Why does `cd` break redirects by `exec`?

Question

I'm trying to redirect parts of the output from a script to a file and have done it like so:

#!/usr/bin/bash
cleanup() {
  echo "Lines in $tmpfile:"
  cat $tmpfile
  rm $tmpfile
}

tmpfile=$(mktemp .tmp.`basename $0`.XXXXXX)
trap cleanup EXIT
for var in one two; do
  echo "$var: Before redirect"
  exec 6>&1           # Save stdout to descriptor #6
  exec >> "$tmpfile"  # Send output to file
  if [ 'a' == 'b' ]; then
    echo "this never happens"
  else
    echo "$var: line in file"
  fi
  exec 1>&6 # Restore stdout
  exec 6>&- # Close descriptor 6
  echo "$var: After redirect"
done

This produces the expected output:

$ ./test-working.sh
one: Before redirect
one: After redirect
two: Before redirect
two: After redirect
Lines in .tmp.test-working.sh.c3noO3:
one: line in file
two: line in file

If I do a small tweak like so however:

$ diff -u test-working.sh test-broken.sh
--- test-working.sh     2021-06-16 12:48:35.852500568 +0200
+++ test-broken.sh      2021-06-16 12:48:35.205483749 +0200
@@ -7,6 +7,7 @@

 tmpfile=$(mktemp .tmp.`basename $0`.XXXXXX)
 trap cleanup EXIT
+cd
 for var in one two; do
   echo "$var: Before redirect"
   exec 6>&1           # Save stdout to descriptor #6
@@ -20,3 +21,4 @@
   exec 6>&- # Close descriptor 6
   echo "$var: After redirect"
 done
+cd -

Then it breaks:

$ ./test-broken.sh
one: Before redirect
one: After redirect
two: Before redirect
two: After redirect
/script/run/path
Lines in .tmp.test-broken.sh.HSgMrS:

Using set -x you can clearly see that echo is writing something somewhere:

$ bash -x test-broken.sh
+++ basename test-broken.sh
++ mktemp .tmp.test-broken.sh.XXXXXX
+ tmpfile=.tmp.test-broken.sh.c6mf6c
+ trap cleanup EXIT
+ cd
+ for var in one two
+ echo 'one: Before redirect'
one: Before redirect
+ exec
+ exec
+ '[' a == b ']'
+ echo 'one: line in file'
+ exec
+ exec
+ echo 'one: After redirect'
one: After redirect
+ for var in one two
+ echo 'two: Before redirect'
two: Before redirect
+ exec
+ exec
+ '[' a == b ']'
+ echo 'two: line in file'
+ exec
+ exec
+ echo 'two: After redirect'
two: After redirect
+ cd -
/script/run/path
+ cleanup
+ echo 'Lines in .tmp.test-broken.sh.c6mf6c:'
Lines in .tmp.test-broken.sh.c6mf6c:
+ cat .tmp.test-broken.sh.c6mf6c
+ rm .tmp.test-broken.sh.c6mf6c

But it does not end up in the expected file. I'm struggling to see what I'm doing wrong. Why is cd breaking the redirect?

You're creating another file in the home directory with the same name and redirecting stdout to it. What else were you expecting to happen? Use `mktemp -p '' whatever` instead. — oguz ismail, Jun 16 '21 at 11:11

score 0 · Answer 1 · answered Jun 16 '21 at 11:29

As pointed out in the comments, my code reads and writes to the same filename, but in different relative paths. Absolute paths fixes it:

$ diff -u test-broken.sh test-broken-fixed.sh
--- test-broken.sh      2021-06-16 13:21:30.301815257 +0200
+++ test-broken-fixed.sh        2021-06-16 13:22:03.222670969 +0200
@@ -5,7 +5,7 @@
   rm $tmpfile
 }

-tmpfile="$(mktemp .tmp.`basename $0`.XXXXXX)"
+tmpfile="$PWD/$(mktemp .tmp.`basename $0`.XXXXXX)"
 trap cleanup EXIT
 cd
 for var in one two; do

Using mktemp -p '' filename would also work, then the file would land in /tmp.

Why does `cd` break redirects by `exec`?

1 Answers1