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If a Bash backup works in your terminal but fails from cron, the usual cause is not aws s3 sync itself. Cron starts a non-interactive job with a smaller environment, normally runs the command through /bin/sh, may use a different HOME and AWS profile, and starts in an unexpected directory. Make the job deterministic: use a Bash script with a shebang, absolute executable and file paths, an explicit PATH, deliberate credentials and region settings, an explicit cd, and redirected logs. Test the exact command as the crontab owner, inspect an --dryrun, then enable the real sync. If it still fails, --debug shows whether the problem is shell parsing, PATH, credentials, IAM authorization, quoting, or endpoint connectivity.
Why does a Bash backup work manually but fail from cron?
Interactive shells load startup files and inherit settings that cron does not reproduce. Linux cron documentation states that SHELL is normally /bin/sh, while HOME and LOGNAME come from the crontab owner’s account entry. Cron can also receive explicit environment assignments in the crontab, but it does not automatically recreate your terminal’s aliases, exported variables, virtual environment, working directory, or login profile.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThe command portion of a crontab entry is interpreted by the configured shell. Bash-only constructs such as arrays, [[ ... ]], and source can therefore fail when the line is run by /bin/sh. An unescaped percent sign is another cron-specific hazard: cron treats the first unescaped % in the command as the start of standard input, and later percent signs become newline-separated input. A date expression that works interactively can consequently be split or truncated in cron.
AWS configuration is environment-dependent too. The AWS CLI may be found through a terminal-only PATH, and a profile may be read from the interactive user’s home directory while cron runs as another account or with a different HOME. Relative source paths can point at an empty directory, and a region, proxy, DNS configuration, or network route can differ between an interactive session and the cron execution context.
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What should a reliable cron-to-S3 design do?
A robust design makes six properties explicit rather than relying on an interactive shell:
| Property | Fragile assumption | Deterministic repair |
|---|---|---|
| Shell fidelity | The crontab line will use Bash because the script is called a “Bash” script. | Put a Bash shebang at the top of the file and invoke that file by absolute path. |
| Environment | Terminal PATH, HOME, profile, and exported variables will be inherited. |
Set the required PATH and HOME; select the AWS profile or credential source explicitly. |
| File context | Cron starts in the directory where you usually run the command. | Use absolute paths and cd to the intended source directory before syncing. |
| Observability | A failed job will be visible in the terminal. | Redirect both output streams, add timestamps and an exit-status record, and use --debug temporarily. |
| Authorization | The identity used in the terminal is the identity used by cron. | Verify the account, profile, and IAM permissions in the cron context. |
| Data safety | A sync command cannot remove anything unexpectedly. | Review a dry run and leave --delete disabled until direction and scope are proven. |
| Network correctness | Region, endpoint, DNS, proxy, and routing are identical everywhere. | Set the region when appropriate and test connectivity as the same user on the same host. |
How do I write a cron-safe Bash S3 backup script?
Place the logic in a file instead of putting a long, shell-sensitive command directly in the crontab. The following pattern deliberately uses placeholders for paths and bucket names that you must replace with values valid on your host.
- Create the file and select its owner. Store it somewhere such as
/usr/local/sbin/s3-backup.sh. The account that owns the crontab must be able to read and execute it, read the source directory, and write the log. - Use a Bash shebang and strict error handling. A shebang tells the kernel which interpreter to use when cron launches the file.
set -Eeuo pipefailmakes unset variables, failed commands, and pipeline failures visible instead of silently continuing. - Set a known environment. Use a conservative absolute
PATH, defineHOMEfor the account whose AWS configuration is intended, and assign the absolute path to the AWS executable. - Change to the intended source directory. Keep the source path absolute even after
cd; this prevents an accidental change of working directory from selecting the wrong files. - Log start, completion, and exit status. The cron entry can redirect output to a file, while the script records when it started and whether the command returned zero.
