How to manually expand a special variable (ex: ~ tilde) in bash

Question

I have a variable in my bash script whose value is something like this:

~/a/b/c

Note that it is unexpanded tilde. When I do ls -lt on this

Answer 1

Simplest: replace 'magic' with 'eval echo'.

$ eval echo "~"
/whatever/the/f/the/home/directory/is

Problem: You're going to run into issues with other variables because eval is evil. For instance:

$ # home is /Users/Hacker$(s)
$ s="echo SCARY COMMAND"
$ eval echo $(eval echo "~")
/Users/HackerSCARY COMMAND

Note that the issue of the injection doesn't happen on the first expansion. So if you were to simply replace magic with eval echo, you should be okay. But if you do echo $(eval echo ~), that would be susceptible to injection.

Similarly, if you do eval echo ~ instead of eval echo "~", that would count as twice expanded and therefore injection would be possible right away.

Answer 2

Here is the POSIX function equivalent of Håkon Hægland's Bash answer

expand_tilde() {
    tilde_less="${1#\~/}"
    [ "$1" != "$tilde_less" ] && tilde_less="$HOME/$tilde_less"
    printf '%s' "$tilde_less"
}

2017-12-10 edit: add '%s' per @CharlesDuffy in the comments.

Answer 3

You might find this easier to do in python.

(1) From the unix command line:

python -c 'import os; import sys; print os.path.expanduser(sys.argv[1])' ~/fred

Results in:

/Users/someone/fred

(2) Within a bash script as a one-off - save this as test.sh:

#!/usr/bin/env bash

thepath=$(python -c 'import os; import sys; print os.path.expanduser(sys.argv[1])' $1)

echo $thepath

Running bash ./test.sh results in:

/Users/someone/fred

(3) As a utility - save this as expanduser somewhere on your path, with execute permissions:

#!/usr/bin/env python

import sys
import os

print os.path.expanduser(sys.argv[1])

This could then be used on the command line:

expanduser ~/fred

Or in a script:

#!/usr/bin/env bash

thepath=$(expanduser $1)

echo $thepath

Answer 4

Expanding (no pun intended) on birryree's and halloleo's answers: The general approach is to use eval, but it comes with some important caveats, namely spaces and output redirection (>) in the variable. The following seems to work for me:

mypath="$1"

if [ -e "`eval echo ${mypath//>}`" ]; then
    echo "FOUND $mypath"
else
    echo "$mypath NOT FOUND"
fi

Try it with each of the following arguments:

'~'
'~/existing_file'
'~/existing file with spaces'
'~/nonexistant_file'
'~/nonexistant file with spaces'
'~/string containing > redirection'
'~/string containing > redirection > again and >> again'

Explanation

• The ${mypath//>} strips out > characters which could clobber a file during the eval.
• The eval echo ... is what does the actual tilde expansion
• The double-quotes around the -e argument are for support of filenames with spaces.

Perhaps there's a more elegant solution, but this is what I was able to come up with.

Answer 5

why not delve straight into getting the user's home directory with getent?

$ getent passwd mike | cut -d: -f6
/users/mike

Answer 6

Plagarizing myself from a prior answer, to do this robustly without the security risks associated with eval:

expandPath() {
  local path
  local -a pathElements resultPathElements
  IFS=':' read -r -a pathElements <<<"$1"
  : "${pathElements[@]}"
  for path in "${pathElements[@]}"; do
    : "$path"
    case $path in
      "~+"/*)
        path=$PWD/${path#"~+/"}
        ;;
      "~-"/*)
        path=$OLDPWD/${path#"~-/"}
        ;;
      "~"/*)
        path=$HOME/${path#"~/"}
        ;;
      "~"*)
        username=${path%%/*}
        username=${username#"~"}
        IFS=: read -r _ _ _ _ _ homedir _ < <(getent passwd "$username")
        if [[ $path = */* ]]; then
          path=${homedir}/${path#*/}
        else
          path=$homedir
        fi
        ;;
    esac
    resultPathElements+=( "$path" )
  done
  local result
  printf -v result '%s:' "${resultPathElements[@]}"
  printf '%s\n' "${result%:}"
}

...used as...

path=$(expandPath '~/hello')

---

Alternately, a simpler approach that uses eval carefully:

expandPath() {
  case $1 in
    ~[+-]*)
      local content content_q
      printf -v content_q '%q' "${1:2}"
      eval "content=${1:0:2}${content_q}"
      printf '%s\n' "$content"
      ;;
    ~*)
      local content content_q
      printf -v content_q '%q' "${1:1}"
      eval "content=~${content_q}"
      printf '%s\n' "$content"
      ;;
    *)
      printf '%s\n' "$1"
      ;;
  esac
}