Warning: Work in progress! Leave feedback on Zulip or Github if you'd like this doc to be updated.

OSH Help

This doc describes every aspect of OSH briefly. It underlies the help builtin, and is indexed by keywords.

Navigate it with the index of OSH help topics.

Table of Contents
Overview
Usage
Lexing
Command Language
Commands
Conditional
Iteration
Control Flow
Grouping
Concurrency
Redirects
Other Command
Assigning Variables
Operators
Compound Data
Builtins
Word Language
Quotes
Substitutions
Var Ops
Other Shell Sublanguages
Arithmetic
Boolean
Patterns
Other Sublang
Builtin Commands
I/O
Run Code
Set Options
Working Dir
Completion
Shell Process
Child Process
External
Introspection
Word Lookup
Interactive
Shell Options
Errors
Globbing
Debugging
Interactive
Other Option
Environment Variables
Shell Options
Other Env
Oil Paths
Special Variables
Special
POSIX Special
Other Special
Oil Special
Platform
Call Stack
Tracing
Process State
Process Stack
Shell State
Completion
Functions
Other Special
Plugins and Hooks
Signals
Traps
Words
Completion
Other Plugin

Overview

Usage

bin/osh Usage

Usage: osh [OPTION]... SCRIPT [ARG]...
       osh [OPTION]... -c COMMAND [ARG]...

The command line accepted by bin/osh is compatible with /bin/sh and bash.

osh -c 'echo hi'
osh myscript.sh
echo 'echo hi' | osh

It also has a few enhancements:

osh -n -c 'hello'                    # pretty-print the AST
osh --ast-format text -n -c 'hello'  # print it full

osh accepts POSIX sh flags, with these additions:

-n parse the program but don't execute it. Print the AST. --ast-format what format the AST should be in

Configuring the Shell

If the --rcfile flag is specified, that file will be executed on startup. Otherwise:

Pass --rcfile /dev/null to disable this behavior.

Startup Files

History is read?

Lexing

comment

A comment starts with # and goes until the end of the line.

echo hi  # print a greeting

line-continuation

A backslash \ at the end of a line continues the line without executing it:

ls /usr/bin \
   /usr/lib \
   ~/src        # A single command split over three lines

Command Language

Commands

simple-command

Commands are composed of words. The first word may by the name of a shell builtin, an Oil proc / shell "function", an external command, or an alias:

echo hi               # a shell builtin doesn't start a process
ls /usr/bin ~/src     # starts a new process
myproc "hello $name"
myshellfunc "hello $name"
myalias -l

Redirects are also allowed in any part of the command:

echo 'to stderr' >&2
echo >&2 'to stderr'

echo 'to file' > out.txt
echo > out.txt 'to file'

semicolon ;

Run two commands in sequence like this:

echo one; echo two

or this:

echo one
echo two

Conditional

case

Match a string against a series of glob patterns. Execute code in the section below the matching pattern.

path='foo.py'
case "$path" in
  *.py)
    echo 'python'
    ;;
  *.sh)
    echo 'shell'
    ;;
esac

if

Test if a command exited with status zero (true). If so, execute the corresponding block of code.

Shell:

if test -d foo; then
  echo 'foo is a directory'
elif test -f foo; then
  echo 'foo is a file'
else
  echo 'neither'
fi

Oil:

if test -d foo {
  echo 'foo is a directory'
} elif test -f foo {
  echo 'foo is a file'
} else {
  echo 'neither'
}

true

Do nothing and return status 0.

if true; then
  echo hello
fi

false

Do nothing and return status 1.

if false; then
  echo 'not reached'
else
  echo hello
fi

colon

Like true: do nothing and return status 0.

bang

Invert an exit code:

if ! test -d /tmp; then   
  echo "No temp directory
fi

and

mkdir -p /tmp && cp foo /tmp

or

ls || die "failed"

Iteration

while

POSIX

until

POSIX

for

For loops iterate over words.

Oil style:

var mystr = 'one'
var myarray = %(two three)

for i in $mystr @myarray *.py {
  echo $i
}

Shell style:

local mystr='one'
local myarray=(two three)

for i in "mystr" "${myarray[@]}" *.py; do
  echo $i
done

Both fragments output 3 lines and then Python files on remaining lines.

for-expr-sh

A bash/ksh construct:

for (( i = 0; i < 5; ++i )); do
  echo $i
done

Control Flow

These are keywords in Oil, not builtins!

break

Break out of a loop. (Not used for case statements!)

continue

Continue to the next iteration of a loop.

return

Return from a function.

exit

Exit the shell process with the given status:

exit 2

Grouping

sh-func

POSIX:

f() {
  echo args "$@"
}
f 1 2 3

sh-block

POSIX:

{ echo one; echo two; }

Note the trailing ; -- which isn't necessary in Oil.

subshell

( echo one; echo two )

Use forkwait in Oil instead.

