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Bash Examples: Append to File

April 16, 2021
 

>> Operator

The >> operator allows us to append the standard output (stdout) of a command to the end of a file. The syntax is:

Bash
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command >> file
$
command >> file

We can also leave out the spaces. All three lines below do the same thing:

Bash
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command>>file
command >>file
command>> file
$
$
$
command>>file
command >>file
command>> file

It is also possible to append a command's standard error to a file using the same operator.

If file does not already exist, it will be created.

Append Text to File

Using echo

With Newline

To append "Hello, world!" to the end of the file myfile.log, we can write:

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echo "Hello, world!" >> myfile.log
$
echo "Hello, world!" >> myfile.log

Because the echo command automatically adds a newline (line break) at the end of the text, this will add a newline to the end of the file as well.

No Newline

If we want to append just the text without a newline at the end, we can use the echo command's -n option:

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echo -n "Hello, world!" >> myfile.log
$
echo -n "Hello, world!" >> myfile.log

Using printf

An alternative to using echo is to use the printf command, which allows you to specify an output format using special character sequences like %s for "insert text string here" and \n for newline (line break).

With Newline

The first argument of printf is the string that specifies the output format. To indicate that we want to display a text string (%s) followed by a newline (\n), we can use the format string %s\n. Then, we can supply the content of the text string as the next argument to printf. Putting it together, to append Hello, world! followed by a newline, we can run the following command:

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printf "%s\n" "Hello, world!" >> myfile.log
$
printf "%s\n" "Hello, world!" >> myfile.log

No Newline

If we don't want the newline at the end, we can simply leave it out of the format string:

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printf %s "Hello, world!" >> myfile.log
$
printf %s "Hello, world!" >> myfile.log

Append One File to Another File

To append the contents of file1.log to the end of file2.log:

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cat file1.log >> file2.log
$
cat file1.log >> file2.log

The cat command prints the contents of file1.log to standard output, and the >> operator appends that output to the end of file2.log.

cat does not automatically add a newline at the end of file2.log. To add a newline, we can combine it with printf:

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printf "%s\n" "$(cat file1.log)" >> file2.log
$
printf "%s\n" "$(cat file1.log)" >> file2.log

This tells Bash to first store the output of the command cat file1.log to a string and then pass that string into the printf "%s\n" command we saw earlier.

Standard Error (stderr)

The above examples have all shown how to append standard output from a command (echo, printf and cat) to a file. But what if you want to append standard error (stderr)? For that, we can specify standard error by its POSIX file descriptor number 2, as shown below.

Both Standard Output and Standard Error

To append both standard output (which is always assigned file descriptor 1 in POSIX) and standard error (file descriptor 2) from a command to a file, we can write:

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command >>file 2>&1
$
command >>file 2>&1

This first tells Bash to append standard output from command to file (>> file), and then tells it to redirect standard error to whereever standard output is going (i.e., append to file).

Although standard output is implied when we write >>file, we can also write 1 explicitly:

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command 1>>file 2>&1
$
command 1>>file 2>&1

This example does the exact same thing as the previous example.

Standard Error Only

To append only standard error from a command to a file, we can write:

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command 2>>file
$
command 2>>file

Standard output will be left alone, so if we run this command from a terminal command line, standard output will show up on screen while standard error will be appended to file.

To Different Files

To append standard output to out.log and standard error to err.log, we can write:

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command 1>>out.log 2>>err.log
$
command 1>>out.log 2>>err.log

We can also leave out 1 since it is implied:

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command >>out.log 2>>err.log
$
command >>out.log 2>>err.log

References

See Also