In Bash, repeat cat with append redirection inside a fixed-count loop:
for ((i=0; i<3; i++)); do
cat -- source.txt >> combined.txt
doneThis appends source.txt three times to combined.txt. Replace 3 with the number of repetitions you need.
Append repeatedly without deleting existing content
The >> operator opens the destination in append mode. If the destination does not exist, the shell creates it; existing content remains intact.
Start with a fresh destination
If the result must contain only the new repetitions, truncate the destination once before the loop:
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: > combined.txt
for ((i=0; i<3; i++)); do
cat -- source.txt >> combined.txt
doneThe : command succeeds without producing output, while > empties or creates combined.txt. Do this only when replacing earlier output is intentional.
Choose append or overwrite deliberately
| Operator | Effect | Typical use |
|---|---|---|
>> |
Appends output and preserves existing destination data | Adding each repeated copy to one file |
> |
Truncates the destination before writing | Creating a replacement result, usually once before a loop |
Running an append loop again adds another set of copies. Clear the destination first when you need a reproducible, freshly generated file.
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Repeat several source files in order
cat writes each named input to standard output in the order supplied. Put all source files in one command and place that command inside the loop:
for ((i=0; i<3; i++)); do
cat -- part1.txt part2.txt part3.txt >> combined.txt
doneEach iteration appends part1.txt, then part2.txt, then part3.txt. The sequence is repeated three times.
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Repeat generated command output
The same pattern works for a command that writes to standard output:
for ((i=0; i<3; i++)); do
some_command >> combined.txt
doneFor a pipeline that must append to more than one destination, GNU tee -a appends its standard input to each named file:
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some_command | tee -a first.log second.log > /dev/nullWithout -a, tee overwrites its output files instead of appending.
Avoid destroying the input file
Never redirect a command’s output to the same file it is reading:
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cat source.txt > source.txtThe shell opens and truncates the output file before cat starts reading it, so the original contents can be lost. Use a different destination, and verify that no input path is also the output path.
Appending is safe only when the source and destination are different files. For example, cat doc.end >> doc reads doc.end while appending to doc.
Checks before running the loop
- Confirm the repetition count; Bash’s arithmetic loop starts at zero, so
i<3runs three times. - Use
>>when existing destination content must remain. - Use one initial
: > destinationonly when a fresh result is required. - Check that every input path differs from the destination path.
- Remember that rerunning an append loop adds more data.
Shell compatibility
The examples use Bash arithmetic for syntax. In another shell, use that shell’s loop form, while retaining append redirection where supported. The essential operations are concatenating inputs to standard output and sending that output to the destination with >>.
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