Related
I am trying to understand what a parameter expansion does inside a bash script.
third_party_bash_script
#!/bin/sh
files="${*:--}"
# For my understanding I tried to print the contents of files
echo $files
pkill bb_stream
if [ "x$VERBOSE" != "" ]; then
ARGS=-v1
fi
while [ 1 ]; do cat $files; done | bb_stream $ARGS
When I run ./third_party_bash_script, all it prints is a hyphen - and nothing else. Since it did not make sense to me, I also tried to experiment with it in the terminal
$ set one="1" two="2" three="3"
$ files="${*:--}"
$ echo $files
one="1" two="2" three="3"
$ set four="4"
$ files="${*:--}"
four="4"
I can't seem to understand what it's doing. Could someone kindly help me with the interpretation of ${*:--} by the sh?
"$#" is an array of the arguments passed to your script, "$*" is a string of all of those arguments concatenated with blanks in between.
"${*:--}" is the string of arguments if any were provided (:-), or - otherwise which means "take input from stdin" otherwise.
"${#:--}" is the array of arguments if any were provided (:-), or - otherwise which means "take input from stdin" otherwise.
$ cat file
foo
bar
$ cat tst.sh
#!/usr/bin/env bash
awk '{ print FILENAME, $0 }' "${#:--}"
When an arg is provided to the script, "$#" contains "file" so that is the arg that awk is called with:
$ ./tst.sh file
file foo
file bar
When no arg is provided to the script, "$#" is empty so awk is called with - (meaning read from stdin) as it's arg:
$ cat file | ./tst.sh
- foo
- bar
You almost always want to use "${#:--}" rather than "${*:--}" in this context, see https://unix.stackexchange.com/questions/41571/what-is-the-difference-between-and for more info on "$#" vs "$*".
${param:-default} expands to the value of $param if $param is set and not empty, otherwise it expands to default.
$* is all the command-line arguments to the script.
In ${*:--}, param is * and default is -. If $* is not an empty string, it expands to the script arguments. If it's empty, it expands to the default value -.
This can be used in a script to implement the common behavior that a program reads from the files listed in its arguments, and reads from standard input if no filename arguments are given. Many commands treat an input filename argument - as standing for the standard input.
NOTE: addressing OP's original, pre-edited post ...
See shell parameter expansion for a brief review of different options.
While the other answers reference the use of ${*:--} (and ${#:--}) as a alternate means of reading from stdin, OP's sample script is a bit simpler ... if the variable $* (ie, script's command line args) is empty then replace with the literal string -.
We can see this with a few examples:
$ third_party_bash_script
-
$ third_party_bash_script a b c
a b c
$ echo 'a b c' | third_party_bash_script
-
If we replace ${*:--} with ${*:-REPLACEMENT}:
$ third_party_bash_script
REPLACEMENT
$ third_party_bash_script a b c
a b c
$ echo 'a b c' | third_party_bash_script
REPLACEMENT
I'm guessing in OP's actual script there's more going on with the $files variable so in order to know for sure how the ${*:--} is being processed we'd need to see the actual script and how it's referencing the $files variable.
As for OP's set|files=|echo code snippets:
$ set one="1" two="2" three="3"
$ files="${*:--}"
$ echo $files
one=1 two=2 three=3
We can see the same behavior from the script with:
$ third_party_bash_script one="1" two="2" three="3"
one=1 two=2 three=3
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 variable (call it $VAR), I get no such directory. I want to let bash interpret/expand this variable without executing it. In other words, I want bash to run eval but not run the evaluated command. Is this possible in bash?
How did I manage to pass this into my script without expansion? I passed the argument in surrounding it with double quotes.
Try this command to see what I mean:
ls -lt "~"
This is exactly the situation I am in. I want the tilde to be expanded. In other words, what should I replace magic with to make these two commands identical:
ls -lt ~/abc/def/ghi
and
ls -lt $(magic "~/abc/def/ghi")
Note that ~/abc/def/ghi may or may not exist.
If the variable var is input by the user, eval should not be used to expand the tilde using
eval var=$var # Do not use this!
The reason is: the user could by accident (or by purpose) type for example var="$(rm -rf $HOME/)" with possible disastrous consequences.
