specifics.md

Things not working in all modern shells

Most often the common denominator is dash(1) (or e.g. bsd equivalents -- so let's start with such) examples:

dollar-single

e.g. var=$'line 1\nline 2 with\ttab'

(notice "' quote switch before ${#var} in lines below -- as an attempt to simplify presentation)

$ dash -c "var=\$'\n'; printf '%s %s\n' "' ${#var} $var'
3 $\n

$ bash -c "var=\$'\n'; printf '%s %s\n' "' ${#var} $var'
1

$ zsh -c "var=\$'\n'; printf '%s %s\n' "' ${#var} $var'
1
 

By default zsh does not use $IFS to split (and to drop $IFS characters on) unquoted $variables, so there is one "extra" newline above in zsh output.

[[ ]]

Not in dash, present in bash, zsh & ksh (might have sligthly different behaviour, to be tested).

todo: examples

test "$var1" == "$var2"

To test (string) equality, the comparison operator is single =. Use it! (also as [ "$var1" = "$var2" ]) The test 16 (test_eqeq) in portabilitytest shows that in (linux) shells, dash(1) does not support double equals sign.

function keyword

function keyword when defining functions is not supported in dash, and is optional in rest of the shells (where supported). Better not use it.

using ^ to negate in case pattern

Does not work in dash (may even not be defined in any standard). there is no need to use it, [!...] works in all modern shells. See "Default deny" in snippets for an example.

stdout & stderr redirection

While convenient in bash and zsh interactive usage, &> (or >&) should not be used in shell scripts, use

sh -c '(echo to-out; echo to-err >&2) >/dev/null 2>&1'

(i.e. >/dev/null 2>&1, the above is test example) instead (play by removing redirections in the example).

$(< file)

Many shells have internal cat command substitution that replaces e.g. var=$(cat file). The replacement syntax var=$(cat file) sure is faster, so in scripts that does not need to be portable to all modern shells it can be used. Those are easy to recognize and get replaced if need arises (sed 's/$(< */$(cat /g' seems to work).

something not working in bash

Single command shell function:

$ dash -c 'pl () printf %s\\n "$@"; pl einz deux kol'
einz
deux
kol

$ bash -c 'pl () printf %s\\n "$@"; pl einz deux kol'
bash: -c: line 0: syntax error near unexpected token `printf'
bash: -c: line 0: `pl () printf %s\\n "$@"; pl einz deux kol'

$ zsh -c 'pl () printf %s\\n "$@"; pl einz deux kol'
einz
deux
kol

$ ksh -c 'pl () printf %s\\n "$@"; pl einz deux kol'
einz
deux
kol