How can I split a shell command over multiple lines when using an IF statement?

Question

How can I split a command over multiple lines in the shell, when the command is part of an if statement?

This works:

if ! fab --fabfile=.deploy/fabfile.py --forward-agent --disable-known-hosts deploy:$target; then rc=1                                                                        fi 

This doesn't work:

# does not work: if ! fab --fabfile=.deploy/fabfile.py \    --forward-agent \   --disable-known-hosts deploy:$target; then      rc=1 fi 

Instead of the whole command executing, I get:

./script.sh: line 73: --forward-agent: command not found 

More importantly, what is missing from my understanding of Bash that will help me understand this and similar issues in the future?

Answer 1

The line-continuation will fail if you have whitespace (spaces or tab characters¹) after the backslash and before the newline. With no such whitespace, your example works fine for me:

$ cat test.sh if ! fab --fabfile=.deploy/fabfile.py \    --forward-agent \    --disable-known-hosts deploy:$target; then      echo failed else      echo succeeded fi  $ alias fab=true; . ./test.sh succeeded $ alias fab=false; . ./test.sh failed 

Some detail promoted from the comments: the line-continuation backslash in the shell is not really a special case; it is simply an instance of the general rule that a backslash "quotes" the immediately-following character, preventing any special treatment it would normally be subject to. In this case, the next character is a newline, and the special treatment being prevented is terminating the command. Normally, a quoted character winds up included literally in the command; a backslashed newline is instead deleted entirely. But otherwise, the mechanism is the same. Most importantly, the backslash only quotes the immediately-following character; if that character is a space or tab, you just get a literal space or tab; the backslash will have no effect on a subsequent newline.

¹ or carriage returns, for that matter, as Czechnology points out. The POSIX shell does not get along with Windows-formatted text files, not even in WSL. Or Cygwin, but at least their Bash port has added an igncr option that you can set -o to make it carriage-return-tolerant.