A better (and safer) way is to use Bash parameter expansion:
var="${var/#\~/$HOME}"
Due to the nature of StackOverflow, I can't just make this answer unaccepted, but in the intervening 5 years since I posted this there have been far better answers than my admittedly rudimentary and pretty bad answer (I was young, don't kill me).
The other solutions in this thread are safer and better solutions. Preferably, I'd go with either of these two:
Charle's Duffy's solution
Håkon Hægland's solution
Original answer for historic purposes (but please don't use this)
If I'm not mistaken, "~" will not be expanded by a bash script in that manner because it is treated as a literal string "~". You can force expansion via eval like this.
#!/bin/bash
homedir=~
eval homedir=$homedir
echo $homedir # prints home path
Alternatively, just use ${HOME} if you want the user's home directory.
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
}
How about this:
path=`realpath "$1"`
Or:
path=`readlink -f "$1"`
A safe way to use eval is "$(printf "~/%q" "$dangerous_path")". Note that is bash specific.
#!/bin/bash
relativepath=a/b/c
eval homedir="$(printf "~/%q" "$relativepath")"
echo $homedir # prints home path
See this question for details
Also, note that under zsh this would be as as simple as echo ${~dangerous_path}
Here is a ridiculous solution:
$ echo "echo $var" | bash
An explanation of what this command does:
create a new instance of bash, by... calling bash;
take the string "echo $var" and substitute $var with the value of the variable (thus after the substitution the string will contain the tilde);
take the string produced by step 2 and send it to the instance of bash created in step one, which we do here by calling echo and piping its output with the | character.
Basically the current bash instance we're running takes our place as the user of another bash instance and types in the command "echo ~..." for us.
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.
why not delve straight into getting the user's home directory with getent?
$ getent passwd mike | cut -d: -f6
/users/mike
I believe this is what you're looking for
magic() { # returns unexpanded tilde express on invalid user
local _safe_path; printf -v _safe_path "%q" "$1"
eval "ln -sf ${_safe_path#\\} /tmp/realpath.$$"
readlink /tmp/realpath.$$
rm -f /tmp/realpath.$$
}
Example usage:
$ magic ~nobody/would/look/here
/var/empty/would/look/here
$ magic ~invalid/this/will/not/expand
~invalid/this/will/not/expand
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' "$tilde_less"
}
2017-12-10 edit: add '%s' per #CharlesDuffy in the comments.
Here's my solution:
#!/bin/bash
expandTilde()
{
local tilde_re='^(~[A-Za-z0-9_.-]*)(.*)'
local path="$*"
local pathSuffix=
if [[ $path =~ $tilde_re ]]
then
# only use eval on the ~username portion !
path=$(eval echo ${BASH_REMATCH[1]})
pathSuffix=${BASH_REMATCH[2]}
fi
echo "${path}${pathSuffix}"
}
result=$(expandTilde "$1")
echo "Result = $result"
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.
For anyone's reference, a function to mimic python's os.path.expanduser() behavior (no eval usage):
# _expand_homedir_tilde ~/.vim
/root/.vim
# _expand_homedir_tilde ~myuser/.vim
/home/myuser/.vim
# _expand_homedir_tilde ~nonexistent/.vim
~nonexistent/.vim
# _expand_homedir_tilde /full/path
/full/path
And the function:
function _expand_homedir_tilde {
(
set -e
set -u
p="$1"
if [[ "$p" =~ ^~ ]]; then
u=`echo "$p" | sed 's|^~\([a-z0-9_-]*\)/.*|\1|'`
if [ -z "$u" ]; then
u=`whoami`
fi
h=$(set -o pipefail; getent passwd "$u" | cut -d: -f6) || exit 1
p=`echo "$p" | sed "s|^~[a-z0-9_-]*/|${h}/|"`
fi
echo $p
) || echo $1
}
Just to extend birryree's answer for paths with spaces: You cannot use the eval command as is because it seperates evaluation by spaces. One solution is to replace spaces temporarily for the eval command:
mypath="~/a/b/c/Something With Spaces"
expandedpath=${mypath// /_spc_} # replace spaces
eval expandedpath=${expandedpath} # put spaces back
expandedpath=${expandedpath//_spc_/ }
echo "$expandedpath" # prints e.g. /Users/fred/a/b/c/Something With Spaces"
ls -lt "$expandedpath" # outputs dir content
This example relies of course on the assumption that mypath never contains the char sequence "_spc_".