Concurrency

pipe

ampersand

CMD &

The & language construct runs CMD in the background as a job, immediately returning control to the shell.

The resulting PID is recorded in the $! variable.

Redirects

redir-file

Three variants of redirecting stdout:

echo foo > out.txt    # write to a file
echo foo >> out.txt   # append to a file
echo foo >| out.txt   # clobber the file even if set -o noclobber

Redirect stdin:

cat < in.txt

redir-desc

Redirect to a file descriptor:

echo 'to stderr' >&2

here-doc

TODO: unbalanced HTML if we use <<?

cat <<EOF
here doc with $double ${quoted} substitution
EOF

myfunc() {
        cat <<-EOF
        here doc with one tab leading tab stripped
        EOF
}

cat <<< 'here string'

Other Command

dparen ((

h4

time [-p] pipeline

Measures the time taken by a command / pipeline. It uses the getrusage() function from libc.

Note that time is a KEYWORD, not a builtin!

Assigning Variables

Operators

sh-assign

sh-append

Compound Data

sh-array

Array literals in shell accept any sequence of words, just like a command does:

ls $mystr "$@" *.py

# Put it in an array
a=(ls $mystr "$@" *.py)

In Oil, use oil-array.

sh-assoc

In Oil, use oil-dict.

Builtins

local

export

unset

shift

declare

typeset

Alias for declare.

Word Language

Quotes

quotes

Also see oil-string.

Substitutions

com-sub

Evaluates to the stdout of a command. If a trailing newline is returned, it's stripped:

$ hostname
example.com

$ x=$(hostname)
$ echo $x
example.com

var-sub

Evaluates to the value of a variable:

$ x=X
$ echo $x ${x}
X X

arith-sub

Shell has C-style arithmetic:

$ echo $(( 1 + 2*3 ))
7

tilde-sub

Used as a shortcut for a user's home directory:

~/src     # my home dir
~bob/src  # user bob's home dir

Var Ops

op-test

op-strip

op-replace

op-index

${a[i+1]}

op-slice

op-format

${x@P} evaluates x as a prompt string, e.g. the string that would be printed if PS1=$x.

Other Shell Sublanguages

Arithmetic

arith-context

sh-numbers

sh-arith

sh-logical

sh-bitwise

Boolean

dbracket

Compatible with bash.

bool-expr

bool-infix

bool-path

bool-str

bool-other

Patterns

glob

extglob

regex

Part of dbracket

Other Sublang

braces

histsub

char-escapes

These backslash escape sequences are used in echo -e, printf, and in C-style strings like $'foo\n':

\\         backslash
\a         alert (BEL)
\b         backspace
\c         stop processing remaining input
\e         the escape character \x1b
\f         form feed
\n         newline
\r         carriage return
\t         tab
\v         vertical tab
\xHH       the byte with value HH, in hexadecimal
\uHHHH     the unicode char with value HHHH, in hexadecimal
\UHHHHHHHH the unicode char with value HHHHHHHH, in hexadecimal

Also:

\"         Double quote.

Inconsistent octal escapes:

\0NNN      echo -e '\0123'
\NNN       printf '\123'
           echo $'\123'

TODO: Verify other differences between echo -e, printf, and $''. See frontend/lexer_def.py.

Builtin Commands

I/O

These builtins take input and output. They're often used with redirects.

read

read FLAG* VAR*

Read a line from stdin, split it into tokens with the $IFS algorithm, and assign the tokens to the given variables. When no VARs are given, assign to $REPLY.

Note: When writing Oil, prefer the extensions documented in oil-read. The read builtin is confusing because -r needs to be explicitly enabled.

Flags:

-a ARRAY  assign the tokens to elements of this array
-d CHAR   use DELIM as delimiter, instead of newline
-n NUM    read up to NUM characters, respecting delimiters
-p STR    print the string PROMPT before reading input
-r        raw mode: don't let backslashes escape characters
-s        silent: do not echo input coming from a terminal
-t NUM    time out and fail after TIME seconds
          -t 0 returns whether any input is available
-u FD     read from file descriptor FD instead of 0 (stdin)

echo

echo FLAG* ARG*

Prints ARGs to stdout, separated by a space, and terminated by a newline.