#!/usr/bin/env bash
set -Eeuo pipefail
PATH=/usr/local/bin:/usr/bin:/bin
export PATH
# Set this to the home directory that contains the intended AWS configuration.
export HOME=/home/backup
AWS=/usr/local/bin/aws
DATE=/usr/bin/date
SOURCE=/srv/backup
DEST='s3://example-bucket/nightly'
PROFILE='backup'
REGION='us-east-1'
log() {
printf '[%s] %sn' "$($DATE -u +'%Y-%m-%dT%H:%M:%SZ')" "$*"
}
status=0
trap 'status=$?; log "backup exited with status $status"' EXIT
log "backup started"
cd "$SOURCE"
"$AWS" s3 sync "$SOURCE" "$DEST" \
--profile "$PROFILE" \
--region "$REGION"
log "backup completed"
Change /usr/local/bin/aws, /home/backup, /srv/backup, the bucket URI, profile, and region to match your installation. Do not assume the AWS binary is in that location: as the intended account, run command -v aws, verify the result is executable, and use that absolute path. If your system installs Bash somewhere other than the path resolved by /usr/bin/env, use the host’s absolute Bash path in the shebang instead.
Save the file, then make it executable:
chmod 750 /usr/local/sbin/s3-backup.shKeep secrets out of the script and crontab. Use the host’s approved AWS credential mechanism, an instance or workload identity where available, or a profile whose files are readable only by the backup account. A profile name without the matching HOME and credential files is not sufficient.
How do I make cron use the right AWS profile and credentials?
Pass the profile explicitly and make its configuration location unambiguous. In the example, --profile backup tells the AWS CLI which named configuration to use, while HOME=/home/backup ensures the CLI looks under the intended account’s home directory. The crontab must belong to that account, or the account must have deliberate access to the configuration and source files.
Check the identity outside the sync
Run an AWS identity check as the same user who owns the crontab, with the same HOME, PATH, profile, and region. This separates “the CLI cannot find credentials” from “the identity is authenticated but not authorized.” Do not substitute your personal terminal account for the service account when testing.
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sudo -u backup env HOME=/home/backup PATH=/usr/local/bin:/usr/bin:/bin
/usr/local/bin/aws sts get-caller-identity --profile backupIf the host uses a role, metadata service, web identity, or another non-profile source, follow that mechanism’s requirements instead of forcing a shared credentials file. The important test is that the credential source is reachable and resolves to the identity intended for this backup.
Interpret an AccessDenied response correctly
AccessDenied can mean either that cron selected a different identity or that the selected identity lacks an API action required by the operation. A sync can need permission to inspect a bucket and objects as well as upload objects; options that delete or use multipart transfers can require additional actions. Compare the identity and profile first, then adjust the IAM policy to the minimum actions required by the chosen direction and options. Do not grant broad administrator access merely to make a cron test pass.
What does aws s3 sync actually copy?
The AWS CLI describes s3 sync as synchronizing the contents of a bucket and a directory, or the contents of two buckets. It copies files that are missing or considered outdated at the destination; it does not blindly upload every source file on every run.
Choose the direction deliberately
For a local backup, the usual direction is local directory first and S3 URI second:
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/usr/local/bin/aws s3 sync
'/srv/backup'
's3://example-bucket/nightly'
--profile backup
--region us-east-1Reversing the arguments makes S3 the source and the local directory the destination. A syntactically valid command can therefore do the opposite of what you intended. Quote both paths so spaces, wildcard characters, and shell metacharacters are passed as literal path data.
Use --delete only after reviewing the scope
Without --delete, objects that exist only at the destination are retained. With --delete, destination objects absent from the source are removed. That can be useful for a mirror, but it is dangerous for a backup if the source is incomplete, mounted incorrectly, filtered, or temporarily empty. Keep the option off until a reviewed dry run proves the source, destination, and exclusions are correct.
