🚨 Bash Error Guide: 'cannot create temp file for here-document' — Fix It

Overview

Bash prints cannot create temp file for here-document when it tries to spool a here-document (<<EOF) to disk and the filesystem backing its temp directory refuses:

./report.sh: line 30: cannot create temp file for here-document: No space left on device

To feed a here-document to a command’s stdin, Bash writes the body to a temporary file (under $TMPDIR, default /tmp) and redirects from it. If that directory is full, out of inodes, read-only, or not writable by the user, the temp file can’t be created and the whole command fails. The trailing reason (No space left on device, Permission denied, Read-only file system) tells you which.

Symptoms

• Scripts using <<EOF here-documents suddenly fail while simpler commands still work.
• The message always ends with an OS reason: No space left on device, Permission denied, or Read-only file system.
• It fires on a specific host (a full /tmp, a hardened mount) but not others.
• df -h /tmp shows 100% used, or df -i /tmp shows 100% inodes even with free bytes.
• Cron jobs fail here while interactive runs succeed, or vice versa, because TMPDIR differs.

Common Root Causes

• /tmp (or $TMPDIR) is full — no free bytes; the temp file write fails with No space left on device.
• Inodes exhausted — plenty of free space but no free inodes (millions of tiny files), same symptom.
• $TMPDIR points somewhere unwritable — a stale or wrong TMPDIR export, or a directory owned by another user.
• Read-only or full container layer — a container /tmp mounted read-only or a tmpfs sized to near zero.
• Quota exceeded — a per-user disk quota hit even though the volume has room.
• Permissions/sticky-bit damage on /tmp — someone changed /tmp from 1777 so non-root users can’t create files.

Diagnostic Workflow

Find out which directory Bash uses and whether it’s writable:

echo "TMPDIR=${TMPDIR:-/tmp}"
touch "${TMPDIR:-/tmp}/.probe.$$" && echo "writable" && rm -f "${TMPDIR:-/tmp}/.probe.$$"

Check space and inodes — either being full causes the error:

df -h  "${TMPDIR:-/tmp}"     # bytes free
df -i  "${TMPDIR:-/tmp}"     # inodes free

Confirm mount flags (read-only? noexec?):

findmnt -T "${TMPDIR:-/tmp}"

Reproduce minimally:

cat <<EOF
probe
EOF

Find what’s filling /tmp:

du -xhd1 /tmp 2>/dev/null | sort -h | tail
find /tmp -xdev -type f -printf '%s %p\n' 2>/dev/null | sort -rn | head

Example Root Cause Analysis

A nightly reporting job failed only after midnight:

report.sh: line 30: cannot create temp file for here-document: No space left on device

The script piped a here-document into mail:

mail -s "Nightly report" ops@example.com <<EOF
$(generate_summary)
EOF

df -h /tmp showed 100% used, but df -h / showed 40% free — /tmp was a small dedicated tmpfs. du found the cause: a sibling job wrote multi-GB scratch files to /tmp and never cleaned them:

6.1G  /tmp/exporter-scratch

Two fixes applied together. First, point this job at a roomier, self-cleaning temp dir:

export TMPDIR="$(mktemp -d /var/tmp/report.XXXXXX)"
trap 'rm -rf "$TMPDIR"' EXIT

Second, make the offending exporter clean up with its own trap ... EXIT and use mktemp instead of fixed names. With /tmp no longer saturated, the here-document spooled normally.

Prevention Best Practices

• Monitor /tmp space and inodes — alert well before 100%; both df -h and df -i matter.
• Set a sane TMPDIR for jobs that produce large here-documents — e.g. /var/tmp (persistent, usually larger) rather than a tiny tmpfs /tmp.
• Always clean up temp dirs with trap 'rm -rf "$TMPDIR"' EXIT so scratch files don’t accumulate.
• Use mktemp -d for per-run temp directories instead of fixed paths that collide or leak.
• Verify container /tmp is writable and sized — mount a tmpfs with enough capacity, not read-only.
• Keep /tmp at mode 1777 with the sticky bit so all users can create files safely.

Quick Command Reference

df -h "${TMPDIR:-/tmp}"          # bytes free on the temp filesystem
df -i "${TMPDIR:-/tmp}"          # inodes free (often the hidden cause)
findmnt -T "${TMPDIR:-/tmp}"     # mount flags: ro? noexec? tmpfs size?
export TMPDIR="$(mktemp -d /var/tmp/job.XXXXXX)"   # roomier per-run tmp
trap 'rm -rf "$TMPDIR"' EXIT     # guarantee cleanup
du -xhd1 /tmp | sort -h | tail   # find what filled /tmp

Related Guides

• Bash Error Guide: ‘Permission denied’ — when the reason is access rights, not free space.
• Bash Error Guide: ‘ambiguous redirect’ — other redirection failures that abort a command.
• Bash & Python Error Guide: ‘Argument list too long’ (E2BIG) — a related resource limit that stops commands from running.

Conclusion

cannot create temp file for here-document is Bash failing to spool a <<EOF body to disk, and the trailing OS reason names the cause: a full or inode-exhausted /tmp, an unwritable or read-only TMPDIR, or a quota. Check df -h and df -i on the temp filesystem, verify it’s writable, and point large jobs at a roomier TMPDIR with a cleanup trap. Keep /tmp monitored and self-cleaning and here-documents stop failing under load.