Append Lines to a File in Linux
First, we’ll examine the most common commands like echo, printf, and cat. Second, we’ll take a look at the tee command, a lesser-known but useful Bash utility.
2. The echo Command
The echo command is one of the most commonly and widely used built-in commands for Linux Bash. Usually, we can use it to display a string to standard output, which is the terminal by default:
echo "This line will be displayed to the terminal"
Now, we’re going to change the default standard output and divert the input string to a file. This capability is provided by the redirection operator (>). If the file specified below already contains some data, the data will be lost:
echo "This line will be written into the file" > file.txt
In order to append a line to our file.txt and not overwrite its contents, we need to use another redirection operator (>>):
echo "This line will be appended to the file" >> file.txt
Note that the ‘>’ and ‘>>’ operators are not dependent on the echo command and they can redirect the output of any command:
ls -al >> result.txt find . -type f >> result.txt
Moreover, we can enable the interpretation of backslash escapes using the -e option. So, some special characters like the new line character ‘\n’ will be recognized and we can append multiple lines to a file:
echo -e "line3\n line4\n line5\n" >> file.txt
3. The printf Command
The printf command is similar to the C function with the same name. It prints any arguments to standard output in the format:
printf FORMAT [ARGUMENTS]
Let’s build an example and append a new line to our file using the redirection operator:
printf "line%s!" "6" >> file.txt
Unlike the echo command, we can see that the printf‘s syntax is simpler when we need to append multiple lines. Here, we don’t have to specify special options in order to use the newline character:
printf "line7\nline8!" >> file.txt
4. The cat Command
The cat command concatenates files or standard input to standard output.
It uses a syntax similar to the echo command:
cat [OPTION] [FILE(s)]
The difference is that, instead of a string as a parameter, cat accepts one or more files and copies their contents to the standard output, in the specified order.
Let’s suppose we already have some lines in file1.txt and we want to append them to result.txt
cat file1.txt >> result.txt cat file1.txt file2.txt file3.txt >> result.txt
Next, we’re going to remove the input file from the command:
cat >> file.txt
In this case, the cat command will read from the terminal and appends the data to the file.txt.
So, let’s then type something into the terminal including new lines and then press CTRL + D to exit:
[email protected]:~/Desktop/baeldung/append-lines-to-a-file$ cat >> file.txt line1 using cat command line2 using cat command <press CTRL+D to exit>
This will add two lines to the end of file.txt.
5. The tee Command
Another interesting and useful Bash command is the tee command. It reads data from standard input and writes it to the standard output and to files:
tee [OPTION] [FILE(s)] tee file1.txt tee file1.txt file2.txt
In order to append the input to the file and not overwrite its contents, we need to apply the -a option:
[email protected]:~/Desktop/baeldung/append-lines-to-a-file$ tee -a file.txt line1 using tee command
Once we hit Enter, we’ll actually see our same line repeated back to us:
[email protected]:~/Desktop/baeldung/append-lines-to-a-file$ tee -a file.txt line1 using tee command line1 using tee command
This is because, by default, the terminal acts as both standard input and standard output.
We can continue to input how many lines we want and hit the Enter key after each line. We’ll notice that each line will be duplicated in the terminal and also appended to our file.txt:
[email protected]:~/Desktop/baeldung/append-lines-to-a-file$ tee -a file.txt line1 using tee command line1 using tee command line2 using tee command line2 using tee command <press CTRL+D to exit>
Now, let’s suppose we don’t want to append the input to the terminal, but only to a file. This is also possible with the tee command. Thus, we can remove the file parameter and redirect the standard input to our file.txt by using the redirection operator:
[email protected]:~/Desktop/baeldung/append-lines-to-a-file$ tee >> file.txt line3 using tee command <press CTRL+D to exit>
Finally, let’s take a look at the file.txt‘s contents:
This line will be written into the file This line will be appended to the file line3 line4 line5 line6 line7 line8 line1 using cat command line2 using cat command line1 using tee command line2 using tee command line3 using tee command
First, we’ve studied the echo, printf and cat Bash commands and learned how we can combine them with the redirection operator in order to append some text to a file. We’ve also learned how to append the content of one or more files to another file.
Second, we’ve examined the lesser-known tee command that already has a built-in mechanism of appending some text to a file and doesn’t necessarily need the redirection operator.