One line if/else condition in linux shell scripting

To summarize the other answers, for general use:

Multi-line if...then statement

if [ foo ]; then
    a; b
elif [ bar ]; then
    c; d
else
    e; f
fi

Single-line version

if [ foo ]; then a && b; elif [ bar ]; c && d; else e && f; fi

Using the OR operator

( foo && a && b ) || ( bar && c && d ) || e && f;

Notes

Remember that the AND and OR operators evaluate whether or not the result code of the previous operation was equal to true/success (0). So if a custom function returns something else (or nothing at all), you may run into problems with the AND/OR shorthand. In such cases, you may want to replace something like ( a && b ) with ( [ a == 'EXPECTEDRESULT' ] && b ), etc.

Also note that ( and [ are technically commands, so whitespace is required around them.

Instead of a group of && statements like then a && b; else, you could also run statements in a subshell like then $( a; b ); else, though this is less efficient. The same is true for doing something like result1=$( foo; a; b ); result2=$( bar; c; d ); [ "$result1" -o "$result2" ] instead of ( foo && a && b ) || ( bar && c && d ). Though at that point you'd be getting more into less-compact, multi-line stuff anyway.

It looks as if you were on the right track. You just need to add the else statement after the ";" following the "then" statement. Also I would split the first line from the second line with a semicolon instead of joining it with "&&".

maxline='cat journald.conf | grep "#SystemMaxUse="'; if [ $maxline == "#SystemMaxUse=" ]; then sed 's/\#SystemMaxUse=/SystemMaxUse=50M/g' journald.conf > journald.conf2 && mv journald.conf2 journald.conf; else echo "This file has been edited. You'll need to do it manually."; fi

Also in your original script, when declaring maxline you used back-ticks "`" instead of single quotes "'" which might cause problems.

It's not a direct answer to the question but you could just use the OR-operator

( grep "#SystemMaxUse=" journald.conf > /dev/null && sed -i 's/\#SystemMaxUse=/SystemMaxUse=50M/g' journald.conf ) || echo "This file has been edited. You'll need to do it manually."