Lustre Script Coding Style

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 test function 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
• Use $TMP/ for temporary non-Lustre files instead of /tmp/

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, rather than being used as $1 $2 $3 in the function

local facet=$1 local file="$2" local size=$3 "

• 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

Subtests

• test files should be named $tfile for the filename, or base name like $tfile.1 or $tfile.source to simplify debugging
• test directories should be named $tdir, and should be used if a large number of files are created for the subtest
• small/few test files/dirs do not need to be explicitly deleted at the end of the test, that is done by test-framework.sh
• large (over 1MB)/many (over 50) test files/dirs in a subtest should be cleaned up explicitly with a stack_trap so that they are not filling the filesystem if the test exits with an error

stack_trap "rm -f $DIR/$tfile.big" fallocate -l 100M $DIR/$tfile.big || error "$tfile.big create failed" stack_trap "unlinkmany $DIR/$tdir/$tfile- 1000" createmany -o $DIR/$tdif/$tfile- 1000 || error "$tfile creation failed"

• creating large test files is by far the fastest with "fallocate" *when supported* (ldiskfs only), as determined by check_set_fallocate
• use test_mkdir to add some variety to directory creation (random local, striped, remote) if directory location is not critical to the test
• ensure that directory location and MDS facet are aligned. Since 2.14.54 directories may be created on any MDT, so "do_facet mds1 ..." may be on the wrong MDS.
• Use "mkdir_on_mdt0 $DIR/$tdir" to create directories on MDT0000 for use with mds1, or "$LFS getdirstripe -m $DIR/$tdir" to determine MDT index, and "mds$((idx+1))" for facet name.
• use $SECONDS to get the current time when measuring test durations:

<nowiki>

   local start=$SECONDS

   do something
   local elapsed=$((SECONDS - start))