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shellguide.md

@@ -15,11 +15,68 @@ Revision 2.00
 
 Authored, revised and maintained by many Googlers.
 
-[TOC]
+<details>
+  <summary>Table of Contents</summary>
+
+-   [1 Background](#s1-background)
+    *   [1.1 Which Shell to Use](#s1.1-which-shell-to-use)
+-   [2 Shell Files and Interpreter Invocation](#s2-shell-files-and-interpreter-invocation)
+    *   [2.1 File Extensions](#s2.1-file-extensions)
+    *   [2.2 SUID/SGID](#s2.2-suid-sgid)
+-   [3 Environment](#s3-environment)
+    *   [3.1 STDOUT vs STDERR](#s3.1-stdout-vs-stderr)
+-   [4 Comments](#s4-comments)
+    *   [4.1 File Header](#s4.1-file-header)
+        +   [4.1.1 $Id$](#s4.1.1-id)
+        +   [4.1.2 $Id$ Pros](#s4.1.2-id-pros)
+        +   [4.1.3 $Id$ Cons](#s4.1.3-id-cons)
+        +   [4.1.4 $Id$ Decision](#s4.1.4-id-decision)
+    *   [4.2 Function Comments](#s4.2-function-comments)
+    *   [4.3 Implementation Comments](#s4.3-implementation-comments)
+    *   [4.4 TODO Comments](#s4.4-todo-comments)
+-   [5 Formatting](#s5-formatting)
+    *   [5.1 Indentation](#s5.1-indentation)
+    *   [5.2 Line Length and Long Strings](#s5.2-line-length-and-long-strings)
+    *   [5.3 Pipelines](#s5.3-pipelines)
+    *   [5.4 Loops](#s5.4-loops)
+    *   [5.5 Case statement](#s5.5-case-statement)
+    *   [5.6 Variable expansion](#s5.6-variable-expansion)
+    *   [5.7 Quoting](#s5.7-quoting)
+-   [6 Features and Bugs](#s6-features-and-bugs)
+       *   [6.1 ShellCheck](#s6.1-shellcheck)
+    *   [6.2 Command Substitution](#s6.2-command-substitution)
+    *   [6.3 Test, `[ … ]`, and `[[ … ]]`](#s6.3-tests)
+    *   [6.4 Testing Strings](#s6.4-testing-strings)
+    *   [6.5 Wildcard Expansion of Filenames](#s6.5-wildcard-expansion-of-filenames)
+    *   [6.6 Eval](#s6.6-eval)
+    *   [6.7 Arrays](#s6.7-arrays)
+        +   [6.7.1 Arrays Pros](#s6.7.1-arrays-pros)
+        +   [6.7.2 Arrays Cons](#s6.7.2-arrays-cons)
+        +   [6.7.3 Arrays Decision](#s6.7.3-arrays-decision)
+    *   [6.8 Pipes to While](#s6.8-pipes-to-while)
+    *   [6.9 Arithmetic](#s6.9-arithmetic)
+-   [7 Naming Conventions](#s7-naming-conventions)
+    *   [7.1 Function Names](#s7.1-function-names)
+    *   [7.2 Variable Names](#s7.2-variable-names)
+    *   [7.3 Constants and Environment Variable Names](#s7.3-constants-and-environment-variable-names)
+    *   [7.4 Source Filenames](#s7.4-source-filenames)
+    *   [7.5 Read-only Variables](#s7.5-read-only-variables)
+    *   [7.6 Use Local Variables](#s7.6-use-local-variables)
+    *   [7.7 Function Location](#s7.7-function-location)
+    *   [7.8 main](#s7.8-main)
+-   [8 Calling Commands](#s8-calling-commands)
+    *   [8.1 Checking Return Values](#s8.1-checking-return-values)
+    *   [8.2 Builtin Commands vs. External Commands](#s8.2-builtin-commands-vs-external-commands)
+-   [9 Conclusion](#s9-conclusion)
+
+</details>
+
+<a id="s1-background"></a>
 
 ## Background
 
 
+<a id="s1.1-which-shell-to-use"></a>
 
 ### Which Shell to Use
 
@@ -38,6 +95,8 @@ The only exception to this is where you're forced to by whatever
 you're coding for. One example of this is Solaris SVR4 packages
 which require plain Bourne shell for any scripts.
 
+<a id="s1.1-which-shell-to-use"></a>
+
 ### When to use Shell
 
 Shell should only be used for small utilities or simple wrapper
@@ -62,8 +121,12 @@ Some guidelines:
     to switch languages) consider whether the code is easily
     maintainable by people other than its author.
 
+<a id="s2-shell-files-and-interpreter-invocation"></a>
+
 ## Shell Files and Interpreter Invocation
 
+<a id="s2.1-file-extensions"></a>
+
 ### File Extensions
 
 Executables should have no extension (strongly preferred) or a
@@ -80,6 +143,8 @@ languages. This allows library files with identical purposes but
 different languages to be identically named except for the
 language-specific suffix.
 
