How to Write to a File in Bash

Writing to files is a fundamental task in Bash scripting. You can use it to save program output, store user input, or create configuration files. Knowing how to write to a file using Bash is useful for automation and scripting tasks.

This article covers various methods for writing to files in Bash, from simple redirection to using loops and command substitution for more advanced tasks.

Use Output Redirection

In Bash, you can use the > and >> output redirection operators to write a file.

Overwrite a File Using >

Use the > operator to completely overwrite an existing file or create a new one.

$ echo "Hello from Vultr!" > output.txt

This command:

• Creates output.txt if it doesn't exist and writes Hello from Vultr!.
• If the file exists, its content is replaced with Hello from Vultr!.

Append to a File Using >>

Use the >> operator to append data without overwriting its existing content.

$ echo "Greetings from Vultr!" >> output.txt

This appends the line Greetings from Vultr! to output.txt, preserving any existing content. It helps maintain logs or incrementally store data in a file.

Write Output of a Command to a File

You can redirect the output of any command to a file.

• Overwrite a file with the command's output.

  console Copy

  $ ls -l /home/user > bash_file.txt
  

  This command saves the list of files in the /home/user directory to bash_file.txt, overwriting any existing content.

• Append a file with the command's output.

  console Copy

  $ df -kh >> bash_file.txt
  

  This keeps the existing content and adds new entries to the end of the file.

• You can also redirect both standard output and standard error to a file. In Bash, 1 represents stdout and 2 represents stderr.

  console Copy

  $ ls /home/user /nonexistent > bash_output.txt 2>&1
  

  Here, the 2>&1 redirects the standard error (stderr) to the same place as stdout. This is helpful when capturing complete command output, including errors.

Use tee to Write and Display Output

• The tee command allows you to write output to both the terminal and one or more files.

  console Copy

  $ echo "Logged line" | tee bash_log.txt
  

• To append instead of overwrite, use the -a flag.

  console Copy

  $ echo "Another line" | tee -a bash_log.txt
  

  This is useful when debugging or when output needs to be both stored and monitored in real time.

Use a Here Document

Heredoc allows you to pass multi-line text to a command. You can use it with cat or tee to write multi-line content to a file in Bash. It starts from << followed by the specified delimiter (e.g., EOF).

cat <<EOF > bash_config.txt
server=localhost
port=8080
enabled=true
EOF

• The <<EOF starts the Heredoc, and declares EOF as the delimiter, meaning the command reads the line until it encounters EOF.
• The > is the output redirector.

This command creates or overwrites bash_config.txt with the specified configuration block.

If you want to append the lines instead of overwriting, use the >> redirector.

cat <<EOF >> bash_config.txt
# Additional settings
log_level=debug
EOF

Using this method, the script can generate structured configuration files.

Write to a File in a Loop

You can write to a file multiple times inside a loop. This is useful for generating logs or creating structured files.

for i in {1..3}; do
    echo "Line $i" >> bash_loop_output.txt
done

Each iteration of the loop appends a new line to bash_loop_output.txt.

Handle Errors When Writing to Files

This section shows how to manage the script when a write operation fails in your script.

• To ensure the write was successful, check the command's exit status.

  bash Copy

  echo "Test write" > bashfile.txt
  if [ $? -eq 0 ]; then
      echo "Write successful."
  else
      echo "Write failed."
  fi
  

  The $? variable stores the exit status of the previous command. You can also add a line set -e before the write command. This stops the script execution when any command fails.

• You can also check file permissions before attempting a write using -w.

  bash Copy

  if [ -w bashfile.txt ]; then
      echo "Writable"
  else
      echo "No write permission"
  fi
  

  This checks if bashfile.txt is writable and outputs a message accordingly

Use File Descriptors for Advanced Control

If your script writes output to multiple files (or output streams), you can label them with a numeric value using the exec command. This created a file descriptor. To close it, you need to add a line exec <descriptor>&-.

As mentioned previously, in Bash, 1 represents stdout and 2 represents stderr. So you can allocate your output streams values greater than 2.

exec 3> bash_custom_output.txt
echo "Writing with FD 3" >&3
exec 3>&-

• This opens file descriptor 3 for writing
• Writes Writing with FD 3 to it.
• Closes the file descriptor 3.

Conclusion

You now understand multiple ways to write to files in Bash - from redirection and command output to using tee, loops, Heredoc, and file descriptors. Practice combining these techniques to build more powerful and flexible scripts.