Add chn(1df)

1 files changed, 70 insertions, 0 deletions

diff --git a/bin/chn b/bin/chn
new file mode 100755
index 00000000..46a8a27a
--- /dev/null
+++ b/bin/chn
@@ -0,0 +1,70 @@
+#!/bin/sh
+# Repeat a command to filter input several times
+self=chn
+
+# Check arguments.
+if [ "$#" -lt 2 ] ; then
+    printf >&2 '%s: Need a count and a program name\n' "$self"
+    exit 2
+fi
+
+# Shift off the repetition count.
+c=$1
+shift
+
+# Check the repetition count looks sane. Zero is fine!
+if [ "$c" -lt 0 ] ; then
+    printf >&2 '%s: Nonsensical negative count\n' "$self"
+    exit 2
+fi
+
+# If the count is zero, just run the input straight through!
+if [ "$c" -eq 0 ] ; then
+    cat
+    exit
+fi
+
+# Create a temporary directory with name in $td, and handle POSIX-ish traps to
+# remove it when the script exits.
+td=
+cleanup() {
+    [ -n "$td" ] && rm -fr -- "$td"
+    if [ "$1" != EXIT ] ; then
+        trap - "$1"
+        kill "-$1" "$$"
+    fi
+}
+for sig in EXIT HUP INT TERM ; do
+    # shellcheck disable=SC2064
+    trap "cleanup $sig" "$sig"
+done
+td=$(mktd "$self") || exit
+
+# Define and create input and output files
+if=$td/if of=$td/of
+touch -- "$if" "$of"
+
+# Iterate through the count
+while [ "${n=1}" -le "$c" ] ; do
+
+    # Start a subshell so we can deal with FDs cleanly
+    (
+        # If this isn't the first iteration, our input comes from $if
+        [ "$n" -eq 1 ] ||
+            exec <"$if"
+
+        # If this isn't the last iteration, our output goes to $of
+        [ "$n" -eq "$c" ] ||
+            exec >"$of"
+
+        # Run the command with the descriptors above; if the command fails, the
+        # subshell will exit, which will in turn exit the program
+        "$@"
+    ) || exit
+
+    # Copy the output file over the input one
+    cp -- "$of" "$if"
+
+    # Increment the counter for the next while loop run
+    n=$((n+1))
+done