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
Just use eval correctly: with validation.
case $1${1%%/*} in
([!~]*|"$1"?*[!-+_.[:alnum:]]*|"") ! :;;
(*/*) set "${1%%/*}" "${1#*/}" ;;
(*) set "$1"
esac&& eval "printf '%s\n' $1${2+/\"\$2\"}"
I have done this with variable parameter substitution after reading in the path using read -e (among others). So the user can tab-complete the path, and if the user enters a ~ path it gets sorted.
read -rep "Enter a path: " -i "${testpath}" testpath
testpath="${testpath/#~/${HOME}}"
ls -al "${testpath}"
The added benefit is that if there is no tilde nothing happens to the variable, and if there is a tilde but not in the first position it is also ignored.
(I include the -i for read since I use this in a loop so the user can fix the path if there is a problem.)
for some reason when the string is already quoted only perl saves the day
#val="${val/#\~/$HOME}" # for some reason does not work !!
val=$(echo $val|perl -ne 's|~|'$HOME'|g;print')
I think that
thepath=( ~/abc/def/ghi )
is easier than all the other solutions... or I am missing something? It works even if the path does not really exists.
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 variable (call it $VAR), I get no such directory. I want to let bash interpret/expand this variable without executing it. In other words, I want bash to run eval but not run the evaluated command. Is this possible in bash?
How did I manage to pass this into my script without expansion? I passed the argument in surrounding it with double quotes.
Try this command to see what I mean:
ls -lt "~"
This is exactly the situation I am in. I want the tilde to be expanded. In other words, what should I replace magic with to make these two commands identical:
ls -lt ~/abc/def/ghi
and
ls -lt $(magic "~/abc/def/ghi")
Note that ~/abc/def/ghi may or may not exist.
If the variable var is input by the user, eval should not be used to expand the tilde using
eval var=$var # Do not use this!
The reason is: the user could by accident (or by purpose) type for example var="$(rm -rf $HOME/)" with possible disastrous consequences.
A better (and safer) way is to use Bash parameter expansion:
var="${var/#\~/$HOME}"
Due to the nature of StackOverflow, I can't just make this answer unaccepted, but in the intervening 5 years since I posted this there have been far better answers than my admittedly rudimentary and pretty bad answer (I was young, don't kill me).
The other solutions in this thread are safer and better solutions. Preferably, I'd go with either of these two:
Charle's Duffy's solution
Håkon Hægland's solution
Original answer for historic purposes (but please don't use this)
If I'm not mistaken, "~" will not be expanded by a bash script in that manner because it is treated as a literal string "~". You can force expansion via eval like this.
#!/bin/bash
homedir=~
eval homedir=$homedir
echo $homedir # prints home path
Alternatively, just use ${HOME} if you want the user's home directory.
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
}
How about this:
path=`realpath "$1"`
Or:
path=`readlink -f "$1"`
A safe way to use eval is "$(printf "~/%q" "$dangerous_path")". Note that is bash specific.
#!/bin/bash
relativepath=a/b/c
eval homedir="$(printf "~/%q" "$relativepath")"
echo $homedir # prints home path
See this question for details
Also, note that under zsh this would be as as simple as echo ${~dangerous_path}
Here is a ridiculous solution:
$ echo "echo $var" | bash
An explanation of what this command does:
create a new instance of bash, by... calling bash;
take the string "echo $var" and substitute $var with the value of the variable (thus after the substitution the string will contain the tilde);
take the string produced by step 2 and send it to the instance of bash created in step one, which we do here by calling echo and piping its output with the | character.
Basically the current bash instance we're running takes our place as the user of another bash instance and types in the command "echo ~..." for us.
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.
why not delve straight into getting the user's home directory with getent?
