Bdtyktl

Here's a solution that is independent of the length of the string (bash):

string="|abcdefg|"

echo "${string:1:${#string}-2}"

Going off a few posts listed here it seems the simplest way to do it is:

string="|abcdefg|"

echo ${string:1:-1}

edit: works on ubuntu with bash 4.2; does not work on centOS with bash 4.1

Another way is to use head & tail commands:

$ echo -n "|abcdefg|" | tail -c +2 | head -c -2

abcdefg

And another one:

string="|abcdefg|"

echo "${string//|/}"

You can also use sed to remove the | not just referencing the symbol itself but using positional references as in:

$ echo "|abcdefg|" | sed 's:^.(.*).$:1:'

abcdefg

Where ':' are the delimiters (you can replace them with / or any character not in the query, any sign following the s will do it) Here ^ (caret) means at the beginning of the input string and $ (dollar) means at the end. The . (point) that it's after the caret and the one that it's before the dollar sign represents a single character. So in other words we are deleting the first and last characters.
Take in mind this will delete any characters even if | it's not present in the string.

EX:

$ echo "abcdefg" | sed 's:^.(.*).$:1:'

bcdef

shell function

A bit more verbose approach, but works on any sort of first and last character, doesn't have to be the same. Basic idea is that we are taking a variable, reading it character by character, and appending only those
we want to a new variable

Here's that whole idea formatted into a nice function

crop_string_ends() {

    STR="$1" 

    NEWSTR="" 

    COUNT=0 

    while read -n 1 CHAR 

    do

        COUNT=$(($COUNT+1)) 

        if [ $COUNT -eq 1 ] || [ $COUNT -eq ${#STR} ] 

        then

            continue 

        fi 

        NEWSTR="$NEWSTR"$CHAR 

    done <<<"$STR" 

    echo $NEWSTR 

}

And here is that same function in action:

$> crop_string_ends "|abcdefg|"                                                                                       

abcdefg

$> crop_string_ends "HelloWorld"                                                                                      

elloWorl

Python

>>> mystring="|abcdefg|"

>>> print(mystring[1:-1])

abcdefg

or on command line:

$ python -c 'import sys;print sys.stdin.read()[1:-2]' <<< "|abcdefg|"                                             

abcdefg

AWK

$ echo "|abcdefg|" | awk '{print substr($0,2,length($0)-2)}'                                                      

abcdefg

Ruby

$ ruby -ne 'print $_.split("|")[1]' <<< "|abcdefg|"                                                               

abcdefg

Small and universal solution:

expr "|abcdef|" : '.(.*).'

Special in this case and allowing that the '|' character may be there or not:

expr "|abcdef" : '|*([^|]*)|*'