Flags:

-e  enable interpretation of backslash escapes
-n  omit the trailing newline

See char-escapes.

printf

printf FLAG* FMT ARG*

Formats values and prints them. The FMT string contain three types of objects:

  1. Literal Characters
  2. Character escapes like \t. See char-escapes.
  3. Percent codes like %s that specify how to format each each ARG.

If not enough ARGS are passed, the empty string is used. If too many are passed, the FMT string will be "recycled".

Flags:

-v VAR  Write output in variable VAR instead of standard output.

Format specifiers:

%%  Prints a single "%".
%b  Interprets backslash escapes while printing.
%q  Prints the argument escaping the characters needed to make it reusable
    as shell input.
%d  Print as signed decimal number.
%i  Same as %d.
%o  Print as unsigned octal number.
%u  Print as unsigned decimal number.
%x  Print as unsigned hexadecimal numbe with lower-case hex-digits (a-f).
%X  Same as %x, but with upper-case hex-digits (A-F).
%f  Print as floating point number.
%e  Print as a double number, in "±e" format (lower-case e).
%E  Same as %e, but with an upper-case E.
%g  Interprets the argument as double, but prints it like %f or %e.
%G  Same as %g, but print it like %E.
%c  Print as a single char, onlye the first character is printed.
%s  Print as string
%n  The number of characters printed so far is stored in the variable named
    in the argument.
%a  Interprets the argument as double, and prints it like a C99 hexadecimal
    floating-point literal.
%A  Same as %a, but print it like %E.
%(FORMAT)T  Prints date and time, according to FORMAT as a format string
            for strftime(3). The argument is the number of seconds since
            epoch. It can also be -1 (current time, also the default value
            if there is no argument) or -2 (shell startup time).

readarray

Alias for mapfile.

mapfile

mapfile FLAG* ARRAY?

Reads lines from stdin into the variable named ARRAY (default ${MAPFILE[@]}).

Flags:

-t       Remove the trailing newline from every line

Run Code

source

source SCRIPT ARG*

Executes SCRIPT with given ARGs in the context of the current shell. It will modify existing variables.

eval

eval ARG+

Creates a string by joining ARGs with a space, then runs it as a shell command.

Example:

 # Create the string echo "hello $name" and run it.
 a='echo'
 b='"hello $name"'
 eval $a $b

Tips:

eval is usually unnecessary in Oil code. Using it can confuse code and user-supplied data, leading to security issues.

Prefer passing single string ARG to eval.

trap

trap FLAG* CMD SIGNAL*

Registers the shell string CMD to be run after the SIGNALs are received. If the CMD is empty, then the signal is ignored.

Flags:

-l  Lists all signals and their signal number
-p  Prints a list of the installed signal handlers

Tip:

Prefer passing the name of a shell function to trap.

Set Options

set

The set builtin modifies the shell's configuration and behavior.

shopt

The shopt builtin configures the shell.

Working Dir

cd

cd FLAG* DIR

Changes the working directory of the current shell process to DIR.

If DIR isn't specified, change to $HOME. If DIR is -, change to $OLDPWD (a variable that the sets to the previous working directory.)

Flags:

-L  Follow symbolic links, i.e. change to the TARGET of the symlink.
    (default).
-P  Don't follow symbolic links.

pwd

pwd FLAG*

Prints the current working directory.

Flags:

-L  Follow symbolic links if present (default)
-P  Don't follow symbolic links.  Print the link instead of the target.

pushd

pushd DIR

Add DIR to the directory stack, then change the working directory to DIR. Typically used with popd and dirs.

popd

popd

Removes a directory from the directory stack, and changes the working directory to it. Typically used with pushd and dirs.

dirs

dirs FLAG*

Shows the contents of the directory stack. Typically used with pushd and popd.

Flags:

-c  Clear the dir stack.
-l  Show the dir stack, but with the real path instead of ~.
-p  Show the dir stack, but formatted as one line per entry.
-v  Like -p, but numbering each line.

Completion

complete

The complete builtin registers completion policies for different commands.

compgen

The compgen builtin generates completion candidates inside a user-defined completion function.

