How do I manipulate $PATH elements in shell scripts?

by

Addressing the proposed solution from dmckee:

  1. While some versions of Bash may allow hyphens in function names, others (MacOS X) do not.
  2. I don’t see a need to use return immediately before the end of the function.
  3. I don’t see the need for all the semi-colons.
  4. I don’t see why you have path-element-by-pattern export a value. Think of export as equivalent to setting (or even creating) a global variable – something to be avoided whenever possible.
  5. I’m not sure what you expect ‘replace-path PATH $PATH /usr‘ to do, but it does not do what I would expect.

Consider a PATH value that starts off containing:

.
/Users/jleffler/bin
/usr/local/postgresql/bin
/usr/local/mysql/bin
/Users/jleffler/perl/v5.10.0/bin
/usr/local/bin
/usr/bin
/bin
/sw/bin
/usr/sbin
/sbin

The result I got (from ‘replace-path PATH $PATH /usr‘) is:

.
/Users/jleffler/bin
/local/postgresql/bin
/local/mysql/bin
/Users/jleffler/perl/v5.10.0/bin
/local/bin
/bin
/bin
/sw/bin
/sbin
/sbin

I would have expected to get my original path back since /usr does not appear as a (complete) path element, only as part of a path element.

This can be fixed in replace-path by modifying one of the sed commands:

export $path=$(echo -n $list | tr ":" "\n" | sed "s:^$removestr\$:$replacestr:" |
               tr "\n" ":" | sed "s|::|:|g")

I used ‘:’ instead of ‘|’ to separate parts of the substitute since ‘|’ could (in theory) appear in a path component, whereas by definition of PATH, a colon cannot. I observe that the second sed could eliminate the current directory from the middle of a PATH. That is, a legitimate (though perverse) value of PATH could be:

PATH=/bin::/usr/local/bin

After processing, the current directory would no longer be on the PATH.

A similar change to anchor the match is appropriate in path-element-by-pattern:

export $target=$(echo -n $list | tr ":" "\n" | grep -m 1 "^$pat\$")

I note in passing that grep -m 1 is not standard (it is a GNU extension, also available on MacOS X). And, indeed, the-n option for echo is also non-standard; you would be better off simply deleting the trailing colon that is added by virtue of converting the newline from echo into a colon. Since path-element-by-pattern is used just once, has undesirable side-effects (it clobbers any pre-existing exported variable called $removestr), it can be replaced sensibly by its body. This, along with more liberal use of quotes to avoid problems with spaces or unwanted file name expansion, leads to:

# path_tools.bash
#
# A set of tools for manipulating ":" separated lists like the
# canonical $PATH variable.
#
# /bin/sh compatibility can probably be regained by replacing $( )
# style command expansion with ` ` style
###############################################################################
# Usage:
#
# To remove a path:
#    replace_path         PATH $PATH /exact/path/to/remove
#    replace_path_pattern PATH $PATH <grep pattern for target path>
#
# To replace a path:
#    replace_path         PATH $PATH /exact/path/to/remove /replacement/path
#    replace_path_pattern PATH $PATH <target pattern> /replacement/path
#
###############################################################################

# Remove or replace an element of $1
#
#   $1 name of the shell variable to set (e.g. PATH)
#   $2 a ":" delimited list to work from (e.g. $PATH)
#   $3 the precise string to be removed/replaced
#   $4 the replacement string (use "" for removal)
function replace_path () {
    path=$1
    list=$2
    remove=$3
    replace=$4        # Allowed to be empty or unset

    export $path=$(echo "$list" | tr ":" "\n" | sed "s:^$remove\$:$replace:" |
                   tr "\n" ":" | sed 's|:$||')
}

# Remove or replace an element of $1
#
#   $1 name of the shell variable to set (e.g. PATH)
#   $2 a ":" delimited list to work from (e.g. $PATH)
#   $3 a grep pattern identifying the element to be removed/replaced
#   $4 the replacement string (use "" for removal)
function replace_path_pattern () {
    path=$1
    list=$2
    removepat=$3
    replacestr=$4        # Allowed to be empty or unset

    removestr=$(echo "$list" | tr ":" "\n" | grep -m 1 "^$removepat\$")
    replace_path "$path" "$list" "$removestr" "$replacestr"
}

I have a Perl script called echopath which I find useful when debugging problems with PATH-like variables:

#!/usr/bin/perl -w
#
#   "@(#)$Id: echopath.pl,v 1.7 1998/09/15 03:16:36 jleffler Exp $"
#
#   Print the components of a PATH variable one per line.
#   If there are no colons in the arguments, assume that they are
#   the names of environment variables.

