diff --git a/pkgs/applications/science/math/sage/spkg-giac.patch b/pkgs/applications/science/math/sage/spkg-giac.patch
index 15b91433d254..c79d4422133d 100644
--- a/pkgs/applications/science/math/sage/spkg-giac.patch
+++ b/pkgs/applications/science/math/sage/spkg-giac.patch
@@ -1,10 +1,19 @@
---- old/build/pkgs/giac/spkg-install	2017-07-21 14:10:00.000000000 -0500
-+++ new/build/pkgs/giac/spkg-install	2017-10-15 15:55:55.321237645 -0500
-@@ -4,6 +4,8 @@
+diff --git a/build/pkgs/giac/spkg-install b/build/pkgs/giac/spkg-install
+index bdd8df6cb8..3fd7a3ef8a 100644
+--- a/build/pkgs/giac/spkg-install
++++ b/build/pkgs/giac/spkg-install
+@@ -2,6 +2,15 @@
  ## Giac
  ###########################################
  
-+find . -type f -exec sed -e 's@/bin/cp@cp@g' -i '{}' ';' && echo "Patching input parser" && find . -iname 'input_parser.cc'
++# Fix hardcoded paths, while making sure to only update timestamps of actually
++# changed files (otherwise confuses make)
++grep -rlF '/bin/cp' . | while read file
++do
++	sed -e 's@/bin/cp@cp@g' -i "$file"
++done
++
++# Fix input parser syntax
 +sed -e 's@yylex (&yylval)@yylex (\&yyval, scanner)@gp' -i 'src/src/input_parser.cc'
  
  if [ "$SAGE_LOCAL" = "" ]; then
diff --git a/pkgs/applications/science/math/sage/spkg-git.patch b/pkgs/applications/science/math/sage/spkg-git.patch
index ff9a7b2e491a..74f552dd3c36 100644
--- a/pkgs/applications/science/math/sage/spkg-git.patch
+++ b/pkgs/applications/science/math/sage/spkg-git.patch
@@ -1,12 +1,17 @@
 diff --git a/build/pkgs/git/spkg-install b/build/pkgs/git/spkg-install
-index 8469cb58c2..d0dc9a1db9 100755
+index 87874de3d8..b0906245fa 100644
 --- a/build/pkgs/git/spkg-install
 +++ b/build/pkgs/git/spkg-install
-@@ -35,6 +35,8 @@ fi
+@@ -33,6 +33,13 @@ fi
  
  cd src
  
-+find . -type f -exec sed -e 's@/usr/bin/perl@perl@g' -i '{}' ';'
++# Fix hardcoded paths, while making sure to only update timestamps of actually
++# changed files (otherwise confuses make)
++grep -rlF '/usr/bin/perl' . | while read file
++do
++	sed -e 's@/usr/bin/perl@perl@g' -i "$file"
++done
 +
  # We don't want to think about Fink or Macports
  export NO_FINK=1
diff --git a/pkgs/applications/science/math/sage/spkg-singular.patch b/pkgs/applications/science/math/sage/spkg-singular.patch
index d561768600b4..606ffcd3ad4e 100644
--- a/pkgs/applications/science/math/sage/spkg-singular.patch
+++ b/pkgs/applications/science/math/sage/spkg-singular.patch
@@ -1,11 +1,17 @@
---- old/build/pkgs/singular/spkg-install	2017-10-15 10:35:41.826540964 -0500
-+++ new/build/pkgs/singular/spkg-install	2017-10-15 10:36:40.613743443 -0500
-@@ -4,6 +4,9 @@
+diff --git a/build/pkgs/singular/spkg-install b/build/pkgs/singular/spkg-install
+index 8caafb1699..3c34e6608a 100644
+--- a/build/pkgs/singular/spkg-install
++++ b/build/pkgs/singular/spkg-install
+@@ -2,6 +2,13 @@
  ## Singular
  ###########################################
  
-+find . -type f -exec sed -e 's@/bin/rm@rm@g' -i '{}' ';'
-+#echo '#!/usr/bin/env bash\nIgnoring missing $1' > src/build-aux/missing
++# Fix hardcoded paths, while making sure to only update timestamps of actually
++# changed files (otherwise confuses make)
++grep -rlF '/bin/rm' . | while read file
++do
++	sed -e 's@/bin/rm@rm@g' -i "$file"
++done
 +
  if [ -z "$SAGE_LOCAL" ]; then
      echo >&2 "Error: SAGE_LOCAL undefined -- exiting..."