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nix-super/doc/manual/src/command-ref/nix-hash.md
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Name

nix-hash - compute the cryptographic hash of a path

Synopsis

nix-hash [--flat] [--base32] [--truncate] [--type hashAlgo] path…

nix-hash --to-base16 hash…

nix-hash --to-base32 hash…

Description

The command nix-hash computes the cryptographic hash of the contents of each path and prints it on standard output. By default, it computes an MD5 hash, but other hash algorithms are available as well. The hash is printed in hexadecimal. To generate the same hash as nix-prefetch-url you have to specify multiple arguments, see below for an example.

The hash is computed over a serialisation of each path: a dump of the file system tree rooted at the path. This allows directories and symlinks to be hashed as well as regular files. The dump is in the NAR format produced by nix-store --dump. Thus, nix-hash path yields the same cryptographic hash as nix-store --dump path | md5sum.

Options

  • --flat
    Print the cryptographic hash of the contents of each regular file path. That is, do not compute the hash over the dump of path. The result is identical to that produced by the GNU commands md5sum and sha1sum.

  • --base32
    Print the hash in a base-32 representation rather than hexadecimal. This base-32 representation is more compact and can be used in Nix expressions (such as in calls to fetchurl).

  • --truncate
    Truncate hashes longer than 160 bits (such as SHA-256) to 160 bits.

  • --type hashAlgo
    Use the specified cryptographic hash algorithm, which can be one of md5, sha1, sha256, and sha512.

  • --to-base16
    Don’t hash anything, but convert the base-32 hash representation hash to hexadecimal.

  • --to-base32
    Don’t hash anything, but convert the hexadecimal hash representation hash to base-32.

Examples

Computing the same hash as nix-prefetch-url:

$ nix-prefetch-url file://<(echo test)
1lkgqb6fclns49861dwk9rzb6xnfkxbpws74mxnx01z9qyv1pjpj
$ nix-hash --type sha256 --flat --base32 <(echo test)
1lkgqb6fclns49861dwk9rzb6xnfkxbpws74mxnx01z9qyv1pjpj

Computing hashes:

$ mkdir test
$ echo "hello" > test/world

$ nix-hash test/ (MD5 hash; default)
8179d3caeff1869b5ba1744e5a245c04

$ nix-store --dump test/ | md5sum (for comparison)
8179d3caeff1869b5ba1744e5a245c04  -

$ nix-hash --type sha1 test/
e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6

$ nix-hash --type sha1 --base32 test/
nvd61k9nalji1zl9rrdfmsmvyyjqpzg4

$ nix-hash --type sha256 --flat test/
error: reading file `test/': Is a directory

$ nix-hash --type sha256 --flat test/world
5891b5b522d5df086d0ff0b110fbd9d21bb4fc7163af34d08286a2e846f6be03

Converting between hexadecimal and base-32:

$ nix-hash --type sha1 --to-base32 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6
nvd61k9nalji1zl9rrdfmsmvyyjqpzg4

$ nix-hash --type sha1 --to-base16 nvd61k9nalji1zl9rrdfmsmvyyjqpzg4
e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6