@ARGV = $ENV{PATH} unless @ARGV;

foreach $arg (@ARGV)
{
    $var = $arg;
    $var = $ENV{$arg} if $arg =~ /^[A-Za-z_][A-Za-z_0-9]*$/;
    $var = $arg unless $var;
    @lst = split /:/, $var;
    foreach $val (@lst)
    {
            print "$val\n";
    }
}

When I run the modified solution on the test code below:

echo
xpath=$PATH
replace_path xpath $xpath /usr
echopath $xpath

echo
xpath=$PATH
replace_path_pattern xpath $xpath /usr/bin /work/bin
echopath xpath

echo
xpath=$PATH
replace_path_pattern xpath $xpath "/usr/.*/bin" /work/bin
echopath xpath

The output is:

.
/Users/jleffler/bin
/usr/local/postgresql/bin
/usr/local/mysql/bin
/Users/jleffler/perl/v5.10.0/bin
/usr/local/bin
/usr/bin
/bin
/sw/bin
/usr/sbin
/sbin

.
/Users/jleffler/bin
/usr/local/postgresql/bin
/usr/local/mysql/bin
/Users/jleffler/perl/v5.10.0/bin
/usr/local/bin
/work/bin
/bin
/sw/bin
/usr/sbin
/sbin

.
/Users/jleffler/bin
/work/bin
/usr/local/mysql/bin
/Users/jleffler/perl/v5.10.0/bin
/usr/local/bin
/usr/bin
/bin
/sw/bin
/usr/sbin
/sbin

This looks correct to me – at least, for my definition of what the problem is.

I note that echopath LD_LIBRARY_PATH evaluates $LD_LIBRARY_PATH. It would be nice if your functions were able to do that, so the user could type:

replace_path PATH /usr/bin /work/bin

That can be done by using:

list=$(eval echo '$'$path)

This leads to this revision of the code:

# path_tools.bash
#
# A set of tools for manipulating ":" separated lists like the
# canonical $PATH variable.
#
# /bin/sh compatibility can probably be regained by replacing $( )
# style command expansion with ` ` style
###############################################################################
# Usage:
#
# To remove a path:
#    replace_path         PATH /exact/path/to/remove
#    replace_path_pattern PATH <grep pattern for target path>
#
# To replace a path:
#    replace_path         PATH /exact/path/to/remove /replacement/path
#    replace_path_pattern PATH <target pattern> /replacement/path
#
###############################################################################

# Remove or replace an element of $1
#
#   $1 name of the shell variable to set (e.g. PATH)
#   $2 the precise string to be removed/replaced
#   $3 the replacement string (use "" for removal)
function replace_path () {
    path=$1
    list=$(eval echo '$'$path)
    remove=$2
    replace=$3            # Allowed to be empty or unset

    export $path=$(echo "$list" | tr ":" "\n" | sed "s:^$remove\$:$replace:" |
                   tr "\n" ":" | sed 's|:$||')
}

# Remove or replace an element of $1
#
#   $1 name of the shell variable to set (e.g. PATH)
#   $2 a grep pattern identifying the element to be removed/replaced
#   $3 the replacement string (use "" for removal)
function replace_path_pattern () {
    path=$1
    list=$(eval echo '$'$path)
    removepat=$2
    replacestr=$3            # Allowed to be empty or unset

    removestr=$(echo "$list" | tr ":" "\n" | grep -m 1 "^$removepat\$")
    replace_path "$path" "$removestr" "$replacestr"
}

The following revised test now works too:

echo
xpath=$PATH
replace_path xpath /usr
echopath xpath

echo
xpath=$PATH
replace_path_pattern xpath /usr/bin /work/bin
echopath xpath

echo
xpath=$PATH
replace_path_pattern xpath "/usr/.*/bin" /work/bin
echopath xpath

It produces the same output as before.

Leave a Comment