Portable Shell constructs¶
Utilities used in a shell script.¶
The Gnu coding standard - Utilities in Makefiles list them:
The configure script and the Makefile rules for building and installation should not use any utilities directly except these: awk cat cmp cp diff echo egrep expr false grep install-info ln ls mkdir mv printf pwd rm rmdir sed sleep sort tar test touch tr true
Converting bash syntax to dash¶
This table is extracted from Bashism look at the original for more info.
| construct | bash | dash |
|---|---|---|
| functions | function f { echo
hello world; } |
f() { echo hello world; } |
| cases | ;;& ;& etc |
None. Duplicate the case (use a function to avoid code duplication) |
| C-like for loop | for ((i=0; i<3;
i++)); do echo "$i"
done |
i=0 ; while [ "$i" -lt 3 ];
do echo "$i" ; i=$(($i+1))
done |
| expand sequences | echo
$'hello\tworld' |
printf "hello\tworld\n" |
| extended glob | +( ) @( ) !( ) *(
) |
sometimes you can use several globs, sometimes you can use find(1) |
| select | select |
implement the menu yourself, use a command like dialog |
| Brace Expansion | {a,b,c} or
{1..10} |
|
| process substitutions | foo <(bar) |
mkfifo fifo; bar > fifo &
foo < fifo |
${name:n:l} |
$(expr "x$name" :
"x.\{,$n\}\(.\{,$l\}\)") |
|
${name/foo/bar} |
$(printf '%s\n' "$name" |
sed 's/foo/bar/') |
|
${!name} |
use eval (dangerous) | |
| arrays | positional parameters, use
IFS and set -f,
eval (dangerous) |
|
| simple test | [[ |
use [ and use double
quotes around the expansions
[ "$var" = "" ] |
| pattern matching | [[ foo = *glov
]] |
use case or expr or grep |
| equality with test | == |
use = instead |
| compare lexicographically. | <> |
no change |
| modification times | [[ file1 -nt file2
]] or -ot |
[ "$(find 'file1' -prune
-newer 'file2')" ] or [
"file1" -nt "file2" ] |
| files are the same hardlink | [[ file1 -ef file2
]] |
[ "file1" -ef "file2" ] |
| (( )) | var=((3+1)) |
var=$((3+1)) |
| (( )) | ((3 + 1 < 5)) |
[ $((3 + 1)) -lt 5 ] or
[ "$((3 + 1 < 5))" -ne 0 ] |
| pre/post increment/decrement | ++ -- |
i=$((i+1)) or
: $((i+=1)) |
| comma operator | , (ok in ash) |
: "$((...))"; cmd
"$((...))" |
| exponentiation | ** (ok in ash) |
|
| let | let x=15;
let x-- |
x=15; x=$((x - 1)) |
| redirect both stdout and stderr | >& and &> |
command > file 2>&1 or
command 2>&1 | command2 |
| duplicate and close | m>&n- m<&n- |
m>&n n>&- |
| herestring | <<<"string" |
echo "string"| command |
| source | source
lib/xxx.sh |
. lib/xxx.sh |
expand ~ in PATH |
PATH="~/bin:$PATH" |
`` PATH=”$HOME/bin:$PATH”`` |