Replace command in Bash script from another script

Question

I'm trying to setup a script that will fetch changes from github where a script has a command that's not executable in Mac OS X.
What I'm aiming for is to use sed etc to replace the command within that script.

These are the commands:

Original command (to be replaced in script):

DIR=$(dirname "$(readlink -f $0)")

New command (I want to replace with):

DIR="$(cd "$(dirname "$0")" && pwd -P)"

What I've tried so far:

StartStopScript="/path/to/script.sh"

DIRnew=""$(cd "$(dirname "$0")" && pwd -P)""

DIRold=""$(dirname "$(readlink -f $0)")""

echo "$StartStopScript" | sed -e 's/"$DIRold"/"$DIRnew"/g'

This fails and the commands seems to be executed instead of interpreted as a string.
Figure it comes down to quoting in sed.

Grateful for any help I can get with this.

Answer 1

OrigScript="/path/to/script.sh"
echo $OrigScript
/path/to/script.sh
#note: no quotes
OrigScript='"/path/to/script.sh"'
echo $OrigScript
"/path/to/script.sh"
#note: with quotes, but not good for filesnames, rather use "$filename" in the command argument


DIRnew='"$(cd "$(dirname "$0")" \&\& pwd -P)"'
echo $DIRnew
"$(cd "$(dirname "$0")" \&\& pwd -P)"
#note: with quotes, escaping & needed for suppressing interpretation by sed

DIRold='"$(dirname "$(readlink -f $0)")"'
echo $DIRold
"$(dirname "$(readlink -f $0)")"
#note: in your question the original DIR=.. did NOT have double quotes,
#I assume a typo

sed -e "s/$DIRold/$DIRnew/g" "$OrigScript"
#note: $OrigScript should NOT have the double quotes in it!
#note: I assume $OrigScript == $StartStopScript

#test:
echo $DIRold | sed "s/$DIRold/$DIRnew/"
"$(cd "$(dirname "$0")" && pwd -P)"
#result includes double quotes!

1) for declaring a variable with (double) quotes, you need to escape the quotes to have the quotes in the string, better use single quotes to make sure the string inside is exactly just the string and no variables are interpreted. This also simplifies the use of &, and $() as the are then not interpreted by the shell

2) for echoing (double) quotes, escape them

3) for allowing sed to interpret a shell variable, you MUST use double quotes around the sed command ( with single quotes, the command sed 's/$a/$b/' will try to replace the actual string '$a' with the string '$b' , also be aware of the special meaning of $ in sed), double quotes around the variable within the sed command is not needed, as sed is for treating strings with any character (with restrictions regarding the special command characters of sed).

4) echo "$StartStopScript" | sed -e 's/"$DIRold"/"$DIRnew"/g' will echo the variable $StartStopScript ( which I assume is simply the file name of the script ) and try to replace the variables in that string (i.e. the file name). For applying sed to a file, use sed 's/a/b/g' file (output written to stout), for overwriting the file, use -i (use with care, i.e. test for the right substitutions first)

5) & has a special meaning in sed, it reprints the initial (matching) pattern:

echo ABC | sed 's/A/&12/'
A12BC

You will have to escape it in the replacement pattern to suppress interpretation:

echo ABC | sed 's/A/\&12/'
&12BC

Thus the variable $DIRnew needs to have the escape backslashes in its string.

6) Apart from that, I would suggest to rather use an if loop identifying the OS and to adapt the commands accordingly. That way you only need one script for both OSes. I.e.

if [ OS = Mac ] ; then
    dir=MacDir
else
    dir=LinuxDir
fi

Answer 2

It's probably more:

DIR="$(CDPATH= cd -P -- "$(dirname -- "$0")" && pwd -P)"

That you want.

(the outer quotes are not strictly necessary here when the result is used in the target of a scalar variable assignment but would be in other contexts).

You were missing:

• the -- end-of-option marker without which it won't work properly if $0 starts with -
• the -P without which cd doesn't do a straight chdir() but treats .. components specially (and in a different way from the way the shell resolved the path of the script as stored in $0 when opening it to interpret its contents)
• CDPATH= without which for relative paths cd could go into a directory relative to something other than the current working directory if there was a CDPATH variable in the environment.

Pretty annoying that cd in Korn/POSIX-like shells doesn't have something like a -r (raw) so it can do just a plain chdir(). Even with all those work arounds above, it still fails for - (or -1, -3, +1 in zsh) where cd chdir()s to a past directory instead.

If you could switch to zsh, you could just do DIR=$0:h:P which gives you the realPath of the head of $0 without having to do all that gymnastic.

Then, to replace arbitrary strings, perl would be a much petter choice than sed.

StartStopScript=/path/to/script.sh
DIRold='"$(dirname "$(readlink -f $0)")"'
DIRnew='"$(CDPATH= cd -P -- "$(dirname -- "$0")" && pwd -P)"'

OLD=$DIRold NEW=$DIRnew perl -pe '
  s/\Q$ENV{OLD}\E/$ENV{NEW}/g' < "$StartStopScript"

Doing the same reliably with sed is much more difficult. See How to ensure that string interpolated into `sed` substitution escapes all metachars for details (and where that perl approach is described as well).