When displaying and grep-ing text in the Linux terminal, syntax highlighting is very nice to have. In this post I'll give my preferred method, with output like:

$ less util.c    # yielding...:

$ less util.c|grep "present" -A4    # yielding...:

My preference is GNU's source-highlight:

$ sudo apt-get install source-highlight

Setting it up

The manual suggests setting the LESSOPEN environment variable in ~/.bashrc in order to get syntax highlighting in the Linux terminal through the less command:

# source-highlight's suggested addition to .bashrc:
# (This is not my preferred method, read on.)
export LESSOPEN="| /path/to/src-hilite-lesspipe.sh %s"
export LESS=' -R '

However, this will break some other functionality of less in some distros, e.g. Ubuntu. These distributions define their own LESSOPEN in .bashrc, giving extended uses of less (less-ing pdf's, images, .deb's, compressed files and more) and depending on which definition is done first, one will override the other.

Can we get both? Yes. Leave the LESSOPEN untouched. What it does is adding a preprocessor to less processing, namely lesspipe/lessfile. Luckily this script allows for additional user-defined pre-processors in the file ~/.lessfilter if it exists.

Create it, give it execution rights, and give it the contents:

# file ~/.lessfilter:

file -b -L "$1" | grep -q text && \
/usr/share/source-highlight/src-hilite-lesspipe.sh "$1"

This script will be called from lessfile/lesspipe. If it is a text file it will call the source-highlight script, otherwise it will exit with bad exit status which will leave further processing to the lessfile/lesspipe script.

However we do need to add the line

export LESS=' -R '

to ~/.bash_aliases (if your distro doesn't support this file, add to ~/.bashrc).

The less command will now yield the output shown in the images above.

'grep'-ing highlighted source code

Note that grep-ing text in source works as well, and according to the image above, you will get the highlight both from syntax coloring and the grep matches.

Often you wish to paginate the output from grep. This requires a trick, for two reasons:

  • piping through less will not provide syntax highlighting
  • when output from grep is piped, grep will not highlight the matches by default

The remedy is to tell grep to keep the highlighting, although its output is piped, and before grep-ing send the file through less for syntax coloring:

$ less util.c|grep --color=always "const" -C3 |less    # yielding...:

Since I often pipe grep output to less I have defined the alias

alias grepc='grep --color=always'

in ~/.bashrc or ~/.bash_aliases 1. This enables short-hand grep-ing with colored, paginated output, e.g.

$ less util.c|grepc "const" -C3 |less

Will source-highlight color my HDL as well?

There is no language definition for VHDL included, but I have created a rudimentary definition file (download) that can be used. Place it in /usr/share/source-highlight and then edit lang.map in the same directory and add the lines

vhdl = vhdl.lang
vhd = vhdl.lang

Are there other alternatives to GNU's soruce-highlight?

Yes, several. Among them is Vim, with the alias

alias vless='vim -u /usr/share/vim/vim74/macros/less.vim'

in ~/.bashrc or ~/.bash_aliases. Then

$ vless utils.c

provides a paginated, source-highlighted output. Vim has built-in support for a large number of file types, including VHDL, Verilog, Tcl and Specman - but unfortunately it won't be possible to grep on source-highlighted output. That's why I rather prefer GNU's source-highlight.


  1. Don't be tempted to alias 'grep' to 'grep --color=always'! This might break scripts.