1 files changed, 25 insertions, 0 deletions
diff --git a/tools/filter b/tools/filter
new file mode 100755
index 0000000..94258d0
--- /dev/null
+++ b/tools/filter
@@ -0,0 +1,25 @@
+#!/bin/sh
+
+if [ -z "$1" ]; then
+    echo "Usage: $0 <command>"
+    echo ""
+    echo "Run the given test expression for each newline-separated line in stdin, and echo only the lines that return zero."
+    echo "(Think filter in many programming languages)."
+    echo 'The iterator variable is called "$ELEM", and the index variable is called "$IDX".'
+    exit 1
+fi
+
+i=0
+while IFS=$'\n' read -r line; do
+    j=0
+    for item in $line; do
+        eval "X${j}='$item'"
+        j=$((j+1))
+    done
+
+    IDX=$i ELEM="$line" eval "test $1"
+    if [ $? -eq 0 ]; then
+        echo "$line"
+    fi
+    i=$((i+1))
+done