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switch to using --check #4

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merged 3 commits into from
Nov 1, 2022
Merged

switch to using --check #4

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94c5c97
switch to using --check
gavinuhma Oct 29, 2022
291e606
Use var for hash function
gavinuhma Nov 1, 2022
b011567
Update readme with new checksum function
gavinuhma Nov 1, 2022
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13 changes: 6 additions & 7 deletions checksum.sh
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Original file line number Diff line number Diff line change
@@ -1,18 +1,17 @@
#!/bin/bash

function checksum() {
local s
s=$(curl -fsSL "$1")
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@dundarious dundarious Oct 31, 2022 •
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Note that shell truncates trailing newlines from variables, so if a script ends with several newlines, it'll only keep the last one. Example exhibiting the problem:

# In another shell, run `python -m http.server --bind localhost 8980`
$ printf %s\\n 1 2 3 "" "" "" > f  # File contents: "1\n2\n3\n\n\n\n"
$ printf %s\\n 1 2 3 > t  # File contents: "1\n2\n3\n"
$ shasum -a 256 f
09bc489de9097269db796e13a5d79c0bdb021a4ba90e1ac1e7f56aecc60b5b7c *f
$ shasum -a 256 t
14c5e74c4b96ccef41cd94db73a9ec3348038ac094feca4fd897cecffa07cdae *t
$ curl -fsSL http://localhost:8980/f | shasum -a 256
09bc489de9097269db796e13a5d79c0bdb021a4ba90e1ac1e7f56aecc60b5b7c *-
$ curl -fsSL http://localhost:8980/t | shasum -a 256
14c5e74c4b96ccef41cd94db73a9ec3348038ac094feca4fd897cecffa07cdae *-
$ t=$(curl -fsSL http://localhost:8980/t)
$ f=$(curl -fsSL http://localhost:8980/f)
$ printf %s\\n "$t" | shasum -a 256
14c5e74c4b96ccef41cd94db73a9ec3348038ac094feca4fd897cecffa07cdae *-
$ printf %s\\n "$f" | shasum -a 256
14c5e74c4b96ccef41cd94db73a9ec3348038ac094feca4fd897cecffa07cdae *-

So printf %s\\n "$f" | shasum -a 256 does not give the same as curl -fsSL http://localhost:8980/f | shasum -a 256, it gives the same result as curl -fsSL http://localhost:8980/t | shasum -a 256 and printf %s\\n "$t" | shasum -a 256.

It's quite likely there exist scripts in the wild that have trailing newlines, so the shell variable capture is probably non-viable.

(I ran this experiment in msys zsh on windows, but ran it based on knowledge of the newline chomping of POSIX sh -- I just double-checked and ran it on bash on linux and it was the same result, only difference being the weird "*" in the shasum output is now another space as expected)

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the shell variable capture is probably non-viable.

@dundarious given that, downloading the script to a tmp file may be the best solution

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#6

if ! command -v shasum >/dev/null
then
shasum() { sha1sum "$@"; }
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sha1sum doesn't support the -a option you always give below. You just shouldn't pass -a. I only used it when contriving an example (where I had to mock having something to download which has a hash already).

And you can just use a variable for the command (you don't need eval or anything like that to do such things).

local hasher
if command -v shasum >/dev/null ; then
  hasher=shasum
else
  hasher=sha1sum
fi
...
printf %s\\n "$s" | "$hasher" --check --status ...

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Updated. Great suggestion. Thanks!

fi
c=$(printf %s\\n "$s" | shasum | awk '{print $1}')
if [ "$c" = "$2" ]
if [ ! "$2" ]
then
printf %s\\n "$s"
else
echo "invalid checksum $c != $2" 1>&2;
printf %s\\n "$s" | shasum -a 256 | awk '{print $1}'
return 1;
fi
unset s
unset c
printf %s\\n "$s" | shasum -a 256 --check --status <(printf '%s -\n' "$2") || { echo "checksum failed" >&2; return 1; }
printf %s\\n "$s"
}