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Oil Language Idioms

This is an informal, lightly-organized list of recommended idioms for the Oil language. Each section has snippets labeled No and Yes.

• Use the Yes style when you want to write in Oil, and don't care about compatibility with other shells.
• The No style is discouraged in new code, but Oil will run it. The OSH language is compatible with POSIX and bash.

Use Simple Word Evaluation to Avoid "Quoting Hell"

Substitute Variables

No:

local x='my song.mp3'
ls "$x"  # quotes required to avoid mangling

Yes:

var x = 'my song.mp3'
ls $x  # no quotes needed

Splice Arrays

No:

local myflags=( --all --long )
ls "${myflags[@]}" "$@"

Yes:

var myflags = %( --all --long )
ls @myflags @ARGV

Explicitly Split, Glob, and Omit Empty Args

Oil doesn't split arguments after variable expansion.

No:

local packages='python-dev gawk'
apt install $packages

Yes:

var packages = 'python-dev gawk'
apt install @split(packages)

Even better:

var packages = %(python-dev gawk)  # array literal
apt install @packages              # splice array

Oil doesn't glob after variable expansion.

No:

local pat='*.py'
echo $pat

Yes:

var pat = '*.py'
echo @glob(pat)   # explicit call

Oil doesn't omit unquoted words that evaluate to the empty string.

No:

local e=''
cp $e other $dest            # cp gets 2 args, not 3, in sh

Yes:

var e = ''
cp @maybe(e) other $dest     # explicit call

Iterate a Number of Times (Split Command Sub)

No:

local n=3
for x in $(seq $n); do  # No implicit splitting of unquoted words in Oil
  echo $x
done

Yes:

var n = 3
for x in @(seq $n) {   # Explicit splitting
  echo $x
}

Note that {1..3} works in bash and Oil, but the numbers must be constant.

Avoid Ad Hoc Parsing and Splitting

In other words, avoid groveling through backslashes and spaces in shell.

Instead, emit and consume the QSN and QTSV interchange formats.

• QSN is a JSON-like format for byte string literals
• QTSV is a convention for embedding QSN in TSV files.

Custom parsing and serializing should be limited to "the edges" of your Oil programs.

Use New Builtins That Support Structured I/O

These are discussed in the next two sections, but here's a summary.

write --qsn        # also -q
read --qsn :mystr  # also -q

read --line --qsn :myline     # read a single line

That is, take advantage of the the invariants that the IO builtins respect. (doc in progress)

More Strategies For Structured Data

• Wrap and Adapt External Tools. Parse their output, and emit QSN and QTSV.
  • These can be one-off, "bespoke" wrappers in your program, or maintained programs. Use the proc construct and flagspec!
  • Example: uxy wrappers.
  • TODO: Examples written in Oil and in other languages.
• Patch Existing Tools.
  • Enhance GNU grep, etc. to emit QSN and QTSV. Add a --qsn flag.
• Write Your Own Structured Versions.
  • For example, you can write a structured subset of ls in Python with little effort.

The write Builtin Is Simpler Than printf and echo

Write an Arbitrary Line

No:

printf '%s\n' "$mystr"

Yes:

write -- $mystr

The write builtin accepts -- so it doesn't confuse flags and args.

Write Without a Newline

No:

echo -n "$mystr"  # breaks if mystr is -e

Yes:

write --end '' -- $mystr
write -n -- $mystr  # -n is an alias for --end ''

Write an Array of Lines

var myarray = %(one two three)
write -- @myarray

New Long Flags on the read builtin

Read a Line

No:

read line     # Bad because it mangles your backslashes!
read -r line  # Better, but easy to forget

Yes:

read --line   # also faster because it's a buffered read

Read a Whole File

No:

read -d ''      # harder to read, easy to forget -r

Yes:

read --all :mystr

Read Until \0 (consume find -print0)

No:

# Obscure syntax that bash accepts, but not other shells
read -r -d '' myvar

Yes:

read -0 :myvar

Oil Enhancements to Builtins

Use shopt Instead of set

bin/oil uses the set keyword for assignments, although OSH code will often use setvar. Using shopt "clears the way" for this upgrade.

