Shell - Write variable contents to a file
None of the answers above work if your variable:
- starts with
-e
- starts with
-n
- starts with
-E
- contains a
\
followed by ann
- should not have an extra newline appended after it
and so they cannot be relied upon for arbitrary string contents.
In bash, you can use "here strings" as:
cat <<< "$var" > "$destdir"
As noted in the comment by Ash below, @Trebawa's answer (formulated in the same room as mine!) using printf
is a better approach than cat
.
All of the above work, but also have to work around a problem (escapes and special characters) that doesn't need to occur in the first place: Special characters when the variable is expanded by the shell. Just don't do that (variable expansion) in the first place. Use the variable directly, without expansion.
Also, if your variable contains a secret and you want to copy that secret into a file, you might want to not have expansion in the command line as tracing/command echo of the shell commands might reveal the secret. Means, all answers which use $var
in the command line may have a potential security risk by exposing the variable contents to tracing and logging of the shell.
For variables that are already exported, use this:
printenv var >file
That means, in case of the OP question:
printenv var >"$destfile"
Note: variable names are case sensitive.
Warning: It is not a good idea to export a variable just for the sake of printing it with printenv. If you have a non-exported script variable that contains a secret, exporting it will expose it to all future child processes (unless unexported, for example using export -n
).
Use the echo
command:
var="text to append";
destdir=/some/directory/path/filename
if [ -f "$destdir" ]
then
echo "$var" > "$destdir"
fi
The if
tests that $destdir
represents a file.
The >
appends the text after truncating the file. If you only want to append the text in $var
to the file existing contents, then use >>
instead:
echo "$var" >> "$destdir"
The cp
command is used for copying files (to files), not for writing text to a file.
echo
has the problem that if var
contains something like -e
, it will be interpreted as a flag. Another option is printf
, but printf "$var" > "$destdir"
will expand any escaped characters in the variable, so if the variable contains backslashes the file contents won't match. However, because printf
only interprets backslashes as escapes in the format string, you can use the %s
format specifier to store the exact variable contents to the destination file:
printf "%s" "$var" > "$destdir"