+<a id="s2.2-suid-sgid"></a>
+
 ### SUID/SGID
 
 SUID and SGID are *forbidden* on shell scripts.
@@ -91,9 +156,12 @@ which is why we're being explicit about banning it.
 
 Use `sudo` to provide elevated access if you need it.
 
+<a id="s3-environment"></a>
 
 ## Environment
 
+<a id="s3.1-stdout-vs-stderr"></a>
+
 ### STDOUT vs STDERR
 
 All error messages should go to `STDERR`.
@@ -114,9 +182,12 @@ if ! do_something; then
 fi
 ```
 
+<a id="s4-comments"></a>
 
 ## Comments
 
+<a id="s4.1-file-header"></a>
+
 ### File Header
 
 Start each file with a description of its contents.
@@ -135,6 +206,8 @@ Example:
 ```
 
 
+<a id="s4.2-function-comments"></a>
+
 ### Function Comments
 
 Any function that is not both obvious and short must be commented. Any
@@ -194,6 +267,8 @@ function del_thing(arg) {
 }
 ```
 
+<a id="s4.3-implementation-comments"></a>
+
 ### Implementation Comments
 
 Comment tricky, non-obvious, interesting or important parts of your
@@ -203,6 +278,8 @@ This follows general Google coding comment practice. Don't comment
 everything. If there's a complex algorithm or you're doing something
 out of the ordinary, put a short comment in.
 
+<a id="s4.4-todo-comments"></a>
+
 ### TODO Comments
 
 Use TODO comments for code that is temporary, a short-term solution, or
@@ -227,12 +304,15 @@ Examples:
 # TODO(mrmonkey): Handle the unlikely edge cases (bug ####)
 ```
 
+<a id="s5-formatting"></a>
 
 ## Formatting
 
 While you should follow the style that's already there for files that
 you're modifying, the following are required for any new code.
 
+<a id="s5.1-indentation"></a>
+
 ### Indentation
 
 Indent 2 spaces. No tabs.
@@ -241,6 +321,8 @@ Use blank lines between blocks to improve readability. Indentation is
 two spaces. Whatever you do, don't use tabs. For existing files, stay
 faithful to the existing indentation.
 
+<a id="s5.2-line-length-and-long-strings"></a>
+
 ### Line Length and Long Strings
 
 Maximum line length is 80 characters.
@@ -263,6 +345,8 @@ long_string="I am an exceptionally
 long string."
 ```
 
+<a id="s5.3-pipelines"></a>
+
 ### Pipelines
 
 Pipelines should be split one per line if they don't all fit on one
@@ -286,6 +370,8 @@ command1 \
   | command4
 ```
 
+<a id="s5.4-loops"></a>
+
 ### Loops
 
 Put `; do` and `; then` on the same line as the
@@ -320,6 +406,8 @@ for dir in "${dirs_to_cleanup[@]}"; do
 done
 ```
 
+<a id="s5.5-case-statement"></a>
+
 ### Case statement
 
 *   Indent alternatives by 2 spaces.
@@ -374,6 +462,8 @@ while getopts 'abf:v' flag; do
 done
 ```
 
+<a id="s5.6-variable-expansion"></a>
+
 ### Variable expansion
 
 In order of precedence: Stay consistent with what you find; quote your
@@ -430,6 +520,8 @@ They are listed in order of precedence.
 NOTE: Using braces in `${var}` is *not* a form of quoting. "Double quotes" must
 be used *as well*.
 
+<a id="s5.7-quoting"></a>
+
 ### Quoting
 
 *   Always quote strings containing variables, command substitutions, spaces or
@@ -516,14 +608,22 @@ grep -cP '([Ss]pecial|\|?characters*)$' ${1:+"$1"}
 (set -- 1 "2 two" "3 three tres"; echo $#; set -- "$@"; echo "$#, $@")
 ```
 
+<a id="s6-features-and-bugs"></a>
+
 ## Features and Bugs
 
+<a id="s6.1-shelllint"></a>
+
+<a id="s6.1-shellcheck"></a>
+
 ### ShellCheck
 
 The [ShellCheck project](https://www.shellcheck.net/) identifies common bugs and
 warnings for your shell scripts. It is recommended for all scripts, large or
 small.
 
+<a id="s6.2-command-substitution"></a>
+
 ### Command Substitution
 
 Use `$(command)` instead of backticks.
@@ -544,6 +644,8 @@ var="$(command "$(command1)")"
 var="`command \`command1\``"
 ```
 
+<a id="s6.3-tests"></a>
+
 <a id="tests"></a>
 ### Test, `[ … ]`, and `[[ … ]]` 
 
@@ -578,6 +680,8 @@ fi
 
 For the gory details, see E14 at http://tiswww.case.edu/php/chet/bash/FAQ
 
+<a id="s6.4-testing-strings"></a>
+
 ### Testing Strings
 
 Use quotes rather than filler characters where possible.
@@ -664,6 +768,8 @@ if [[ "${my_var}" > 3 ]]; then
 fi
 ```
 
+<a id="s6.5-wildcard-expansion-of-filenames"></a>
+
 ### Wildcard Expansion of Filenames
 
 Use an explicit path when doing wildcard expansion of filenames.
@@ -690,6 +796,8 @@ rm: cannot remove `./somedir': Is a directory
 removed `./somefile'
 ```
 
