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up vote 8 down vote favorite

When I uss less file1 file2 I get both files shown in the "less buffer viewer", but less file1 file2 | cat prints the content of both files appended to stdout. How does less know if it should show the "less buffer viewer" or produce output to stdout for a next command? What mechanism is used for doing this?

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up vote 15 down vote accepted

less writes stdout to

  • a terminal (/dev/tty?) and opens the default buffer viewer
  • a pipe when piping it to another programm using | (less text | cut -d: -f1)
  • or a file when redirecting stdout to a file with > (less text > tmp)

There is a C function called "isatty" which checks for its output is writing to a tty.

In Bash you can use test (see man test)

  • -t FD file descriptor FD is opened on a terminal
  • -p FILE exists and is a named pipe

something like that

[[ -t 1 ]] && \
    echo 'STDOUT is attached to TTY'

[[ -p /dev/stdout ]] && \
    echo 'STDOUT is attached to a pipe'

[[ ! -t 1 && ! -p /dev/stdout ]] && \
    echo 'STDOUT is attached to a redirection'
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1  
@tfh If STDOUT is not attached to a pipe or a redirection, it's correct that they don't print that STDOUT is attached to a pipe or a redirection. Put all three in a script. Call bash script.sh, bash script.sh | cat, bash script.sh > file, and see what output you get. – hvd 6 hours ago

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