Lustre Script Coding Style

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Revision as of 16:53, 5 January 2021 by Adilger (talk | contribs) (→‎Bash Style: minor updates)
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Bash Style

  • Bash is a programming language. It includes functions. Shell code outside of functions is effectively code in an implicit main() function. An entire function should be fully seen on one page (~70-90 lines) and be readily comprehensible. If you have any doubts, then it is too complicated. Make it easier to understand by separating it into subroutines.
  • The total length of a line (including comment) must not exceed 80 characters. Take advantage of bash's += operator for constants or linefeed escapes \.
    • Lines can be split without the need for a linefeed escape after |, ||, & and && operators.
  • The indentation must use 8-column tabs and not spaces. For line continuation, an additional tab should be used to indent the continued line, or align after [ or ( for continued logic operations.
  • Comments are just as important in a shell script as in C code.
  • Use $(...) instead of `...` for subshell commands:
    • $(...) is easier to see the start and end of the subshell command
    • $(...) avoids confusion between '...' and `...` with a small font
    • $(...) can be nested
  • Use the subshell syntax only when you have to:
    • When you need to capture the output of a separate program
    • Using the construct with functions leads to stray output and/or convoluted code struggling to avoid output pollution
    • It is more computationally efficient to not fork() the Bash process. Bash is slow enough already.
  • Use "here string" like function <<<$var instead of echo $var | function to avoid forking a subshell and pipe
  • Use file arguments like awk '...' $file or input redirection like function << $file instead of a useless use of cat
  • Use built-in Bash Parameter Expansion for variable/string manipulation rather than forking sed/tr:
    • Use ${VAR#prefix} or ${VAR%suffix} to remove prefix or suffix respectively
    • Use ${VAR/pattern/string} to replace pattern with string
  • Avoid use of "grep foo | awk '{ print $2 }'" since "awk '/foo/ { print $2 }' works just as well and avoids a separate fork + pipe
  • If a variable is intended to be used as a boolean, then it must be assigned as follows:
        local mybool=false         # or true

        if $mybool; then
                do_stuff
        fi
 
  • for loops it is possible to avoid a subshell for $(seq 10) using the built-in iterator for fixed-length loops:
    • Unfortunately, {1..$var} does not work, but eval {1..$var} does
        for i in {1..10}; do
                something_with $i
        done

  • Use export FOOBAR=val instead of FOOBAR=val; export FOOBAR for clarity and simplicity
  • Use [[ expr ]] instead of [ expr ]
    • The [[ test understands regular expression matching with the =~ operator
    • The easiest way to use it is by putting the expression in a variable and expanding it after the operator without quotes.
  • Use (( expr )) instead of [ expr ] or let expr when evaluating numerical expressions
    • This can include mathematical operators like $((...))
    • This uses normal <=, >=, == comparisons
  • Use $((...)) for arithmetic expressions instead of expr ...
    • No need for $ when referencing variable names inside $((...))
    • $((...)) can handle hex values and math operators
  • Error checks should prefer the form [[ check ]] || action to avoid leaving a dangling "false" on the return stack
    • Otherwise, [[ check ]] && action will leave a dangling "false" on the stack if check fails and an immediately following return/end of function will return an error

Test Framework

Variables

  • Names of variables local to current script which are not exported to the environment should be declared with "local" and use lowercase letters
  • Names of global variables or variables that exported to the environment should be UPPERCASE letters

Functions

  • Each function must have a section describing what it does and explain the list of parameters
# One line description of this function's purpose
#
# More detailed description of what the function is doing if necessary
#
# usage: function_name [--option argument] {required_argument} ...
# option: meaning of "option" and its argument
# required_argument: meaning of "required_argument"
# 
# expected output and/or return value(s)

  • Function arguments should be given local variable names for clarity like "local facet=$1", rather than being used as $1 in the function
  • Use sleep 0.1 instead of usleep 100000, since usleep is RHEL-specific

Tests and Libraries

  • To avoid clustering a single test-framework.sh file, there should be a <test-lib>.sh file for each test that contains specific functions and variables for that test.
  • Any functions, variables that global to all tests should be put in test-framework.sh
  • A test file only need to source test-framework.sh and necessary <test-lib>.sh file