How to cat <<EOF >> a file containing code?
LinuxUnixShHeredocLinux Problem Overview
I want to print code into a file using cat <<EOF >>
:
cat <<EOF >> brightup.sh
!/bin/bash
curr=`cat /sys/class/backlight/intel_backlight/actual_brightness`
if [ $curr -lt 4477 ]; then
curr=$((curr+406));
echo $curr > /sys/class/backlight/intel_backlight/brightness;
fi
EOF
but when I check the file output, I get this:
!/bin/bash
curr=1634
if [ -lt 4477 ]; then
curr=406;
echo > /sys/class/backlight/intel_backlight/brightness;
fi
I tried putting single quotes but the output also carries the single quotes with it. How can I avoid this issue?
Linux Solutions
Solution 1 - Linux
You only need a minimal change; single-quote the here-document delimiter after <<
.
cat <<'EOF' >> brightup.sh
or equivalently backslash-escape it:
cat <<\EOF >>brightup.sh
Without quoting, the here document will undergo variable substitution, backticks will be evaluated, etc, like you discovered.
If you need to expand some, but not all, values, you need to individually escape the ones you want to prevent.
cat <<EOF >>brightup.sh
#!/bin/sh
# Created on $(date # : <<-- this will be evaluated before cat;)
echo "\$HOME will not be evaluated because it is backslash-escaped"
EOF
will produce
#!/bin/sh
# Created on Fri Feb 16 11:00:18 UTC 2018
echo "$HOME will not be evaluated because it is backslash-escaped"
As suggested by @fedorqui, here is the relevant section from man bash
:
> Here Documents
>
> This type of redirection instructs the shell to read input from the
> current source until a line containing only delimiter (with no
> trailing blanks) is seen. All of the lines read up to that point are
> then used as the standard input for a command.
>
> The format of here-documents is:
>
> <<[-]word
> here-document
> delimiter
>
> No parameter expansion, command substitution, arithmetic expansion,
> or pathname expansion is performed on word. If any characters in word
> are quoted, the delimiter is the result of quote removal on word, and
> the lines in the here-document are not expanded. If word is
> unquoted, all lines of the here-document are subjected to parameter
> expansion, command substitution, and arithmetic expansion. In the
> latter case, the character sequence \<newline>
is ignored, and \
> must be used to quote the characters \
, $
, and `
.
Solution 2 - Linux
This should work, I just tested it out and it worked as expected: no expansion, substitution, or what-have-you took place.
cat <<< '
#!/bin/bash
curr=`cat /sys/class/backlight/intel_backlight/actual_brightness`
if [ $curr -lt 4477 ]; then
curr=$((curr+406));
echo $curr > /sys/class/backlight/intel_backlight/brightness;
fi' > file # use overwrite mode so that you don't keep on appending the same script to that file over and over again, unless that's what you want.
Using the following also works.
cat <<< ' > file
... code ...'
Also, it's worth noting that when using heredocs, such as << EOF
, substitution and variable expansion and the like takes place. So doing something like this:
cat << EOF > file
cd "$HOME"
echo "$PWD" # echo the current path
EOF
will always result in the expansion of the variables $HOME
and $PWD
. So if your home directory is /home/foobar
and the current path is /home/foobar/bin
, file
will look like this:
cd "/home/foobar"
echo "/home/foobar/bin"
instead of the expected:
cd "$HOME"
echo "$PWD"
Solution 3 - Linux
Or, using your EOF markers, you need to quote the initial marker so expansion won't be done:
#-----v---v------
cat <<'EOF' >> brightup.sh
#!/bin/bash
curr=`cat /sys/class/backlight/intel_backlight/actual_brightness`
if [ $curr -lt 4477 ]; then
curr=$((curr+406));
echo $curr > /sys/class/backlight/intel_backlight/brightness;
fi
EOF
IHTH
Solution 4 - Linux
I know this is a two year old question, but this is a quick answer for those searching for a 'how to'.
If you don't want to have to put quotes around anything you can simply write a block of text to a file, and escape variables you want to export as text (for instance for use in a script) and not escape one's you want to export as the value of the variable.
#!/bin/bash
FILE_NAME="test.txt"
VAR_EXAMPLE="\"string\""
cat > ${FILE_NAME} << EOF
\${VAR_EXAMPLE}=${VAR_EXAMPLE} in ${FILE_NAME}
EOF
Will write '"${VAR_EXAMPLE}="string" in test.txt' into test.txt
This can also be used to output blocks of text to the console with the same rules by omitting the file name
#!/bin/bash
VAR_EXAMPLE="\"string\""
cat << EOF
\${VAR_EXAMPLE}=${VAR_EXAMPLE} to console
EOF
Will output '"${VAR_EXAMPLE}="string" to console'
Solution 5 - Linux
cat with <<EOF>>
will create or append the content to the existing file, won't overwrite. whereas cat with <<EOF>
will create or overwrite the content.
cat test.txt
hello
cat <<EOF>> test.txt
> hi
> EOF
cat test.txt
hello
hi
cat <<EOF> test.txt
> haiiiii
> EOF
cat test.txt
haiiiii