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Understand why a successful run can copy zero files
“No files copied” is not automatically an error. The source may already match the destination, the source path may contain no files, filters may exclude everything, timestamps or size comparisons may make objects appear current, or the command may point in the wrong direction. A dry run exposes the planned operations without changing data.
How should I test the command before enabling cron?
Test progressively, using the exact account and absolute paths that cron will use. This catches permission and environment mistakes before a scheduled run can overwrite a destination or produce an incomplete backup.
- Verify local access. As the crontab owner, list the source directory and confirm that the account can read every path that should be backed up.
- Verify the executable. Run the absolute AWS path with
--versionand confirm it executes without relying on an interactive shell function or alias. - Verify credentials. Run an identity check using the intended
HOMEand profile. - Run a dry run. Add
--dryrunto the exact sync command. Review every planned upload, source prefix, destination prefix, and exclusion. - Check the log and exit code. A dry run that prints plausible operations is not proof that the real upload is authorized or reachable; retain the command’s exit status.
- Run one real sync without
--delete. Confirm that the expected objects appear at the destination, then decide whether a reviewed mirror policy truly needs deletion.
/usr/local/bin/aws s3 sync
'/srv/backup'
's3://example-bucket/nightly'
--profile backup
--region us-east-1
--dryrunThe AWS CLI documentation treats --dryrun as a preview. It does not validate every final transfer outcome, so keep the first real run observable and inspect its exit status.
What should the crontab entry look like?
Call the script by its absolute path and redirect both standard output and standard error. For a backup at 02:17 every day:
17 2 * * * /usr/local/sbin/s3-backup.sh >>/var/log/s3-backup.log 2>&1Ensure the cron account can append to that log, or direct it to a user-writable location. You can set variables such as SHELL, PATH, and HOME at the top of a crontab, but keeping the critical values in the script makes the script’s behavior easier to reproduce manually and from other schedulers.
Avoid date substitutions and other percent signs in the crontab command. If a percent sign is unavoidable, escape it as %; placing the date logic inside the Bash script is usually clearer and avoids cron’s special input behavior. A long inline command also makes quoting and shell selection harder to audit than a short script invocation.
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How do I diagnose the common cron S3 failures?
Start with the first meaningful error in the log, not the final generic exit message. This table maps symptoms to the narrowest useful check.
| Symptom | Likely check | Corrective action |
|---|---|---|
aws: command not found |
Cron’s PATH differs from the terminal, the CLI is installed elsewhere, or the file is not executable. |
Use the absolute AWS path, set PATH explicitly, and verify execute permission as the cron user. |
| Bash syntax error | The crontab line is being interpreted by /bin/sh, or the command uses Bash-only syntax. |
Move the logic into a file with a Bash shebang and invoke that file by absolute path. |
| Relative files are missing | Cron’s working directory is different from the interactive directory. | Use absolute paths and perform an explicit cd before syncing. |
| Profile or credentials not found | HOME differs, startup files were not loaded, or the cron user is different. |
Set HOME deliberately, pass --profile, and verify the credential source as the same user. |
AccessDenied |
The identity is different, or its IAM policy lacks an API action used by the sync. | Compare the resolved identity and profile, then grant only the required S3 actions for the selected operation. |
| No files copied | Source and destination are reversed, filters exclude files, paths are wrong, or the destination is already current. | Run --dryrun, temporarily remove filters, and inspect both sides and the planned operations. |
| Endpoint timeout or connection failure | Region, endpoint selection, DNS, proxy, or network access differs in the cron context. | Set --region explicitly when appropriate and test connectivity from the same host, user, and environment. |
| Unexpected deletions | --delete removed destination objects absent from the source. |
Disable --delete and repeat a reviewed dry run after confirming direction and source completeness. |
How does --debug reveal the real failure?
