [PIPESTATUS]: chap-special-var.html#PIPESTATUS
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| 372 | [_pipeline_status]: chap-special-var.html#_pipeline_status
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| 373 |
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| 374 | <h3 id="ampersand" class="osh-topic">ampersand &</h3>
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| 375 |
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| 376 | Start a command as a background job. Don't wait for it to finish, and return
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| 377 | control to the shell.
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| 378 |
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| 379 | The PID of the job is recorded in the `$!` variable.
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| 380 |
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| 381 | sleep 1 &
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| 382 | echo pid=$!
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| 383 | { echo two; sleep 2 } &
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| 384 | wait
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| 385 | wait
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| 386 |
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| 387 | In YSH, use the [fork][] builtin.
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| 388 |
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| 389 | [fork]: chap-builtin-cmd.html#fork
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| 390 |
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| 391 |
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| 392 | <h2 id="Redirects">Redirects</h2>
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| 393 |
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| 394 | ### redir-file
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| 395 |
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| 396 | The operators `>` and `>>` redirect the `stdout` of a process to a disk file.
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| 397 | The `<` operator redirects `stdin` from a disk file.
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| 398 |
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| 399 | ---
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| 400 |
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| 401 | Examples of redirecting the `stdout` of a command:
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| 402 |
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| 403 | echo foo > out.txt # overwrite out.txt
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| 404 | date >> stamp.txt # append to stamp.txt
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| 405 |
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| 406 | <!--
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| 407 | echo foo >| out.txt # clobber the file even if set -o noclobber
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| 408 | -->
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| 409 |
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| 410 | Redirect to the `stdin` of a command:
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| 411 |
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| 412 | cat < in.txt
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| 413 |
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| 414 | Redirects are compatible with POSIX and bash, so they take descriptor numbers
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| 415 | on the left:
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| 416 |
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| 417 | make 2> stderr.txt # '2>' is valid, but '2 >' is not
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| 418 |
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| 419 | Note that the word argument to **file** redirects is evaluated like bash, which
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| 420 | is different than other arguments to other redirects:
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| 421 |
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| 422 | tar -x -z < Python* # glob must expand to exactly 1 file
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| 423 | tar -x -z < $myvar # $myvar is split because it's unquoted
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| 424 |
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| 425 | In other words, it's evaluated **as** a sequence of 1 word, which **produces**
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| 426 | zero to N strings. But redirects are only valid when it produces exactly 1
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| 427 | string.
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| 428 |
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| 429 | (Related: YSH uses `shopt --set simple_word_eval`, which means that globs that
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| 430 | match nothing evaluate to zero strings, not themselves.)
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| 431 |
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| 432 | <!-- They also take a file descriptor on the left -->
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| 433 |
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| 434 |
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| 435 | ### redir-desc
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| 436 |
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| 437 | Redirect to a file descriptor:
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| 438 |
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| 439 | echo 'to stderr' >&2
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| 440 |
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| 441 | <!--
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| 442 | NOTE: >&2 is just like <&2
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| 443 | There's no real difference.
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| 444 | -->
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| 445 |
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| 446 | ### here-doc
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| 447 |
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| 448 | Here documents let you write the `stdin` of a process in the shell program.
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| 449 |
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| 450 | Specify a delimiter word (like EOF) after the redir operator (like `<<`).
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| 451 |
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| 452 | If it's unquoted, then `$` expansion happens, like a double-quoted string:
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| 453 |
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| 454 | cat <<EOF
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| 455 | here doc with $double ${quoted} substitution
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| 456 | EOF
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| 457 |
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| 458 | If the delimiter is quoted, then `$` expansion does **not** happen, like a
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| 459 | single-quoted string:
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| 460 |
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| 461 | cat <<'EOF'
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| 462 | price is $3.99
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| 463 | EOF
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| 464 |
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| 465 | Leading tabs can be stripped with the `<<-` operator:
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| 466 |
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| 467 | myfunc() {
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| 468 | cat <<-EOF
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| 469 | here doc with one tab leading tab stripped
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| 470 | EOF
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| 471 | }
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| 472 |
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| 473 | ### here-str
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| 474 |
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| 475 | The `<<<` operator means that the argument is a `stdin` string, not a
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| 476 | chosen delimiter.
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| 477 |
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| 478 | cat <<< 'here string'
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| 479 |
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| 480 | The string **plus a newline** is the `stdin` value, which is consistent with
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| 481 | GNU bash.