It can also be used in scripts, i.e. outside a completion function.

compopt

The compopt builtin changes completion options inside a user-defined completion function.

compadjust

The compadjust builtin adjusts COMP_ARGV according to specified delimiters, and optionally set variables cur, prev, words (an array), and cword. May also set 'split'.

This is an OSH extension that makes it easier to run the bash-completion project.

Shell Process

exec

exec BIN_PATH ARG*

Replaces the running shell with the binary specified, which is passed ARGs. BIN_PATH must exist on the file system; i.e. it can't be a shell builtin or function.

umask

umask MODE?

Sets the bit mask that determines the permissions for new files and directories. The mask is subtracted from 666 for files and 777 for directories.

Oil currently supports writing masks in octal.

If no MODE, show the current mask.

times

times

Shows the user and system time used by the shell and its child processes.

Child Process

jobs

jobs

Shows all jobs running in the shell and their status.

wait

wait FLAG* ARG

Wait for processes to exit.

If the ARG is a PID, wait only for that job, and return its status.

If there's no ARG, wait for all child processes.

Flags:

-n  Wait for the next process to exit, rather than a specific process.

Wait can be interrupted by a signal, in which case the exit code indicates the signal number.

fg

fg JOB?

Returns a job running in the background to the foreground. If no JOB is specified, use the latest job.

External

test

test OP ARG
test ARG OP ARG
[ OP ARG ]      # [ is an alias for test that requires closing ]
[ ARG OP ARG ]

Evaluates a conditional expression and returns 0 (true) or 1 (false).

Note that [ is the name of a builtin, not an operator in the language. Use 'test' to avoid this confusion.

String expressions:

-n STR           True if STR is not empty.
                 'test STR' is usually equivalent, but discouraged.
-z STR           True if STR is empty.
STR1 = STR2      True if the strings are equal.
STR1 != STR2     True if the strings are not equal.
STR1 < STR2      True if STR1 sorts before STR2 lexicographically.
STR1 > STR2      True if STR1 sorts after STR2 lexicographically.
                 Note: < and > should be quoted like \< and \>

File expressions:

-a FILE          Synonym for -e.
-b FILE          True if FILE is a block special file.
-c FILE          True if FILE is a character special file.
-d FILE          True if FILE is a directory.
-e FILE          True if FILE exists.
-f FILE          True if FILE is a regular file.
-g FILE          True if FILE has the sgid bit set.
-G FILE          True if current user's group is also FILE's group.
-h FILE          True if FILE is a symbolic link.
-L FILE          True if FILE is a symbolic link.
-k FILE          True if FILE has the sticky bit set.
-O FILE          True if current user is the file owner.
-p FILE          True if FILE is a named pipe (FIFO).
-r FILE          True if FILE is readable.
-s FILE          True if FILE has size bigger than 0.
-S FILE          True if FILE is a socket file.
-t FD            True if file descriptor FD is open and refers to a terminal.
-u FILE          True if FILE has suid bit set.
-w FILE          True if FILE is writable.
-x FILE          True if FILE is executable.
FILE1 -nt FILE2  True if FILE1 is newer than FILE2 (mtime).
FILE1 -ot FILE2  True if FILE1 is older than FILE2 (mtime).
FILE1 -ef FILE2  True if FILE1 is a hard link to FILE2.

Arithmetic expressions coerce arguments to integers, then compare:

INT1 -eq INT2    True if they're equal.
INT1 -ne INT2    True if they're not equal.
INT1 -lt INT2    True if INT1 is less than INT2.
INT1 -le INT2    True if INT1 is less or equal than INT2.
INT1 -gt INT2    True if INT1 is greater than INT2.
INT1 -ge INT2    True if INT1 is greater or equal than INT2.

Other expressions:

-o OPTION        True if the shell option OPTION is set.
-v VAR           True if the variable VAR is set.

The test builtin also supports POSIX conditionals like -a, -o, !, and ( ), but these are discouraged.

Oil supports these long flags:

--dir            same as -d
--exists         same as -e
--file           same as -f
--symlink        same as -L

getopts

getopts SPEC VAR ARG*

A single iteration of flag parsing. The SPEC is a sequence of flag characters, with a trailing : to indicate that the flag takes an argument:

ab    # accept  -a and -b
xy:z  # accept -x, -y arg, and -z

The input is "$@" by default, unless ARGs are passed.

On each iteration, the flag character is stored in VAR. If the flag has an argument, it's stored in $OPTARG. When an error occurs, VAR is set to ? and $OPTARG is unset.

