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.
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 going to 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 = $VAR
#Thats end of text file
^[
ZZ
EOFXX
Then file will have 3 lines as below. "i" is to start vi insert mode and similarly to close the file with Esc and ZZ.
#This is my var in text file
var = Test
#Thats end of text file
Related
I have this code
TOKEN=$(cat ./config/token)
echo "$TOKEN"
cat > variables.env <<EOF
TOKEN=`echo "$TOKEN"`
EOF
I am trying to get the content of a file and output it in a new file prefixed by some text. The first echo in the console echoes the output I want, keeping the whitespaces and newlines.
However, in the new file the output is just the first line of the original string, while I'd like the same output I can see in the console with the first echo.
Use printf %q (in ksh or bash) to escape content in such a way that it will always evaluate back to its literal value:
printf 'TOKEN=%q\n' "$(<./config/token)" >variables.env
$(<file) is a ksh and bash extension which acts as a more efficient replacement for $(cat file) (as the regular command substitution needs to fork off a subprocess, set up a FIFO, and spawn an external copy of /bin/cat, whereas the $(<file) form simply tells the shell to read the file directly).
This way a taken containing an otherwise-hostile string such as $(rm -rf ~) or content that could simply be expanded as a variable ($$) will be emitted as literal content.
Providing an explicit example of how this behaves:
printf '%s\n' "first line" "second line" >token # write two lines to the file "token"
printf 'TOKEN=%q\n' "$(<token)" >variables.env # write a shell command which assigns those
# two lines to a variable to variables.env
source variables.env # execute variables.env in the current shell
echo "$TOKEN" # emit the value of TOKEN, as given in the current shell
...when run with bash, will emit the exact output:
first line
second line
...after writing the following (with bash 3.2.48; may vary with other releases) to variables.env:
TOKEN=$'first line\nsecond line'
Useless use of echo
This is what you could write:
cat > variables.env <<EOF
TOKEN=${TOKEN}
EOF
you are doing it in a very convoluted way, there are easier methods
sed '1s/./TOKEN=&/' file > newfile
will insert TOKEN= on the first line. This has an additional benefit of not modifying empty files (at least one char should exist in the original file). If that's not intended you can use unconditional insert.
You can do:
echo "TOKEN=" > newfile && cat ./config/token >> newfile
>> appends to a file.
#!/bin/bash
if [ $# -ne 1 ]
then
echo "USAGE:vitest filename"
else
FILENAME=$1
exec vi $FILENAME <<EOF
i
Line 1.
Line 2.
^[
ZZ
EOF
fi
exit 0
I'm trying to input the Line 1. and Line 2. with Exec vi using the here doc, and commands.
When running the script it gives me the following:
Vim(?):Warning: Input is not from a terminal
Vim: Error reading input, exiting...
Press ENTER or type command to continueVim: Finished.
Vim: Error reading input, exiting...
Vim: Finished.
You want to start vi in ex mode, with a few minor changes to the script.
vi -e "$FILENAME" <<EOF
i
Line 1.
Line 2.
.
wq
EOF
exec is almost certainly unnecessary, especially since you have an exit command following vi. exec is used to replace the current script with the given command; it is not needed simply to execute a command.
A brief history of UNIX text editors:
ed was the original editor, designed to work with a teletype rather than a video terminal.
ex was an extended version of ed, designed to take advantage of a video terminal.
vi was an editor that provided ex with a full-screen visual mode, in contrast with the line-oriented interface employed by ed and ex.
As suggested, ed
ed file << END
1i
line1
line2
.
wq
END
The "dot" line means "end of input".
It can be written less legibly as a one-liner
printf "%s\n" 1i "line1" "line2" . wq | ed file
Use cat.
$ cat file1.txt file2.txt | tee file3.txt
Line 1
Line 2
aaaa
bbbb
cccc
Using sed
If I understand correctly, you want to add two lines to the beginning of a file. In that case, as per Cyrus' suggestion, run:
#!/bin/bash
if [ $# -ne 1 ]
then
echo "USAGE:vitest filename"
exit 1
fi
sed -i.bak '1 s/^/line1\nline2\n/' "$1"
Notes:
When a shell variable is used, it should be in double-quotes unless you want word splitting and pathname expansion to be performed. This is important for file names, for example, as it is now common for them to contain whitespace.
It is best practice to use lower or mixed case names for shell variables. The system uses upper case names for its variables and you don't want to overwrite one of them accidentally.
In the check for the argument, the if statement should include an exit to prevent the rest of the script from being run in the case that no argument was provided. In the above, we added exit 1 which sets the exit code to 1 to signal an error.
Using vi
Let's start with this test file:
$ cat File
some line
Now, let's run vi and see what is in File afterward:
$ vi -s <(echo $'iline1\nline2\n\eZZ') File
$ cat File
line1
line2
some line
The above requires bash or similar.
What is the easiest way to append text to a file in Linux?
I had a look at this question, but the accepted answer uses an additional program (sed) I'm sure there should be an easier way with echo or similar.
How about:
echo "hello" >> <filename>
Using the >> operator will append data at the end of the file, while using the > will overwrite the contents of the file if already existing.