$ getent passwd mike | cut -d: -f6
/users/mike
I believe this is what you're looking for
magic() { # returns unexpanded tilde express on invalid user
local _safe_path; printf -v _safe_path "%q" "$1"
eval "ln -sf ${_safe_path#\\} /tmp/realpath.$$"
readlink /tmp/realpath.$$
rm -f /tmp/realpath.$$
}
Example usage:
$ magic ~nobody/would/look/here
/var/empty/would/look/here
$ magic ~invalid/this/will/not/expand
~invalid/this/will/not/expand
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' "$tilde_less"
}
2017-12-10 edit: add '%s' per #CharlesDuffy in the comments.
Here's my solution:
#!/bin/bash
expandTilde()
{
local tilde_re='^(~[A-Za-z0-9_.-]*)(.*)'
local path="$*"
local pathSuffix=
if [[ $path =~ $tilde_re ]]
then
# only use eval on the ~username portion !
path=$(eval echo ${BASH_REMATCH[1]})
pathSuffix=${BASH_REMATCH[2]}
fi
echo "${path}${pathSuffix}"
}
result=$(expandTilde "$1")
echo "Result = $result"
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.
For anyone's reference, a function to mimic python's os.path.expanduser() behavior (no eval usage):
# _expand_homedir_tilde ~/.vim
/root/.vim
# _expand_homedir_tilde ~myuser/.vim
/home/myuser/.vim
# _expand_homedir_tilde ~nonexistent/.vim
~nonexistent/.vim
# _expand_homedir_tilde /full/path
/full/path
And the function:
function _expand_homedir_tilde {
(
set -e
set -u
p="$1"
if [[ "$p" =~ ^~ ]]; then
u=`echo "$p" | sed 's|^~\([a-z0-9_-]*\)/.*|\1|'`
if [ -z "$u" ]; then
u=`whoami`
fi
h=$(set -o pipefail; getent passwd "$u" | cut -d: -f6) || exit 1
p=`echo "$p" | sed "s|^~[a-z0-9_-]*/|${h}/|"`
fi
echo $p
) || echo $1
}
Just to extend birryree's answer for paths with spaces: You cannot use the eval command as is because it seperates evaluation by spaces. One solution is to replace spaces temporarily for the eval command:
mypath="~/a/b/c/Something With Spaces"
expandedpath=${mypath// /_spc_} # replace spaces
eval expandedpath=${expandedpath} # put spaces back
expandedpath=${expandedpath//_spc_/ }
echo "$expandedpath" # prints e.g. /Users/fred/a/b/c/Something With Spaces"
ls -lt "$expandedpath" # outputs dir content
This example relies of course on the assumption that mypath never contains the char sequence "_spc_".
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
Just use eval correctly: with validation.
case $1${1%%/*} in
([!~]*|"$1"?*[!-+_.[:alnum:]]*|"") ! :;;
(*/*) set "${1%%/*}" "${1#*/}" ;;
(*) set "$1"
esac&& eval "printf '%s\n' $1${2+/\"\$2\"}"
I have done this with variable parameter substitution after reading in the path using read -e (among others). So the user can tab-complete the path, and if the user enters a ~ path it gets sorted.
read -rep "Enter a path: " -i "${testpath}" testpath
testpath="${testpath/#~/${HOME}}"
ls -al "${testpath}"
The added benefit is that if there is no tilde nothing happens to the variable, and if there is a tilde but not in the first position it is also ignored.
(I include the -i for read since I use this in a loop so the user can fix the path if there is a problem.)
for some reason when the string is already quoted only perl saves the day
#val="${val/#\~/$HOME}" # for some reason does not work !!
val=$(echo $val|perl -ne 's|~|'$HOME'|g;print')
I think that
thepath=( ~/abc/def/ghi )
is easier than all the other solutions... or I am missing something? It works even if the path does not really exists.
I'm trying to pass 2 arguments to a command and each argument contains spaces, I've tried escaping the spaces in the args, I've tried wrapping in single quotes, I've tried escaping \" but nothing will work.
Here's a simple example.
