Bash Script – Variable Within a Variable Not Interpreted

bashshell-scriptvariable

I need to extract a path that is set within a config file, to use it in a bash script. This is how that path looks like inside said file:

DIR = "${HOME}/test/tmp"

I need to extract it without quotation marks, and this is how i do it:

TESTVAR="$(cat /home/user/path/to/file.conf | grep ^DIR | grep -o '".*"' | tr -d '"')"

The problem is that commands don't interpret ${HOME} variable "properly". Let's say i call echo $TESTVAR – as a result, instead of this:

/home/user/test/tmp

i get that:

${HOME}/test/tmp

so i can't use it as a parameter of commands inside a script!

Pretty please?

Best Answer

Expansions don't get recursively applied. Doing that would make it impossible to handle arbitrary data with dollar signs embedded. (A related matter is that quotes and redirection and other operators are also just regular characters after expansions.)

A somewhat usual custom is to have config files like that as actual shell script, so (like the ones in Debian's /etc/default), so the file would be

DIR="${HOME}/test/tmp"

and you'd read it with

. configfile

though of course that has the problem that the file is a full shell script, must conform to the shell syntax and the config can run arbitrary commands.

Another possibility would be to run the file through envsubst, e.g. with the file in your question, envsubst < configfile would output:

DIR = "/home/me/test/tmp"

or you could use envsubst '$HOME $OTHER $VARS' to expand just some particular ones.

Note that unlike a shell script would do, envsubst doesn't read any assignments from the input file. E.g. the value of ROOTPATH used on the second line is the one envsubst gets from the environment, it has nothing to do with the "assignment" on the first line:

ROOTPATH=/something/$USER
LOGPATH=$ROOTPATH/logs

Related Solutions

SSH Sudo Expect – Run Command with Sudo on Remote Machine

I think you missed a level of quotes escaping. At this high level of escaping, it is best to simply make a little script for every stage where otherwise quotes would be required.

Otherwise, you could try this modified version (but mind you, I don't encourage this coding style!)

function _remoteInstallation(){
    local retval=1
    local debToInstall=$(basename "$1")
    local remoteMachine="$2"
    spawned=$(expect -d -c "
          set timeout 1800
          spawn \"/usr/bin/ssh -t borrajax@$remoteMachine /usr/bin/sudo /usr/bin/dpkg -i /home/borrajax/Documents/$debToInstall\"
          expect {
                \"Are you sure you want to continue connecting\" { send \"yes\r\"; exp_continue }
                \"password\" { send \"myPassword\r\";  exp_continue }
                \"[sudo] password\" { send \"myPassword\r\";  exp_continue }
                default { exit 1 }
          }
    " )
    retval=$?
    return $retval
}

Bash – Obtain script current directory (so that I can do include files without relative paths and run the script from everywhere)

The $0 variable contains the script's path:

$ cat ~/bin/foo.sh
#!/bin/sh
echo $0

$ ./bin/foo.sh
./bin/foo.sh

$ foo.sh
/home/terdon/bin/foo.sh

$ cd ~/bin
$ foo.sh
./foo.sh

As you can see, the output depends on the way it was called, but it always returns the path to the script relative to the way the script was executed. You can, therefore, do:

## Set mydir to the directory containing the script
## The ${var%pattern} format will remove the shortest match of
## pattern from the end of the string. Here, it will remove the
## script's name,. leaving only the directory. 
mydir="${0%/*}"

# config load
source "$mydir"/config.sh

If the directory is in your $PATH, things are even simpler. You can just run source config.sh. By default, source will look for files in directories in $PATH and will source the first one it finds:

$ help source
source: source filename [arguments]
    Execute commands from a file in the current shell.

Read and execute commands from FILENAME in the current shell.  The
entries in $PATH are used to find the directory containing FILENAME.
If any ARGUMENTS are supplied, they become the positional parameters
when FILENAME is executed.

If you are sure your config.sh is unique or, at least, that it is the first one found in $PATH, you can source it directly. However, I suggest you don't do this and stick to the first method instead. You never know when another config.sh might be in your $PATH.

Best Answer

Related Solutions

SSH Sudo Expect – Run Command with Sudo on Remote Machine

Bash – Obtain script current directory (so that I can do include files without relative paths and run the script from everywhere)

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