Shell Programming and Scripting

adio_idfl_201302_df.txt
dfa_201301_dll.ctl
dalkd_20130301.csv

201302
201301
20130301

 
for i in `ls -1`
do
j=`echo $i | sed 's/_[0-9]*/[0-9]/'`
echo $j
done

 for i in *; do
  j=`echo $i | sed 's/.*_\([0-9]\+\).*/\1/'`
  echo $j
done

 for i in *; do
  j=`echo $i | sed -r 's/.*_([0-9]+).*/\1/'`
  echo $j
done

i='adio_idfl_201302_df.txt'

echo "${i//[!0-9]/}"
201302

ls  | tr -dc '[:digit:]'

for file in *
do
        [[ "$file" =~ [0-9]+ ]] && printf "%s\n" "${BASH_REMATCH[0]}"
done

BASH_REMATCH

An array variable whose members are assigned by the =~ binary operator to the [[ conditional command.  The element with index  0  is  the  portion  of  the
string  matching  the entire regular expression.  The element with index n is the portion of the string matching the nth parenthesized subexpression.  This
variable is read-only.

$ ls
adio_idfl_201302_df.txt  dalkd_20130301.csv  dfa_201301_dll.ctl
$ ls | tr -dc [:digit:]
20130220130301201301
$ ls | tr -dc '[\n[:digit:]]'
201302
20130301
201301