How can I make a custom header in less? Or alternatively, how can I open two files simultaneously in less?

Question

How can I prepend some custom text when opening a file in less? For example, I would like to see something like:

My custom title

Content of the opened file goes here...

I want a command for that like:

echo 'My custom title' | less file

less does not have this feature. But it must be possible to achieve with some workaround. Basically, we need to sequentially combine two files into one and then open it. How can I create a temporary virtual file in RAM that will contain this new file to open in less?

Similar question:

• files - 'less' with header line kept at the top - Unix & Linux Stack Exchange

Answer 1

Why not

( echo 'My custom title'; cat file ) | less

Drop it into a function if you want it available interactively; this version takes precisely one file name (plus any less options you may want to provide):

# bash
myless() {
    local x=("$@")
    local f="${x[-1]}"
    unset x[-1]
    { echo 'My custom title'; cat "$f"; } | less "${x[@]}"
}

For a more specific scenario you can modify the function appropriately. For example, if the first argument is a header and the second is always a file, you could vastly simplify like this:

myless() {
    { printf '%s\n' "$1"; cat -- "$2"; } | less
}

If you want the header to be the file name, it would seem more sensible to use the options already in less to provide this on the prompt line. But alternatively,

myless() {
    while [ "$#" -gt 0 ]
    do
        printf '\n\n%s\n' "$1"
        cat -- "$1"
        shift
    done |
        less
}

Answer 2

The usual, sub-shell based approach:

(echo "custom title"; cat file) | less

works.

I do like my vim (or neovim), so I do things like

#!/bin/ksh
# or /bin/bash or /usr/bin/zsh, really
open_with_title() {
  title="$1"
  shift
  ${EDITOR:-vim} \
       --clean \
       -R \
       --cmd ':set showtablines=2' \
       --cmd ":set tabline='${title}'" \
       -- \
       $@
}

To use vim's "viewer" mode, and set the title of the view. (To prevent confusion, if you've never used ed or vi or derivatives: you'll type :q↵ to quit.)

Answer 3

Setup RAM partition as described here: shell script - How to make a temporary file in RAM? - Unix & Linux Stack Exchange

Add function for opening less with title to .bashrc or .bash_funcs.sh:

open_less_with_title() { 
    local title="$1"
    local file="$2"
    local tmp_dir=""
    local virtual_tmp_dir="/mnt/tmpfs"
    if [ -d "$virtual_tmp_dir" ]; then
        tmp_dir="$virtual_tmp_dir"
    else
        tmp_dir="/tmp"
    fi
    tmp_less_file=$(mktemp "$tmp_dir/tmp_less_file.XXXXXXXXXX")
    echo -e "$title\n\n" > "$tmp_less_file"
    cat "$file" >> "$tmp_less_file"
    less "$tmp_less_file"
    rm "$tmp_less_file"
}

Example of usage:

open_less_with_title "My custom title" /path/to/file