#!/bin/bash -xv
ARG="/tmp/a b/1.txt"
ARG2="/tmp/a b/2.txt"
ARG_BOTH="\"$ARG\" \"$ARG2\""
cat $ARG_BOTH
I'm getting the following when it runs:
ARG_BOTH="$ARG $ARG2"
+ ARG_BOTH='/tmp/a\ b/1.txt /tmp/a\ b/2.txt'
cat $ARG_BOTH
+ cat '/tmp/a\' b/1.txt '/tmp/a\' b/2.txt
cat: /tmp/a\: No such file or directory
cat: b/1.txt: No such file or directory
cat: /tmp/a\: No such file or directory
cat: b/2.txt: No such file or directory
See http://mywiki.wooledge.org/BashFAQ/050
TLDR
Put your args in an array and call your program as myutil "${arr[#]}"
#!/bin/bash -xv
file1="file with spaces 1"
file2="file with spaces 2"
echo "foo" > "$file1"
echo "bar" > "$file2"
arr=("$file1" "$file2")
cat "${arr[#]}"
Output
file1="file with spaces 1"
+ file1='file with spaces 1'
file2="file with spaces 2"
+ file2='file with spaces 2'
echo "foo" > "$file1"
+ echo foo
echo "bar" > "$file2"
+ echo bar
arr=("$file1" "$file2")
+ arr=("$file1" "$file2")
cat "${arr[#]}"
+ cat 'file with spaces 1' 'file with spaces 2'
foo
bar
This might be a good use-case for the generic "set" command, which sets the top-level shell parameters to a word list. That is, $1, $2, ... and so also $* and $# get reset.
This gives you some of the advantages of arrays while also staying all-Posix-shell-compatible.
So:
set "arg with spaces" "another thing with spaces"
cat "$#"
The most straightforward revision of your example shell script that will work correctly is:
#! /bin/sh
ARG="/tmp/a b/1.txt"
ARG2="/tmp/a b/2.txt"
cat "$ARG" "$ARG2"
However, if you need to wrap up a whole bunch of arguments in one shell variable, you're up a creek; there is no portable, reliable way to do it. (Arrays are Bash-specific; the only portable options are set and eval, both of which are asking for grief.) I would consider a need for this as an indication that it was time to rewrite in a more powerful scripting language, e.g. Perl or Python.
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 variable (call it $VAR), I get no such directory. I want to let bash interpret/expand this variable without executing it. In other words, I want bash to run eval but not run the evaluated command. Is this possible in bash?
How did I manage to pass this into my script without expansion? I passed the argument in surrounding it with double quotes.
Try this command to see what I mean:
ls -lt "~"
This is exactly the situation I am in. I want the tilde to be expanded. In other words, what should I replace magic with to make these two commands identical:
ls -lt ~/abc/def/ghi
and
ls -lt $(magic "~/abc/def/ghi")
Note that ~/abc/def/ghi may or may not exist.
If the variable var is input by the user, eval should not be used to expand the tilde using
eval var=$var # Do not use this!
The reason is: the user could by accident (or by purpose) type for example var="$(rm -rf $HOME/)" with possible disastrous consequences.
A better (and safer) way is to use Bash parameter expansion:
var="${var/#\~/$HOME}"
Due to the nature of StackOverflow, I can't just make this answer unaccepted, but in the intervening 5 years since I posted this there have been far better answers than my admittedly rudimentary and pretty bad answer (I was young, don't kill me).
The other solutions in this thread are safer and better solutions. Preferably, I'd go with either of these two:
Charle's Duffy's solution
Håkon Hægland's solution
Original answer for historic purposes (but please don't use this)
If I'm not mistaken, "~" will not be expanded by a bash script in that manner because it is treated as a literal string "~". You can force expansion via eval like this.
#!/bin/bash
homedir=~
eval homedir=$homedir
echo $homedir # prints home path
Alternatively, just use ${HOME} if you want the user's home directory.
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
}
How about this:
path=`realpath "$1"`
Or:
path=`readlink -f "$1"`
A safe way to use eval is "$(printf "~/%q" "$dangerous_path")". Note that is bash specific.
#!/bin/bash
relativepath=a/b/c
eval homedir="$(printf "~/%q" "$relativepath")"
echo $homedir # prints home path
See this question for details
Also, note that under zsh this would be as as simple as echo ${~dangerous_path}
Here is a ridiculous solution:
$ echo "echo $var" | bash
An explanation of what this command does:
create a new instance of bash, by... calling bash;
take the string "echo $var" and substitute $var with the value of the variable (thus after the substitution the string will contain the tilde);
take the string produced by step 2 and send it to the instance of bash created in step one, which we do here by calling echo and piping its output with the | character.