+<a id="s6.6-eval"></a>
+
 ### Eval
 
 `eval` should be avoided.
@@ -707,6 +815,8 @@ eval $(set_my_variables)
 variable="$(eval some_function)"
 ```
 
+<a id="s6.7-arrays"></a>
+
 ### Arrays
 
 Bash arrays should be used to store lists of elements, to avoid quoting
@@ -747,6 +857,8 @@ declare -a files=($(ls /directory))
 mybinary $(get_arguments)
 ```
 
+<a id="s6.7.1-arrays-pros"></a>
+
 #### Arrays Pros
 
 *   Using Arrays allows lists of things without confusing quoting
@@ -755,10 +867,14 @@ mybinary $(get_arguments)
 *   Arrays make it possible to safely store sequences/lists of
     arbitrary strings, including strings containing whitespace.
 
+<a id="s6.7.2-arrays-cons"></a>
+
 #### Arrays Cons
 
 Using arrays can risk a script’s complexity growing.
 
+<a id="s6.7.3-arrays-decision"></a>
+
 #### Arrays Decision
 
 Arrays should be used to safely create and pass around lists. In
@@ -768,6 +884,8 @@ avoid confusing quoting issues. Use quoted expansion –
 advanced data manipulation is required, shell scripting should be
 avoided altogether; see [above](#when-to-use-shell).
 
+<a id="s6.8-pipes-to-while"></a>
+
 ### Pipes to While
 
 Use process substitution or for loops in preference to piping to while.
@@ -833,6 +951,8 @@ while read src dest type opts rest; do
 done < /proc/mounts
 ```
 
+<a id="s6.9-arithmetic"></a>
+
 ### Arithmetic
 
 Always use `(( … ))` or `$(( … ))` rather than
@@ -915,9 +1035,12 @@ sec=30
 echo $(( hr * 3600 + min * 60 + sec )) # prints 7530 as expected
 ```
 
+<a id="s7-naming-conventions"></a>
 
 ## Naming Conventions
 
+<a id="s7.1-function-names"></a>
+
 ### Function Names
 
 Lower-case, with underscores to separate words. Separate libraries with
@@ -947,6 +1070,8 @@ The `function` keyword is extraneous when "()" is present
 after the function name, but enhances quick identification of
 functions.
 
+<a id="s7.2-variable-names"></a>
+
 ### Variable Names
 
 As for function names.
@@ -960,6 +1085,8 @@ for zone in "${zones[@]}"; do
 done
 ```
 
+<a id="s7.3-constants-and-environment-variable-names"></a>
+
 ### Constants and Environment Variable Names
 
 All caps, separated with underscores, declared at the top of the file.
@@ -991,6 +1118,8 @@ done
 readonly VERBOSE
 ```
 
+<a id="s7.4-source-filenames"></a>
+
 ### Source Filenames
 
 Lowercase, with underscores to separate words if desired.
@@ -999,6 +1128,8 @@ This is for consistency with other code styles in Google:
 `maketemplate` or `make_template` but not
 `make-template`.
 
+<a id="s7.5-read-only-variables"></a>
+
 ### Read-only Variables
 
 Use `readonly` or `declare -r` to ensure they're
@@ -1017,6 +1148,8 @@ else
 fi
 ```
 
+<a id="s7.6-use-local-variables"></a>
+
 ### Use Local Variables
 
 Declare function-specific variables with `local`. Declaration
@@ -1056,6 +1189,8 @@ my_func2() {
 }
 ```
 
+<a id="s7.7-function-location"></a>
+
 ### Function Location
 
 Put all functions together in the file just below constants. Don't hide
@@ -1068,6 +1203,8 @@ may be done before declaring functions.
 Don't hide executable code between functions. Doing so makes the code
 difficult to follow and results in nasty surprises when debugging.
 
+<a id="s7.8-main"></a>
+
 ### main
 
 A function called `main` is required for scripts long enough
@@ -1087,9 +1224,12 @@ main "$@"
 Obviously, for short scripts where it's just a linear flow,
 `main` is overkill and so is not required.
 
+<a id="s8-calling-commands"></a>
 
 ## Calling Commands
 
+<a id="s8.1-checking-return-values"></a>
+
 ### Checking Return Values
 
 Always check return values and give informative return values.
@@ -1143,6 +1283,8 @@ if (( return_codes[1] != 0 )); then
 fi
 ```
 
+<a id="s8.2-builtin-commands-vs-external-commands"></a>
+
 ### Builtin Commands vs. External Commands
 
 Given the choice between invoking a shell builtin and invoking a
@@ -1166,6 +1308,7 @@ addition="$(expr ${X} + ${Y})"
 substitution="$(echo "${string}" | sed -e 's/^foo/bar/')"
 ```
 
+<a id="s9-conclusion"></a>
 
 ## Conclusion