Discouraged:

set -o errexit  

Idiomatic:

shopt --set errexit

(As always, using set is useful if your script needs to run under another shell.)

Use : When Mentioning Variable Names

Oil accepts this optional "pseudo-sigil" to make code more explicit.

No:

read -0 record < file.bin
echo $record

Yes:

read -0 :record < file.bin
echo $record

Consider Using --long-flags

Easier to write:

test -d /tmp
test -d / && test -f /vmlinuz

shopt -u extglob

Easier to read:

test --dir /tmp
test --dir / && test --file /vmlinuz

shopt --unset extglob

Use Blocks to Save and Restore Context

Do Something In Another Directory

No:

( cd /tmp; echo $PWD )  # subshell is unnecessary (and limited)

No:

pushd /tmp
echo $PWD
popd

Yes:

cd /tmp {
  echo $PWD
}

Temporarily Set Shell Options

No:

set +o errexit
myfunc  # without error checking
set -o errexit

Yes:

shopt --unset errexit {
  myfunc
}

Use the forkwait builtin for Subshells, not ()

No:

( not_mutated=foo )
echo $not_mutated

Yes:

forkwait {
  setvar not_mutated = 'foo'
}
echo $not_mutated

Use the fork builtin for async, not &

No:

myproc &

{ sleep 1; echo one; sleep 2; } &

Yes:

fork { myproc }

fork { sleep 1; echo one; sleep 2 }

Use Procs (Better Shell Functions)

Use Named Parameters Instead of $1, $2, ...

No:

f() {
  local src=$1
  local dest=${2:-/tmp}

  cp "$src" "$dest"
}

Yes:

proc f(src, dest='/tmp') {   # Python-like default values
  cp $src $dest
}

Use Named Varargs Instead of "$@"

No:

f() {
  local first=$1
  shift

  echo $first
  echo "$@"
}

Yes:

proc f(first, @rest) {  # @ means "the rest of the arguments"
  write -- $first
  write -- @rest        # @ means "splice this array"
}

You can also use the implicit ARGV variable:

proc p {
  cp -- @ARGV /tmp
}

Use "Out Params" instead of declare -n

Out params are one way to "return" values from a proc.

No:

f() {
  local in=$1
  local -n out=$2

  out=PREFIX-$in
}

myvar='init'
f zzz myvar         # assigns myvar to 'PREFIX-zzz'

Yes:

proc f(in, :out) {  # : is an out param, i.e. a string "reference"
  setref out = "PREFIX-$in"
}

var myvar = 'init'
f zzz :myvar        # assigns myvar to 'PREFIX-zzz'.
                    # colon is required

Curly Braces Fix Semantic Problems

Procs Don't Mess With Their Callers

That is, dynamic scope is turned off when procs are invoked.

Here's an example of shell functions reading variables in their caller:

bar() {
  echo $foo_var  # looks up the stack
}

foo() {
  foo_var=x
  bar
}

foo

In Oil, you have to pass params explicitly:

proc bar {
  echo $foo_var  # error, not defined
}

Shell functions can also mutate variables in their caller! But procs can't do this, which makes code easier to reason about.

If and errexit

Bug in POSIX shell, which Oil's strict_errexit warns you of:

if myfunc; then  # oops, errors not checked in myfunc
  echo 'success'
fi

Suggested workaround:

if $0 myfunc; then  # invoke a new shell
  echo 'success'
fi

"$@"

Oil has a run builtin, which re-enables errexit without the extra process:

if run myfunc; then
  echo 'success'
fi

(TODO: decide on this) Or you can also use curly braces for an implicit run:

if myfunc {
  echo 'success'
}

This explicit syntax avoids breaking POSIX shell. You have to opt in to the better behavior.

