Bash Question
Get exact output of a shell command
The
bash
Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted.
Demonstration - 3 characters, newlines first:
$ output="$(printf "\n\nx")"; echo -n "$output" | wc -c
3
Here the newlines are not at the end, and do not get removed, so the count is 3.
Demonstration - 3 characters, newlines last:
$ output="$(printf "x\n\n")"; echo -n "$output" | wc -c
1
Here the newlines are removed from the end, so the count is 1.
TL;DR
What is a robust work-around to get the binary-clean output of a command into a variable?
Bonus points for Bourne shell compatibility.
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Answer Source
The only way to do it in a "Bourne compatible" way is to use external utilities.
Beside writting one in c, you can use xxd and expr (for example):
$ output="$(printf "x\n\n"; printf "X")" # get the output ending in "X".
$ printf '%s' "${output}" | xxd -p # transform the string to hex.
780a0a58
$ hexstr="$(printf '%s' "${output}" | xxd -p)" # capture the hex
$ expr "$hexstr" : '\(.*\)..' # remove the last two hex ("X").
780a0a
$ hexstr="$(expr "$hexstr" : '\(.*\)..') # capture the shorter str.
$ printf "$hexstr" | xxd -p -r | wc -c # convert back to binary.
3
Shortened:
$ output="$(printf "x\n\n"; printf "X")"
$ hexstr="$(printf '%s' "${output}" | xxd -p )"
$ expr "$hexstr" : '\(.*\)..' | xxd -p -r | wc -c
3
The command xxd is being used for its ability to convert back to binary.
Note that wc will fail with many UNICODE characters (multibyte chars):
$ printf "Voilà" | wc -c
6
$ printf "★" | wc -c
3
It will print the count of bytes, not characters.
The length of a variable ${#var} will also fail in older shells.
Of course, to get this to run in a Bourne shell you must use `…` instead of $(…).
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