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Oil vs. Shell 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 QTT interchange formats.

• QSN is a JSON-like format for byte string literals
• QTT is a convention for embedding QSN in TSV files (not yet implemented)

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 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 QTT.
  • 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 QTT. 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

Using a single builtin for all options makes scripts easier to read:

Discouraged:

set -o errexit  
shopt -s dotglob

Idiomatic:

shopt --set errexit
shopt --set dotglob

(As always, set can be used when you care about compatibility with other shells.)

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
}

Batch I/O

No:

echo 1 > out.txt   
echo 2 >> out.txt  # appending is less efficient
                   # because open() and close()

No:

{ echo 1
  echo 2
} > out.txt

Yes:

fopen > out.txt {
  echo 1
  echo 2
}

The fopen builtin is syntactic sugar -- it lets you see redirects before the code that uses them.

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:

( cd /tmp; rm *.sh )

Yes:

forkwait {
  cd /tmp
  rm *.sh
}

Better:

cd /tmp {  # no process created
  rm *.sh
}

Use the fork builtin for async, not &

No:

myfunc &

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

Yes:

fork { myfunc }

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

Note: 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.

Use Modules

Oil has a few lightweight features that make it easier to organize code into files. It doesn't have "namespaces".

Relative Imports

Suppose we are running bin/mytool, and we want BASE_DIR to be the root of the repository so we can do a relative import of lib/foo.sh.

No:

# All of these are common idioms, with caveats
BASE_DIR=$(dirname $0)/..

BASE_DIR=$(dirname ${BASH_SOURCE[0]})/..

BASE_DIR=$(cd $($dirname $0)/.. && pwd)

BASE_DIR=$(dirname (dirname $(readlink -f $0)))

source $BASE_DIR/lib/foo.sh

Yes:

const BASE_DIR = "$this_dir/.."

source $BASE_DIR/lib/foo.sh

# Or simply:
source $_this_dir/../lib/foo.sh

The value of _this_dir is the directory that contains the currently executing file.

Include Guards

No:

# libfoo.sh
if test -z "$__LIBFOO_SH"; then
  return
fi
__LIBFOO_SH=1

Yes:

# libfoo.sh
module libfoo.sh || return 0

Taskfile Pattern

No:

deploy() {
  echo ...
}
"$@"

Yes

proc deploy() {
  echo ...
}
runproc @ARGV  # gives better error messages

Error Handling

Oil Fixes Shell's Error Handling (errexit) once and for all! Here's a comprehensive list of error handling idioms.

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

It also avoids the Trailing && Pitfall mentioned at the end of the error handling doc.

Ignore an Error

No:

ls /bad || true  # OK because ls is external
myfunc || true   # suffers from the "Disabled errexit Quirk"

Yes:

try ls /bad
try myfunc

Retrieve A Command's Status When errexit is On

No:

# set -e is enabled earlier

set +e
mycommand  # this ignores errors when mycommand is a function
status=$?  # save it before it changes
set -e

echo $status

Yes:

try mycommand
echo $_status

Does a Builtin Or External Command Succeed?

These idioms are OK in both shell and Oil:

if ! cp foo /tmp {
  echo 'error copying'  # any non-zero status
}

if ! test -d /bin {
  echo 'not a directory'
}

To be consistent with the idioms below, you can also write them like this:

try cp foo /tmp
if (_status !== 0) {
  echo 'error copying'
}

Does a Function Succeed?

When the command is a shell function, you shouldn't use if myfunc directly. This is because shell has the Disabled errexit Quirk, which is detected by Oil's strict_errexit.

No:

if myfunc; then  # errors not checked in body of myfunc
  echo 'success'
fi

Yes. The $0 Dispatch Pattern is a workaround that works in all shells.

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

"$@"  # Run the function $1 with args $2, $3, ...

Yes. Oil's try builtin sets the special _status variable and returns 0.

try myfunc  # doesn't abort
if (_status === 0) {
  echo 'success'
fi

try Also Takes a Block

A block arg is useful for multiple commands:

try {              # stops at the first error
  chmod +x myfile
  cp myfile /bin
}
if (_status !== 0) {
  echo 'error'
}

Does a Pipeline Succeed?

No:

if ps | grep python; then
  echo 'found'
fi

This is technically correct when pipefail is on, but it's impossible for Oil's strict_errexit to distinguish it from if myfunc | grep python ahead of time (the "meta" pitfall). If you know what you're doing, you can disable strict_errexit.

Yes:

try {
  ps | grep python
}
if (_status === 0) {
  echo 'found'
}

# You can also examine the status of each part of the pipeline
if (_pipeline_status[0] !== 0) {
  echo 'ps failed'
}

Does a Command With Process Subs Succeed?

Similar to the pipeline example above:

No:

if ! comm <(sort left.txt) <(sort right.txt); then
  echo 'error'
fi

Yes:

try {
  comm <(sort left.txt) <(sort right.txt)
}
if (_status !== 0) {
  echo 'error'
}

# You can also examine the status of each process sub
if (_process_sub_status[0] !== 0) {
  echo 'first process sub failed'
}

(I used comm in this example because it doesn't have a true / false / error status like diff.)

Handle Errors in Oil Expressions

try {
  var x = 42 / 0
  echo "result is $[42 / 0]"
}
if (_status !== 0) {
  echo 'divide by zero'
}

Test Boolean Statuses, like grep, diff, test

Oil's boolstatus builtin distinguishes error from false.

No, this is subtly wrong. grep has 3 different return values.

if grep 'class' *.py {       
  echo 'found'               # status 0 means found
} else {
  echo 'not found OR ERROR'  # any non-zero status
}

Yes. boolstatus aborts the program if egrep doesn't return 0 or 1.

if boolstatus grep 'class' *.py {  # may abort
  echo 'found'               # status 0 means found
} else {
  echo 'not found'           # status 1 means not found
}

More flexible style:

try grep 'class' *.py
case $_status {
  (0) echo 'found'
      ;;
  (1) echo 'not found'
      ;;
  (*) echo 'fatal'
      exit $_status
      ;;
}

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

Arrays in Oil look like %(my array) and ['my', 'array'].

No:

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

Yes:

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

var same = ['one', 'two', 'three']
var typed = [1, 2, true, false, null]

Initialize and Assign Dicts

Dicts in Oil look like {key: 'value'}.

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'

Get Values From Arrays and Dicts

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 [[ $x =~ foo-([[:digit:]]+) ]] {
  echo "${BASH_REMATCH[1]}"  # first submatch
}

Yes:

if (x ~ / 'foo-' <d+> /) {   # <> is capture
  echo $_match(1)             # first submatch
}

Glob Matching

No:

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

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

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.
• What Breaks When You Upgrade to Oil. Shell constructs that Oil users should avoid.
• Oil Fixes Shell's Error Handling (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.