Follow Pipe with less – How to Follow a Pipe Using less

lesspipetail

Can less follow (by pressing F) a piped input (similarly to a file)?
For a file that is being written to, the command

less <file>

will follow the file when pressing F.

But if I have a command that pipes output directly into less, like this

command | less

pressing F will do nothing.

So it looks like pipes cannot be followed like files can? Or maybe it has to do with command also writing to STDERR? The effect I'm trying to achieve is always see the latest output of the command: just like keeping PageDown pressed!

A related remark holds for G (go to end): when piping directly to less, it won't work.

Best Answer

Pressing F or G makes less try to reach input EOF. If the input is a pipe, less hangs until the pipe is closed on the other side (and not "does nothing").

This can be worked around by saving the command output to a temporary file in the background, and then by using it as input for less:

command > /tmp/x &
less +F /tmp/x; kill %; rm /tmp/x

There is no option to do this in less only; however, I admit it would be useful.

Related Solutions

Bash Pipe – Using Data Read from a Pipe Instead of a File in Command Options

This is dependent on the command. Some commands that read from a file expect that the file be a regular file, whose size is known in advance, and which can be read from any position and rewinded. This is unlikely if the contents of the file is a list of file names: then the command will probably be content with a pipe which it will just read sequentially from start to finish. There are several ways to feed data via a pipe to a command that expects a file name.

Many commands treat - as a special name, meaning to read from standard input rather than opening a file. This is a convention, not an obligation.
```
ls | command -r -
```
Many unix variants provide special files in /dev that designate the standard descriptors. If /dev/stdin exists, opening it and reading from it is equivalent to reading from standard input; likewise /dev/fd/0 if it exists.
```
ls | command -r /dev/stdin
ls | command -r /dev/fd/0
```
If your shell is ksh, bash or zsh, you can make the shell deal with the business of allocating some file descriptor. The main advantage of this method is that it's not tied to standard input, so you can use standard input for something else, and you can use it more than once.
```
command -r <(ls)
```
If the command expects the name to have a particular form (typically a particular extension), you can try to fool it with a symbolic link.
```
ln -s /dev/fd/0 list.foo
ls | command -r list.foo
```
Or you can use a named pipe.
```
mkfifo list.foo
ls >list.foo &
command -r list.foo
```

Note that generating a list of files with ls is problematic because ls tends to mangle file names when they contain unprintable characters. printf '%s\n' * is more reliable — it'll print every byte literally in file names. File names containing newlines will still cause trouble, but that's unavoidable if the command expects a list of file names separated by newlines.

Less Command – How to Write All Lines to a File

From less, type s then type the file name you want to save to, then Enter.
From the man page, under COMMANDS:

s filename
      Save the input to a file.  This only works if the input is a pipe, not an ordinary file.

man page also states that, depending on your particular installation, the s command might not be available. In that case, you could go to line 1 with:

g or < or ESC-<
      Go to line N in the file, default 1 (beginning of file).

and pipe the whole content to cat with:

| <m> shell-command
      <m> represents any mark letter. Pipes a section of the input file to the
      given shell command. The section of the file to be piped is between the
      first line on the current screen and the position marked by the letter. 
      <m> may also be ^ or $ to indicate beginning or end of file respectively.

so either:

g|$cat > filename

or:

<|$cat > filename

i.e. type g or < (g or less-than) | $ (pipe then dollar) then cat > filename and Enter.
This should work whether input is a pipe or an ordinary file.

Best Answer

Related Solutions

Bash Pipe – Using Data Read from a Pipe Instead of a File in Command Options

Less Command – How to Write All Lines to a File

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