In Bash, how do I add a string after each line in a file?

If your sed allows in place editing via the -i parameter:

sed -e 's/$/string after each line/' -i filename

If not, you have to make a temporary file:

typeset TMP_FILE=$( mktemp )

touch "${TMP_FILE}"
cp -p filename "${TMP_FILE}"
sed -e 's/$/string after each line/' "${TMP_FILE}" > filename

I prefer using awk. If there is only one column, use $0, else replace it with the last column.

One way,

awk '{print $0, "string to append after each line"}' file > new_file

or this,

awk '$0=$0"string to append after each line"' file > new_file

I prefer echo. using pure bash:

cat file | while read line; do echo ${line}$string; done

If you have it, the lam (laminate) utility can do it, for example:

$ lam filename -s "string after each line"

1. Pure POSIX shell and sponge:

   suffix=foobar
   while read l ; do printf '%s\n' "$l" "${suffix}" ; done < file | 
   sponge file
   

2. xargs and printf:

   suffix=foobar
   xargs -L 1 printf "%s${suffix}\n" < file | sponge file
   

3. Using join:

   suffix=foobar
   join file file -e "${suffix}" -o 1.1,2.99999 | sponge file
   

4. Shell tools using paste, yes, head & wc:

   suffix=foobar
   paste file <(yes "${suffix}" | head -$(wc -l < file) ) | sponge file
   

   Note that paste inserts a Tab char before $suffix.

Of course sponge can be replaced with a temp file, afterwards mv'd over the original filename, as with some other answers...