Returns 0 if a flag is parsed, or 1 on end of input or another error.

Example:

while getopts "ab:" flag; do
    case $flag in
        a)   flag_a=1 ;;
        b)   flag_b=$OPTARG" ;;
        '?') echo 'Invalid Syntax'; break ;;
    esac
done

Notes:

kill

TODO

enable

Bash has this, but OSH won't implement it.

Introspection

help

help oil             # list Oil language help topics
help osh             # list OSH language help topics

help TOPIC           # show help on a given topic

help osh-usage       # same as osh --help
help oil-usage       # same as oil --help

View on the web:

https://www.oilshell.org/release/$VERSION/doc/

hash

hash

Display information about remembered commands.

hash FLAG* CMD+

Determine the locations of commands using $PATH, and remember them.

Flag:

-r       Discard all remembered locations.

type

type FLAG* NAME*

Print the type of each NAME. Is it a keyword, shell builtin, shell function, alias, or executable file?

Flags:

-f  Don't look for functions
-P  Only look for executable files in $PATH
-t  Print a single word: alias, builtin, file, function, or keyword

Word Lookup

command

command FLAG* CMD ARG*

Look up CMD as a shell builtin or executable file, and execute it with the given ARGs. That is, the lookup ignores shell functions named CMD.

Flags:

-v  Instead of executing CMD, print a description of it.
    Similar to the 'type' builtin.

builtin

builtin CMD ARG*

Look up CMD as a shell builtin, and execute it with the given ARGs. That is, the lookup ignores shell functions and executables named CMD.

Interactive

alias

alias NAME=CODE

Make NAME a shortcut for executing CODE, e.g. alias hi='echo hello'.

alias NAME

Show the value of this alias.

alias

Show a list of all aliases.

Tips:

Prefer shell functions like:

ls() {
  command ls --color "$@"
}

to aliases like:

alias ls='ls --color'

Functions are less likely to cause parsing problems.

TODO: Turn off aliases in Oil.

unalias

unalias NAME

Remove the alias NAME.

history

history FLAG*

Display and manipulate the shell's history entries.

history NUM

Show the last NUM history entries.

Flags:

-c      Clears the history.
-d POS  Deletes the history entry at position POS.

Shell Options

Errors

Globbing

nullglob

Normally, when no files match a glob, the glob itself is returned:

$ echo L *.py R  # no Python files in this dir
L *.py R

With nullglob on, the glob expands to no arguments:

shopt -s nullglob
$ echo L *.py R
L R

(This option is in GNU bash as well.)

dashglob

Do globs return results that start with -? It's on by default in bin/osh, but off when Oil is enabled.

Turning it off prevents a command like rm * from being confused by a file called -rf.

$ touch -- myfile -rf

$ echo *
-rf myfile

$ shopt -u dashglob
$ echo *
myfile

Debugging

Interactive

Other Option

Environment Variables

Shell Options

SHELLOPTS

For the 'set' builtin.

BASHOPTS

For the 'shopt' builtin.

Other Env

HOME

$HOME is used for:

  1. ~ expansion
  2. ~ abbreviation in the UI (the dirs builtin, \W in $PS1).

Note: The shell doesn't set $HOME. According to POSIX, the program that invokes the login shell sets it based on /etc/passwd.

PATH

A colon-separated string that's used to find executables to run.

IFS

Used for word splitting. And the builtin split() function.

Oil Paths

Special Variables

Special

POSIX Special

Other Special

Oil Special

Platform

Call Stack

Tracing

Process State

Process Stack

Shell State

Completion

COMP_WORDS

An array of words, split by : and = for compatibility with bash. New completion scripts should use COMP_ARGV instead.

COMP_CWORD

Discouraged; for compatibility with bash.

COMP_LINE

Discouraged; for compatibility with bash.

COMP_POINT

Discouraged; for compatibility with bash.

COMPREPLY

User-defined completion functions should Fill this array with candidates. It is cleared on every completion request.

COMP_ARGV

An array of partial command arguments to complete. Preferred over COMP_WORDS. The compadjust builtin uses this variable.

(An OSH extension to bash.)

Functions

Other Special

Plugins and Hooks

Signals

Traps

Words

PS1

First line of a prompt.

PS2

Second line of a prompt.

PS3

For the 'select' builtin (unimplemented).

PS4

For 'set -o xtrace'. The leading character is special.

Completion

Other Plugin


Generated on Sat Feb 19 18:43:17 EST 2022