Error Handling

Use the run Builtin With !, ||, and &&

These constructs require an explicit run:

No:

! myfunc
myfunc || fail
myfunc && echo 'success'

Yes:

! run myfunc
run myfunc || fail
run myfunc && echo 'success'

Although || and && are rare in idiomatic Oil code.

Don't use && Outside of if / while

It's implicit because errexit is on in Oil.

No:

mkdir /tmp/dest && cp foo /tmp/dest

Yes:

mkdir /tmp/dest
cp foo /tmp/dest

Use Oil Expressions, Initializations, and Assignments (var, setvar)

Initialize and Assign Strings and Integers

No:

local mystr=foo
mystr='new value'

local myint=42  # still a string in shell

Yes:

var mystr = 'foo'
setvar mystr = 'new value'

var myint = 42  # a real integer

Expressions on Integers

No:

x=$(( 1 + 2*3 ))
(( x = 1 + 2*3 ))

Yes:

setvar x = 1 + 2*3

Mutate Integers

No:

(( i++ ))  # interacts poorly with errexit
i=$(( i+1 ))

Yes:

setvar i += 1  # like Python, with a keyword

Initialize and Assign Arrays

Container literals in Oil look like %(one two) and %{key: 'value'}.

No:

local -a myarray=(one two three)
myarray[3]='THREE'

Yes:

var myarray = %(one two three)
setvar myarray[3] = 'THREE'

No:

local -A myassoc=(['key']=value ['k2']=v2)
myassoc['key']=V

Yes:

# keys don't need to be quoted
var myassoc = %{key: 'value', k2: 'v2'}
setvar myassoc['key'] = 'V'

Expressions on Arrays

No:

local x=${a[i-1]}
x=${a[i]}

local y=${A['key']}

Yes:

var x = a[i-1]
setvar x = a[i]

var y = A['key']

Conditions and Comparisons

No:

if (( x > 0 )); then
  echo positive
fi

Yes:

if (x > 0) {
  echo 'positive'
}

Substituting Expressions in Words

No:

echo flag=$((1 + a[i] * 3))  # C-like arithmetic

Yes:

echo flag=$[1 + a[i] * 3]    # Arbitrary Oil expressions

# Possible, but a local var might be more readable
echo flag=$['1' if x else '0']

Use Egg Expressions instead of Regexes

Test for a Match

No:

local pat='[[:digit:]]+'
if [[ $x =~ $pat ]]; then
  echo 'number'
fi

Yes:

if (x ~ /digit+/) {
  echo 'number'
}

Or extract the pattern:

var pat = / digit+ /
if (x ~ pat) {
  echo 'number'
}

Extract Submatches

No:

if [[ $x =~ ([[:digit:]]+) ]] {
  echo "${BASH_REMATCH[@]}"
}

Yes:

if (x ~ / <d+> /) {  # <> is capture
  argv.py @M         # special M variable
}

Glob Matching

No:

if [[ $x == *.py ]]; then
  echo Python
fi

TODO: Implement the ~~ operator.

Yes:

if (x ~~ '*.py') {
  echo 'Python'
}

No:

case $x in
  *.py)
    echo Python
    ;;
  *.sh)
    echo Shell
    ;;
esac

Yes (purely a style preference):

case $x {          # curly braces
  (*.py)           # balanced parens
    echo 'Python'
    ;;
  (*.sh)
    echo 'Shell'
    ;;
}

TODO

Source Files and Namespaces

TODO

Distinguish Between Variables and Functions

• $RANDOM vs. random()
• LANG=C vs. shopt --setattr LANG=C

Related Documents

• Shell Language Idioms. This advice applies to shells other than Oil.
• Shell Language Deprecations. Shell constructs that Oil users should avoid.
• Error Handling with set -e / errexit. Oil fixes the flaky error handling in POSIX shell and bash.
• TODO: Go through more of the Pure Bash Bible. Oil provides alternatives for such quirky syntax.