Basically the current bash instance we're running takes our place as the user of another bash instance and types in the command "echo ~..." for us.
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.
why not delve straight into getting the user's home directory with getent?
$ getent passwd mike | cut -d: -f6
/users/mike
I believe this is what you're looking for
magic() { # returns unexpanded tilde express on invalid user
local _safe_path; printf -v _safe_path "%q" "$1"
eval "ln -sf ${_safe_path#\\} /tmp/realpath.$$"
readlink /tmp/realpath.$$
rm -f /tmp/realpath.$$
}
Example usage:
$ magic ~nobody/would/look/here
/var/empty/would/look/here
$ magic ~invalid/this/will/not/expand
~invalid/this/will/not/expand
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' "$tilde_less"
}
2017-12-10 edit: add '%s' per #CharlesDuffy in the comments.
Here's my solution:
#!/bin/bash
expandTilde()
{
local tilde_re='^(~[A-Za-z0-9_.-]*)(.*)'
local path="$*"
local pathSuffix=
if [[ $path =~ $tilde_re ]]
then
# only use eval on the ~username portion !
path=$(eval echo ${BASH_REMATCH[1]})
pathSuffix=${BASH_REMATCH[2]}
fi
echo "${path}${pathSuffix}"
}
result=$(expandTilde "$1")
echo "Result = $result"
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.
For anyone's reference, a function to mimic python's os.path.expanduser() behavior (no eval usage):
# _expand_homedir_tilde ~/.vim
/root/.vim
# _expand_homedir_tilde ~myuser/.vim
/home/myuser/.vim
# _expand_homedir_tilde ~nonexistent/.vim
~nonexistent/.vim
# _expand_homedir_tilde /full/path
/full/path
And the function:
function _expand_homedir_tilde {
(
set -e
set -u
p="$1"
if [[ "$p" =~ ^~ ]]; then
u=`echo "$p" | sed 's|^~\([a-z0-9_-]*\)/.*|\1|'`
if [ -z "$u" ]; then
u=`whoami`
fi
h=$(set -o pipefail; getent passwd "$u" | cut -d: -f6) || exit 1
p=`echo "$p" | sed "s|^~[a-z0-9_-]*/|${h}/|"`
fi
echo $p
) || echo $1
}
Just to extend birryree's answer for paths with spaces: You cannot use the eval command as is because it seperates evaluation by spaces. One solution is to replace spaces temporarily for the eval command:
mypath="~/a/b/c/Something With Spaces"
expandedpath=${mypath// /_spc_} # replace spaces
eval expandedpath=${expandedpath} # put spaces back
expandedpath=${expandedpath//_spc_/ }
echo "$expandedpath" # prints e.g. /Users/fred/a/b/c/Something With Spaces"
ls -lt "$expandedpath" # outputs dir content
This example relies of course on the assumption that mypath never contains the char sequence "_spc_".
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
Just use eval correctly: with validation.
case $1${1%%/*} in
([!~]*|"$1"?*[!-+_.[:alnum:]]*|"") ! :;;
(*/*) set "${1%%/*}" "${1#*/}" ;;
(*) set "$1"
esac&& eval "printf '%s\n' $1${2+/\"\$2\"}"
I have done this with variable parameter substitution after reading in the path using read -e (among others). So the user can tab-complete the path, and if the user enters a ~ path it gets sorted.
read -rep "Enter a path: " -i "${testpath}" testpath
testpath="${testpath/#~/${HOME}}"
ls -al "${testpath}"
The added benefit is that if there is no tilde nothing happens to the variable, and if there is a tilde but not in the first position it is also ignored.
(I include the -i for read since I use this in a loop so the user can fix the path if there is a problem.)
for some reason when the string is already quoted only perl saves the day
#val="${val/#\~/$HOME}" # for some reason does not work !!
val=$(echo $val|perl -ne 's|~|'$HOME'|g;print')
I think that
thepath=( ~/abc/def/ghi )
is easier than all the other solutions... or I am missing something? It works even if the path does not really exists.