How can I write data to a text file automatically by shell scripting in Linux?
I was able to open the file. However, I don't know how to write data to it.
112 Answers
The short answer:
echo "some data for the file" >> fileName However, echo doesn't deal with end of line characters (EOFs) in an ideal way. So, if you're gonna append more than one line, do it with printf:
printf "some data for the file\nAnd a new line" >> fileName The >> and > operators are very useful for redirecting output of commands, they work with multiple other bash commands.
#!/bin/sh FILE="/path/to/file" /bin/cat <<EOM >$FILE text1 text2 # This comment will be inside of the file. The keyword EOM can be any text, but it must start the line and be alone. EOM # This will be also inside of the file, see the space in front of EOM. EOM # No comments and spaces around here, or it will not work. text4 EOM You can redirect the output of a command to a file:
$ cat file > copy_file or append to it
$ cat file >> copy_file If you want to write directly the command is echo 'text'
$ echo 'Hello World' > file #!/bin/bash cat > FILE.txt <<EOF info code info info code info info code info EOF I know this is a damn old question, but as the OP is about scripting, and for the fact that google brought me here, opening file descriptors for reading and writing at the same time should also be mentioned.
#!/bin/bash # Open file descriptor (fd) 3 for read/write on a text file. exec 3<> poem.txt # Let's print some text to fd 3 echo "Roses are red" >&3 echo "Violets are blue" >&3 echo "Poems are cute" >&3 echo "And so are you" >&3 # Close fd 3 exec 3>&- Then cat the file on terminal
$ cat poem.txt Roses are red Violets are blue Poems are cute And so are you This example causes file poem.txt to be open for reading and writing on file descriptor 3. It also shows that *nix boxes know more fd's then just stdin, stdout and stderr (fd 0,1,2). It actually holds a lot. Usually the max number of file descriptors the kernel can allocate can be found in /proc/sys/file-max or /proc/sys/fs/file-max but using any fd above 9 is dangerous as it could conflict with fd's used by the shell internally. So don't bother and only use fd's 0-9. If you need more the 9 file descriptors in a bash script you should use a different language anyways :)
Anyhow, fd's can be used in a lot of interesting ways.
I like this answer:
cat > FILE.txt <<EOF info code info ... EOF but would suggest cat >> FILE.txt << EOF if you want just add something to the end of the file without wiping out what is already exists
Like this:
cat >> FILE.txt <<EOF info code info ... EOF Moving my comment as an answer, as requested by @lycono
If you need to do this with root privileges, do it this way:
sudo sh -c 'echo "some data for the file" >> fileName' For environments where here documents are unavailable (Makefile, Dockerfile, etc) you can often use printf for a reasonably legible and efficient solution.
printf '%s\n' '#!/bin/sh' '# Second line' \ '# Third line' \ '# Conveniently mix single and double quotes, too' \ "# Generated $(date)" \ '# ^ the date command executes when the file is generated' \ 'for file in *; do' \ ' echo "Found $file"' \ 'done' >outputfile I thought there were a few perfectly fine answers, but no concise summary of all possibilities; thus:
The core principal behind most answers here is redirection. Two are important redirection operators for writing to files:
Redirecting Output:
echo 'text to completely overwrite contents of myfile' > myfile Appending Redirected Output
echo 'text to add to end of myfile' >> myfile Here Documents
Others mentioned, rather than from a fixed input source like echo 'text', you could also interactively write to files via a "Here Document", which are also detailed in the link to the bash manual above. Those answers, e.g.
cat > FILE.txt <<EOF` or `cat >> FILE.txt <<EOF make use of the same redirection operators, but add another layer via "Here Documents". In the above syntax, you write to the FILE.txt via the output of cat. The writing only takes place after the interactive input is given some specific string, in this case 'EOF', but this could be any string, e.g.:
cat > FILE.txt <<'StopEverything'` or `cat >> FILE.txt <<'StopEverything' would work just as well. Here Documents also look for various delimiters and other interesting parsing characters, so have a look at the docs for further info on that.
Here Strings
A bit convoluted, and more of an exercise in understanding both redirection and Here Documents syntax, but you could combine Here Document style syntax with standard redirect operators to become a Here String:
Redirecting Output of cat Input
cat > myfile <<<'text to completely overwrite contents of myfile' Appending Redirected Output of cat Input
cat >> myfile <<<'text to completely overwrite contents of myfile' This approach works and is the best
cat > (filename) <<EOF Text1... Text2... EOF Basically the text will search for keyword "EOF" till it terminates writing/appending the file
If you are using variables, you can use
first_var="Hello" second_var="How are you" If you want to concat both string and write it to file, then use below
echo "${first_var} - ${second_var}" > ./file_name.txt Your file_name.txt content will be "Hello - How are you"
Can also use here document and vi, the below script generates a FILE.txt with 3 lines and variable interpolation
VAR=Test vi FILE.txt <<EOFXX i #This is my var in text file var = $VAR #Thats end of text file ^[ ZZ EOFXX Then file will have 3 lines as below. "i" is to start vi insert mode and similarly to close the file with Esc and ZZ.
#This is my var in text file var = Test #Thats end of text file 
