How to extract a substring using sed on OS X? - regex

Question

Im trying to iterate over each file and folder inside a directory and extract part of the file name into a variable, but I can't make sed work correctly. I either get all of the file name or none of it.
This version of the script should capture the entire file name:
#!/bin/bash
for f in *
do
substring=`echo $f | sed -E -n 's/(.*)/\1/'`
echo "sub: $substring"
done
But instead I get nothing:
sub:
sub:
sub:
sub:
...
This version should give me just the first character in the filename:
#!/bin/bash
for f in *
do
substring=`echo $f | sed -E 's/^([a-zA-Z])/\1/'`
echo "sub: $substring"
done
But instead I get the whole file name:
sub: Adlm
sub: Applications
sub: Applications (Parallels)
sub: Desktop
...
I've tried numerous iterations of it and what it basically boils down to is that if I use -n I get nothing and if I don't I get the whole file name.
Can someone show me how to get just the first character?
Or, my overall goal is to be able to extract a substring and store it into a variable, if anybody has a better approach to it, that would be appreciated as well.
Thanks in advance.

Answer 1

If you want to modify a shell parameter, you probably want to use a parameter expansion.
for f in *; do
# This version should expand to the whole parameter
echo "$f"
# This version should expand to the first character in the filename
echo "${f::1}"
done
Parameter expansions are not as powerful as sed, but they are built in to the shell (no launching a separate process or subshell necessary) and there are expansions for:
Substrings (as above)
Replacing and substituting characters
Altering the case of strings (bash 4+)
and more.

Answer 2

This version of the script should capture the entire file name:
sed -E -n 's/(.*)/\1/'
But instead I get nothing.
You used -n so naturally it won't yield anything. Perhaps you should remove -n or add p:
sed -E -n 's/(.*)/\1/p'
This version should give me just the first character in the filename:
sed -E 's/^([a-zA-Z])/\1/'
But instead I get the whole file name,
You didn't replace anything there. Perhaps what you wanted was
sed -E 's/^([a-zA-Z]).*/\1/'
Also I suggest quoting your arguments well:
substring=`echo "$f" | sed ...'`
Finally the simpler method is to use substring expansion if you're using Bash as suggested by kojiro.

Answer 3

You forget to add .* after the capturing group in sed,
$ for i in *; do substring=`echo $i | sed -E 's/^(.).*$/\1/'`; echo "sub: $substring"; done
It's better to use . instead of [a-zA-Z] because it may fail if the first character starts with any special character.

Answer 4

I prefer awk to sed. It seems to be easier for me to understand.
#!/bin/bash
#set -x
for f in *
do
substring=`echo $f | awk '{print substr($1,1,1)}'`
echo "sub: $substring"
done