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| 482 |
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| 483 | ### ysh-here-str
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| 484 |
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| 485 | You can also use YSH multi-line strings as "here strings". For example:
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| 486 |
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| 487 | Double-quoted:
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| 488 |
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| 489 | cat <<< """
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| 490 | double
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| 491 | quoted = $x
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| 492 | """
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| 493 |
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| 494 | Single-quoted:
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| 495 |
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| 496 | cat <<< '''
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| 497 | price is
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| 498 | $3.99
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| 499 | '''
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| 500 |
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| 501 | J8-style with escapes:
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| 502 |
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| 503 | cat <<< u'''
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| 504 | j8 style string price is
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| 505 | mu = \u{3bc}
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| 506 | '''
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| 507 |
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| 508 | In these cases, a trailing newline is **not** added. For example, the first
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| 509 | example is equivalent to:
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| 510 |
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| 511 | write --end '' -- """
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| 512 | double
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| 513 | quoted = $x
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| 514 | """
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| 515 |
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| 516 | ## Other Command
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| 517 |
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| 518 | <h3 id="dparen" class="osh-topic">dparen ((</h3>
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| 519 |
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| 520 | <h3 id="time" class="osh-ysh-topic">time</h3>
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| 521 |
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| 522 | time [-p] pipeline
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| 523 |
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| 524 | Measures the time taken by a command / pipeline. It uses the `getrusage()`
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| 525 | function from `libc`.
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| 526 |
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| 527 | Note that time is a KEYWORD, not a builtin!
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| 528 |
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| 529 | <!-- Note: bash respects TIMEFORMAT -->
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| 530 |
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| 531 |
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| 532 | ## YSH Simple
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| 533 |
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| 534 | ### typed-arg
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| 535 |
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| 536 | Internal commands (procs and builtins) accept typed arguments in parentheses:
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| 537 |
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| 538 | json write (myobj)
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| 539 |
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| 540 | Redirects can also appear after the typed args:
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| 541 |
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| 542 | json write (myobj) >out.txt
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| 543 |
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| 544 | ### lazy-expr-arg
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| 545 |
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| 546 | Expressions in brackets like this:
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| 547 |
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| 548 | assert [42 === x]
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| 549 |
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| 550 | Are syntactic sugar for:
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| 551 |
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| 552 | assert (^[42 === x])
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| 553 |
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| 554 | That is, it's single arg of type `value.Expr`.
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| 555 |
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| 556 | Redirects can also appear after the lazy typed args:
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| 557 |
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| 558 | assert [42 === x] >out.txt
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| 559 |
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| 560 | - Related: [Expr][] type
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| 561 |
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| 562 | [Expr]: chap-type-method.html#Expr
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| 563 |
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| 564 | ### block-arg
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| 565 |
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| 566 | Blocks can be passed to simple commands, either literally:
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| 567 |
|
| 568 | cd /tmp {
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| 569 | echo $PWD # prints /tmp
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| 570 | }
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| 571 | echo $PWD
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| 572 |
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| 573 | Or as an expression:
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| 574 |
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| 575 | var block = ^(echo $PWD)
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| 576 | cd /tmp (; ; block)
|
| 577 |
|
| 578 | Note that `cd` has no typed or named arguments, so the two semicolons are
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| 579 | preceded by nothing.
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| 580 |
|
| 581 | When passed to procs, blocks capture the enclosing stack frame:
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| 582 |
|
| 583 | var x = 42
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| 584 | myproc {
|
| 585 | # lexical scope is respected
|
| 586 | echo "x = $x" # x = 42
|
| 587 | }
|
| 588 |
|
| 589 | ---
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| 590 |
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| 591 | Redirects can appear after the block arg:
|
| 592 |
|
| 593 | cd /tmp {
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| 594 | echo $PWD # prints /tmp
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| 595 | } >out.txt
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| 596 |
|
| 597 |
|
| 598 | Related:
|
| 599 |
|
| 600 | - [sh-block](#sh-block) in OSH.
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| 601 | - [Command][] and [CommandFrag][] types.
|
| 602 |
|
| 603 | [Command]: chap-type-method.html#Command
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| 604 | [CommandFrag]: chap-type-method.html#CommandFrag
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| 605 |
|
| 606 | ## YSH Cond
|
| 607 |
|
| 608 | ### ysh-case
|
| 609 |
|
| 610 | Like the shell case statement, the Ysh case statement has **string/glob** patterns.
|
| 611 |
|
| 612 | var s = 'README.md'
|
| 613 | case (s) {
|
| 614 | *.py { echo 'Python' }
|
| 615 | *.cc | *.h { echo 'C++' }
|
| 616 | * { echo 'Other' }
|
| 617 | }
|
| 618 | # => Other
|
| 619 |
|
| 620 | We also generated it to **typed data** within `()`:
|
| 621 |
|
| 622 | var x = 43
|
| 623 | case (x) {
|
| 624 | (30 + 12) { echo 'the integer 42' }
|
| 625 | (else) { echo 'neither' }
|
| 626 | }
|
| 627 | # => neither
|
| 628 |
|
| 629 | The `else` is a special keyword that matches any value.
