Wednesday, August 27, 2008

Bash, Re-writing a Line

There are instances when writing a shell script that you want to write to the same line over and over. This is mainly used for progress meters that update the same screen position over and over. Most languages represent a carriage return without newline as the \r escape sequence. In shell, it's no different, but there is one small caveat. The echo command requires two arguments.


# simplest progress meter
for ((i=0; i<$items; i+=1)); do
echo -n -e "Processed $i/$items\r"

The -n argument to echo tells it to suppress the implicit newline that it usually includes. The -e argument tells it to respect escape sequences such as \r.


Anonymous said...

This works as well:

printf "Processed $i/$items\r"

Travis Whitton said...

Ah, the ubiquitious printf. Thanks for the tip!

Anonymous said...

\r is pretty nice detail :-)

Unknown said...

Just an fyi that without a sleep command after the echo my shell just hung. I'm using cygwin on XP though so on a real *nix system ymmv