How to iterate over the characters of a string in a POSIX shell script?

问题

A POSIX compliant shell shall provide mechanisms like this to iterate over collections of strings:

for x in $(seq 1 5); do
    echo $x
done

But, how do I iterate over each character of a word?

回答1:

It's a little circuitous, but I think this'll work in any posix-compliant shell. I've tried it in dash, but I don't have busybox handy to test with.

var='ab * cd'

tmp="$var"    # The loop will consume the variable, so make a temp copy first
while [ -n "$tmp" ]; do
    rest="${tmp#?}"    # All but the first character of the string
    first="${tmp%"$rest"}"    # Remove $rest, and you're left with the first character
    echo "$first"
    tmp="$rest"
done

Output:

a
b

*

c
d

Note that the double-quotes around the right-hand side of assignments are not needed; I just prefer to use double-quotes around all expansions rather than trying to keep track of where it's safe to leave them off. On the other hand, the double-quotes in [ -n "$tmp" ] are absolutely necessary, and the inner double-quotes in first="${tmp%"$rest"}" are needed if the string contains "*".

回答2:

You can use the fold command to add a newline after each character and store the result on a variable. You can then iterate over its elements.

word="some_word"

word=$(echo "$word" | fold -w 1)
for x in "$word"; do
    echo $x
done

It will print:

s
o
m
e
_
w
o
r
d

回答3:

This works in dash and busybox:

echo 'ab * cd' | grep -o .

Output:

a
b

*

c
d

来源：https://stackoverflow.com/questions/51052475/how-to-iterate-over-the-characters-of-a-string-in-a-posix-shell-script