Shell Scripts: Difference between revisions

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@@ Line 14: / Line 14: @@
        '';
      };
+</syntaxHighlight>
+== External builder.sh script ==
+Longer bash scripts are usually stored as external script files, and called from Nix:
+<syntaxHighlight lang="nix">
+# default.nix
+{
+  outputTxtDrv = stdenv.mkDerivation rec {
+    name = "output.txt";
+    # disable unpackPhase etc
+    phases = "buildPhase";
+    builder = ./builder.sh;
+    nativeBuildInputs = [ coreutils jq ];
+    PATH = lib.makeBinPath nativeBuildInputs;
+    # only strings can be passed to builder
+    someString = "hello";
+    someNumber = builtins.toString 42;
+    someJson = builtins.toJSON { dst = "world"; };
+  };
+}
+</syntaxHighlight>
+<syntaxHighlight lang="bash">
+# builder.sh
+echo "$someString $(echo "$someJson" | jq -r '.dst') $someNumber" >$out
+</syntaxHighlight>
+See also
+* [https://github.com/NixOS/nixpkgs/issues/23099 make a derivation with no source]
+* [https://nixos.org/guides/nix-pills/working-derivation.html Nix Pills: Chapter 7. Working Derivation]
+=== runCommand + builder.sh ===
+Instead of <code>stdenv.mkDerivation</code>, we can also use <code>runCommand</code> to call an external bash script:
+<syntaxHighlight lang="nix">
+# default.nix
+{
+  outputTxtDrv = runCommand "output.txt" {
+    nativeBuildInputs = [ coreutils jq ];
+    # only strings can be passed to builder
+    someString = "hello";
+    someNumber = builtins.toString 42;
+    someJson = builtins.toJSON { dst = "world"; };
+  } (builtins.readFile ./builder.sh);
+}
 </syntaxHighlight>
@@ Line 48: / Line 97: @@
          --prefix PATH : ${lib.makeBinPath [ bash subversion ]}
      '';
-   };
+   }
+</syntaxHighlight>
+<code>wrapProgram</code> will move the original script to <code>.github-downloader.sh-wrapped</code>
+=== Command not found ===
+for example, the script throws the error <code>svn: command not found</code>, because the dependency <code>subversion</code> is missing.
+when a command is missing, you can use <code>nix-locate</code> to find the package name. for example, the <code>stat</code> command:
+<syntaxHighlight lang="console">
+$ nix-locate bin/stat | grep 'bin/stat$'
+coreutils.out                                         0 s /nix/store/vr96j3cxj75xsczl8pzrgsv1k57hcxyp-coreutils-8.31/bin/stat
 </syntaxHighlight>
-when the dependency <code>subversion</code> is missing, the script throws the error <code>svn: command not found</code>
+== Debugging embedded scripts ==
+When a bash script fails, it prints only an error message, but no code location.
+To trace commands and line numbers, we can use
+<syntaxHighlight lang="nix">
+# test-trace.nix
+{ runCommand, coreutils }:
+  runCommand "output.txt" {
+    nativeBuildInputs = [ coreutils ];
+  } ''
+    # line 5 in nix file = line 1 in bash script -> offset 4
+    PS4='+ Line $(expr $LINENO + 4): '
+    set -o xtrace # print commands
+    echo hello >$out # line 9 in nix file
+    set +o xtrace # hide commands
+  ''
+</syntaxHighlight>
-<code>wrapProgram</code> will move the original script to <code>.github-downloader.sh-wrapped</code>
+<syntaxHighlight lang="console">
+$ nix-build -E 'with import <nixpkgs> { }; callPackage ./test-trace.nix { }'
+this derivation will be built:
+  /nix/store/2v5biwny8plpyk2bv6cfr41ppp0a1i4k-output.txt.drv
+building '/nix/store/2v5biwny8plpyk2bv6cfr41ppp0a1i4k-output.txt.drv'...
+++ Line 9: echo hello
+++ Line 11: set +o xtrace
+/nix/store/ppidmnpd5m762x9kqj8jd3g7df7dknrz-output.txt
+</syntaxHighlight>
 == Posix Shell ==
@@ Line 64: / Line 154: @@
 * [https://nixos.org/manual/nixpkgs/stable/#ssec-stdenv-functions Shell functions section in the Nixpkgs manual]
+* [https://gist.github.com/travisbhartwell/f972aab227306edfcfea nix-shell and Shebang Lines]
+* [https://ertt.ca/nix/shell-scripts/ Shell Scripts with Nix]