I need to know if a file aready has line with contents X in it, if not append line. here's the code I've tried.
if ! $(grep 'eval $(perl -I$HOME/foo/lib/perl5 -Mlocal::lib=$HOME/foo)' ~/.bashrc) then echo 'eval $(perl -I$HOME/foo/lib/perl5 -Mlocal::lib=$HOME/foo)' >> ~/.bashrc fi 
#!/bin/bash LINE='eval $(perl -I$HOME/foo/lib/perl5 -Mlocal::lib=$HOME/foo)' if ! grep -qF "$LINE" file.txt ; then echo "$LINE" >> file.txt ; fi The $(...) will return the result of the command, not the errorlevel value. Use without command substitution to get the proper return code.
! wasn't in Bourne, but it is in POSIX. To search for a literal string:
line='eval $(perl -I$HOME/foo/lib/perl5 -Mlocal::lib=$HOME/foo)' file=~/.bashrc if ! grep -q -x -F -e "$line" <"$file"; then printf '%s\n' "$line" >>"$file" fi -q suppresses grep output (you're only interested in the return status). -x requires the whole line to match. -F searches for a literal string rather than a regexp. -e is a precaution in case $line starts with a -.
You may have better luck with awk's index function: it's an equivalent of strstr, so should be well-suited for comparision (as opposed to grep which is for pattern matching).
You can do this with sed. The issue is that you need to include the string in the code twice. Once to test it, again to insert it.
sed -i '/eval \$(perl -I\$HOME\/foo\/lib\/perl5 -Mlocal::lib=\$HOME\/foo)/!{ ${ s/$/\neval \$(perl -I\$HOME\/foo\/lib\/perl5 -Mlocal::lib=\$HOME\/foo)/; }; }' /path/to/file 