Add --debug temporarily to the AWS CLI command and capture its output in the same log. AWS CLI troubleshooting documentation recommends this option because it exposes credential lookup, request construction, service responses, and output formatting. Search the resulting log for the profile name, credential-provider messages, region, endpoint, HTTP response, AccessDenied, and command-not-found text.
/usr/local/bin/aws s3 sync
'/srv/backup'
's3://example-bucket/nightly'
--profile backup
--region us-east-1
--debugDo not leave verbose debugging enabled indefinitely if logs can expose sensitive request details or create excessive volume. Remove it after identifying the class of failure, and retain normal timestamps and exit-status logging for routine operations.
Separate shell and PATH problems from AWS problems
If the log never reaches AWS CLI output, inspect the shebang, executable permission, absolute path, PATH, working directory, and cron’s percent handling. If the CLI starts and reports credential or HTTP details, the shell has already launched it; focus on profile selection, IAM, region, endpoint, and network behavior.
Separate authorization from connectivity
An immediate AccessDenied response proves that a service endpoint was reached, so investigate identity and IAM rather than DNS first. A timeout, name-resolution error, or inability to connect points instead to endpoint selection, region, proxy, firewall, or routing differences. Run the same diagnostic command as the cron account, not as an administrator with a different network or credential context.
What IAM permissions and data-safety checks belong in production?
Give the backup identity only the bucket and object actions required by its direction and features. A local-to-S3 sync generally needs to inspect the destination and write objects; a mirror using --delete additionally needs permission to remove destination objects. Multipart transfers, encryption settings, bucket policies, and object ownership controls can add requirements specific to your account. Use the service’s denied-action message and policy evaluation process to identify the missing action rather than granting an unrestricted policy.
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- Confirm the bucket name, prefix, region, and account before the first real run.
- Keep
--deleteoff while establishing the job and whenever the source can be incomplete. - Review dry-run output after changing filters, mount points, prefixes, or credentials.
- Make the source readable and the log writable by the cron account, but keep credential files private.
- Monitor the exit status and log size so a failing job is noticed instead of silently accumulating errors.
- Test recovery by restoring a representative object through a separately controlled procedure; a successful upload alone does not prove that every required file was selected.
What if cron variants behave differently?
The documented behavior above describes common Linux and POSIX-style cron implementations. Cron variants can differ in environment handling, mail delivery, permissions, and scheduling details. If a local daemon conflicts with the Linux man-page behavior, consult that daemon’s documentation and inspect the environment it actually supplies. The deterministic script pattern still helps because it removes dependence on login files, aliases, relative paths, and implicit shell choices.
For a final acceptance test, run the script through the same scheduler account and capture the complete environment needed for diagnosis, while avoiding credential values in the log. Compare the scheduled invocation with the successful manual invocation one variable at a time: user, HOME, PATH, working directory, profile, region, endpoint, and network route.
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Can I put AWS access keys directly in a crontab entry?
It is a poor default because crontabs and process environments can be readable to more users than intended and are easy to leak into logs or backups. Prefer the host’s role or workload identity, or a locked-down credential file/profile owned by the backup account, and verify that source under cron.
Why does a dry run list uploads but the real sync still fails?
A dry run primarily previews the comparison and planned operations; it does not prove that every transfer will be authorized, reachable, or accepted by bucket policy. Run one real transfer with normal logging, then use the returned error and exit status to investigate IAM, endpoint, encryption, or object-level restrictions.
Should I set SHELL=/bin/bash in the crontab?
You may set it when the cron implementation supports that variable, but it is not a substitute for a Bash shebang on the script. Calling an executable script with an explicit shebang is easier to test and avoids putting Bash syntax and percent-sensitive logic in the crontab line.
How can I tell whether an empty source is caused by a failed mount?
Log the result of the explicit cd and inspect the directory as the cron user immediately before syncing. A successful cd into an unexpectedly empty local path can produce a valid, empty dry run, so verify the mount, expected filenames, and file count rather than treating a zero-change result as proof of a complete backup.
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