You could also use printf in the same way:
printf "hello" >> <filename>
Note that it can be dangerous to use the above. For instance if you already have a file and you need to append data to the end of the file and you forget to add the last > all data in the file will be destroyed. You can change this behavior by setting the noclobber variable in your .bashrc:
set -o noclobber
Now when you try to do echo "hello" > file.txt you will get a warning saying cannot overwrite existing file.
To force writing to the file you must now use the special syntax:
echo "hello" >| <filename>
You should also know that by default echo adds a trailing new-line character which can be suppressed by using the -n flag:
echo -n "hello" >> <filename>
References
echo(1) - Linux man page
noclobber variable
I/O Redirection
cat >> filename
This is text, perhaps pasted in from some other source.
Or else entered at the keyboard, doesn't matter.
^D
Essentially, you can dump any text you want into the file. CTRL-D sends an end-of-file signal, which terminates input and returns you to the shell.
Other possible way is:
echo "text" | tee -a filename >/dev/null
The -a will append at the end of the file.
If needing sudo, use:
echo "text" | sudo tee -a filename >/dev/null
Follow up to accepted answer.
You need something other than CTRL-D to designate the end if using this in a script. Try this instead:
cat << EOF >> filename
This is text entered via the keyboard or via a script.
EOF
This will append text to the stated file (not including "EOF").
It utilizes a here document (or heredoc).
However if you need sudo to append to the stated file, you will run into trouble utilizing a heredoc due to I/O redirection if you're typing directly on the command line.
This variation will work when you are typing directly on the command line:
sudo sh -c 'cat << EOF >> filename
This is text entered via the keyboard.
EOF'
Or you can use tee instead to avoid the command line sudo issue seen when using the heredoc with cat:
tee -a filename << EOF
This is text entered via the keyboard or via a script.
EOF
I want to write a bash script that will wrap piped input with some text.
Based on Googling and trying to pick from examples.
Here is what I have so far, that does not work:
#!/bin/sh
if readlink /proc/$$/fd/0 | grep -q "^pipe:"; then
echo "{ "template":{"name":"contact sheet template","root":"root","parameters": ["pages"]},"pages":"
cat
echo "}"
fi
I am receiving a JSON list from another program as piped input and I want to output before and afterwards with the above text before I pipe the results to the next program.
program_1 | wrapper.sh | program_2 > outputfile
But it doesn't output anything.
Can someone with more bash expertise point me in the right direction?
Personally I'll search in this way :
myscript.sh
echo 'BEFORE' $(cat) 'AFTER'
Do you mean your script is reading standard input from a pipe, such as
$ other-process | my-script
?
Then the commands in your script will simply inherit standard input from the pipe
#!/bin/sh
# Output preamble
cat <<EOF
{ "template":{"name":"contact sheet template","root":"root","parameters": ["pages"]},"pages":
EOF
cat # This reads from standard input inherited from your script
# Output the closing
cat <<EOF
}
EOF
I have a file with 2 lines and I want to read them into 2 variables respectively. How do I accomplish this in shellscript(bash)?
You can open file descriptors in a shell to read the variables:
#!/bin/bash
# open file
exec 6<tst.txt
read foo <&6
read bar <&6
# close file again
exec 6<&-
echo $foo $bar
EDIT:
As a quick explanation, this is using IO redirection. Normally the file descriptors are handled as follows:
0 stdin (input)
1 stdout (output)
2 stderr (error)
However, there's nothing preventing from using other file descriptors (up to 9), so we're opening the "tst.txt" file in file descriptor 6, and read from it using IO redirection.
So, exec 6<tst.txt opens file descriptor 6 and redirects tst.txt into it, whereas exec 6<&- closes it again.
I'm unfortunately not on linux right now to test, but this would be close.
#!/bin/bash
file="/path/to/file"
# Store the previous IFS so we don't break anything else in the script.
prevIFS='$IFS'
# You need the line break to capture a newline.
IFS='
'
read var1 var2 < $file
echo "Var1: $var1"
echo "Var2: $var2"
# Set IFS back to normal
IFS='$prevIFS'
The simplest answer would be using sed command. Assuming that your file name is file.txt
var1=($(sed '1q;d' file.txt))
var2=($(sed '2q;d' file.txt))
Where 1q and 2q defines the line number.
All the values in Line 1 will be assigned to var1 and similarly to var2.
try this
#!/bin/bash
I=0
while read; do
VAR[$I]=$REPLY
((I++))
done < file
echo ${VAR[0]}
echo ${VAR[1]}
this will work with a file with more than 2 lines
Can you reconfigure the input file (with variables) to work as shell code? i.e.
$ cat varFile
var1=xyz
var2=abc
$ cat myShellScript.sh
#/bin/whatever (bash)?
# source the variable file
. /path/to/varFile
echo $var1
echo $var2
This is a standard concept in shell scripting and makes it much easier to manage configuration issues where you need to control your (unix/linux) environment based on which physical hardware you are running your system. If this is part of you concern, please let me know and I'll update the sample code to extend on this technique.
I hope this helps.