The txt file is :
bar
quux
kabe
Ass
sBo
CcdD
FGH
I would like to grep the words with only one capital letter in this example, but when I use "grep [A-Z]", it shows me all words with capital letters.
Could anyone find the "grep" solution here? My expected output is
Ass
sBo
You seem to have a text file with each word on its own line.
You may use
grep '^[[:lower:]]*[[:upper:]][[:lower:]]*$' file
See the grep online demo.
The ^ matches the start of string (here, line since grep operates on a line by lin basis by default), then [[:lower:]]* matches 0 or more lowercase letters, then an [[:upper:]] pattern matches any uppercase letter, and then [[:lower:]]* matches 0+ lowercase letters and $ asserts the position at the end of string.
If you need to match a whole line with exactly one uppercase letter you may use
grep '^[^[:upper:]]*[[:upper:]][^[:upper:]]*$' file
The only difference from the pattern above is the [^[:upper:]] bracket expression that matches any char but an uppercase letter. See another grep online demo.
To extract words with a single capital letter inside them you may use word boundaries, as shown in mathguy's answer. With GNU grep, you may also use
grep -o '\b[^[:upper:]]*[[:upper:]][^[:upper:]]*\b' file
grep -o '\b[[:lower:]]*[[:upper:]][[:lower:]]*\b' file