|
| 630 |
|
| 631 | case (s) {
|
| 632 | / dot* '.md' / { echo 'Markdown' }
|
| 633 | (else) { echo 'neither' }
|
| 634 | }
|
| 635 | # => Markdown
|
| 636 |
|
| 637 | ### ysh-if
|
| 638 |
|
| 639 | Like shell, you can use a command:
|
| 640 |
|
| 641 | if test --file $x {
|
| 642 | echo "$x is a file"
|
| 643 | }
|
| 644 |
|
| 645 | You can also use an expression:
|
| 646 |
|
| 647 | if (x > 0) {
|
| 648 | echo 'positive'
|
| 649 | }
|
| 650 |
|
| 651 | ## YSH Iter
|
| 652 |
|
| 653 | ### ysh-for
|
| 654 |
|
| 655 | #### Words
|
| 656 |
|
| 657 | This is a shell-style loop over "words":
|
| 658 |
|
| 659 | for word in 'oils' $num_beans {pea,coco}nut {
|
| 660 | echo $word
|
| 661 | }
|
| 662 | # =>
|
| 663 | # oils
|
| 664 | # 13
|
| 665 | # peanut
|
| 666 | # coconut
|
| 667 |
|
| 668 | You can ask for the loop index with `i,`:
|
| 669 |
|
| 670 | for i, name in README.md *.py {
|
| 671 | echo "$i $name"
|
| 672 | }
|
| 673 | # => 0 README.md
|
| 674 | # => 1 foo.py
|
| 675 |
|
| 676 | #### Expressions Over Typed Data
|
| 677 |
|
| 678 | Expressions are enclosed in `()`. You can iterate over a `Range`, `List`,
|
| 679 | `Dict`, or `io.stdin`.
|
| 680 |
|
| 681 | Range:
|
| 682 |
|
| 683 | for i in (3 ..< 5) { # range operator ..<
|
| 684 | echo "i = $i"
|
| 685 | }
|
| 686 | # =>
|
| 687 | # i = 3
|
| 688 | # i = 4
|
| 689 |
|
| 690 | List:
|
| 691 |
|
| 692 | var foods = ['ale', 'bean']
|
| 693 | for item in (foods) {
|
| 694 | echo $item
|
| 695 | }
|
| 696 | # =>
|
| 697 | # ale
|
| 698 | # bean
|
| 699 |
|
| 700 | ---
|
| 701 |
|
| 702 | There are **three** ways of iterating over a `Dict`:
|
| 703 |
|
| 704 | var mydict = {pea: 42, nut: 10}
|
| 705 | for key in (mydict) {
|
| 706 | echo $key
|
| 707 | }
|
| 708 | # =>
|
| 709 | # pea
|
| 710 | # nut
|
| 711 |
|
| 712 | for key, value in (mydict) {
|
| 713 | echo "$key $value"
|
| 714 | }
|
| 715 | # =>
|
| 716 | # pea - 42
|
| 717 | # nut - 10
|
| 718 |
|
| 719 | for i, key, value in (mydict) {
|
| 720 | echo "$i $key $value"
|
| 721 | }
|
| 722 | # =>
|
| 723 | # 0 - pea - 42
|
| 724 | # 1 - nut - 10
|
| 725 |
|
| 726 | That is, if you ask for two things, you'll get the key and value. If you ask
|
| 727 | for three, you'll also get the index.
|
| 728 |
|
| 729 | (One way to think of it: `for` loops in YSH have the functionality Python's
|
| 730 | `enumerate()`, `items()`, `keys()`, and `values()`.)
|
| 731 |
|
| 732 | ---
|
| 733 |
|
| 734 | The `io.stdin` object iterates over lines:
|
| 735 |
|
| 736 | for line in (io.stdin) {
|
| 737 | echo $line
|
| 738 | }
|
| 739 | # lines are buffered, so it's much faster than `while read --raw-line`
|
| 740 |
|
| 741 | ---
|
| 742 |
|
| 743 | (This section is based on [A Tour of YSH](../ysh-tour.html).)
|
| 744 |
|
| 745 | #### Closing Over the Loop Variable
|
| 746 |
|
| 747 | Each iteration of a `for` loop creates a new frame, which may be captured.
|
| 748 |
|
| 749 | var x = 42 # outside the loop
|
| 750 | for i in (0 ..< 3) {
|
| 751 | var j = i + 2
|
| 752 |
|
| 753 | var expr = ^"$x: i = $i, j = $j" # captures x, i, and j
|
| 754 |
|
| 755 | my-task {
|
| 756 | echo "$x: i = $i, j = $j" # also captures x, i, and j
|
| 757 | }
|
| 758 | }
|
| 759 |
|
| 760 | #### Mutating Containers in a `for` Loop
|
| 761 |
|
| 762 | - If you append or remove from a `List` while iterating over it, the loop **will** be affected.
|
| 763 | - If you mutate a `Dict` while iterating over it, the loop will **not** be
|
| 764 | affected.
|
| 765 |
|
| 766 | ### ysh-while
|
| 767 |
|
| 768 | You can use an expression as the condition:
|
| 769 |
|
| 770 | var x = 5
|
| 771 | while (x < 0) {
|
| 772 | setvar x -= 1
|
| 773 | }
|
| 774 |
|
| 775 | You or a command:
|
| 776 |
|
| 777 | while test -f myfile {
|
| 778 | echo 'myfile'
|
| 779 | sleep 1
|
| 780 | }
|
| 781 |
|