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This chapter describes "mini-languages" like glob patterns and brace expansion.

In contrast, the main sub languages of YSH are command, word, and expression.

(in progress)

Other Shell Sublanguages

Arithmetic

arith-context

Arithmetic expressions are parsed and evaluated in many parts of POSIX shell and bash.

Static:

a=$(( x + 1 ))  # POSIX shell

# bash
(( a = x + 1 ))

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

Dynamic:

[[ 5 -eq 3+x ]]   # but not test  5 -eq 3+x

Array index contexts:

echo ${a[i+1]}      # get
echo ${#a[i+1]}     # calculate

a[i+1]=foo          # set

printf -v 'a[i+1]'  # assign to this location
unset 'a[i+1]'      # unset location

echo ${a[@] : i+1 : i+2 }  # bash slicing

bash allows similar array expressions with test -v:

test -v 'array[i+1]'       # is array item set?
test -v 'assoc[$myvar]'    # is assoc array key set?

[[ -v 'array[i+1]' ]]      # ditto
[[ -v 'assoc[$myvar]' ]]

But OSH allows only integers and "bare" string constants:

test -v 'array[42]'        # is array item set?
test -v 'assoc[key]'       # is assoc array key set?

sh-numbers

sh-arith

sh-logical

sh-bitwise

Boolean

bool-expr

Boolean expressions can be use the test builtin:

test ! $x -a $y -o $z

Or the [[ command language:

[[ ! $x && $y || $z ]]

bool-infix

Examples:

test $a -nt $b
test $x == $y

bool-path

Example:

test -d /etc
test -e /
test -f myfile

YSH has long flags:

test --dir /etc
test --exists /
test --file myfile

bool-str

test -n foo  # => status 0 / true -- foo is non-empty
test -z ''   # => status 0 / true -- '' is empty / zero-length

bool-other

Test if a shell option is set:

test -o errexit      

Test the values of variables:

test -v var_name     # is variable defined?
test -v name[index]  # is an entry in a container set?

Notes:

• In name[index], OSH doesn't allow arithmetic expressions / dynamic parsing, as bash does.
• shopt --set strict_word_eval exposes "syntax errors" in name[index], and is recommended.
  • Without this option, test -v will silently return 1 (false) when given nonsense input, like test -v /.

Patterns

glob-pat

Glob patterns look like:

echo *.py    # Ends with .py
echo *.[ch]  # Ends with .c or .h

This syntax is used in:

• "Array of words" contexts
  • simple-command - like echo *.py
  • bash arrays a=( *.py )
  • YSH arrays var a = :| *.py |
  • for loops for x in *.py; do ...
• case patterns
• dbracket - [[ x == *.py ]]
• Word operations
  • op-strip - ${x#*.py}
  • op-patsub - ${x//*.py/replace} -

extglob

Extended globs let you use logical operations with globs.

They may be slow. Regexes and eggexes are preferred.

echo @(*.cc|*.h)   # Show files ending with .cc or .h
echo !(*.cc|*.h)   # Show every file that does NOT end with .cc or .h

Extended globs can appear in most of the places globs can, except op-patsub (because we implement it by translating.

regex

POSIX ERE (extended regular expressions) are part of bash's dbracket:

x=123
if [[ x =~ '[0-9]+ ]]; then
  echo 'looks like a number'
fi

Other Sublang

braces

Brace expansion saves you typing:

$ echo {foo,bar}@example.com
foo@example.com bar@example.com

You can use it with number ranges:

$ echo foo{1..3}
foo1 foo2 foo3

(The numbers must be constant.)

Technically, it does a cartesian product, which is 3 X 2 in this case:

$ for x in foo{1..3}-{X,Y}; do echo $x; done
foo1-X
foo1-Y
foo2-X
foo2-Y
foo3-X
foo3-Y

histsub

History substitution uses !.

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.