Less file1 file2 | cat — why does it work

catlesspipe

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?

Best Answer

less prints text to stdout. stdout goes

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

There is a C function called "isatty" which checks if the output is going to a tty (less 4.81, main.c, line 112). If so, it uses the buffer viewer otherwise it behaves like cat.

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

Example:

[[ -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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cat command – Display File Names with Contents

$ for file in ./tmp/*.txt; do echo "$file";  cat "$file"; done

-or-

$ find ./tmp -maxdepth 1 -name "*.txt" -print -exec cat "{}" \;

Why does pipe not work with cat and locate

locate -e0 '*/pg_type.h' | xargs -r0 cat

locate pg_type.h would find all the files with pg_type.h in their path (so for instance if there was a rpg_type.horn directory, you'd end up displaying all the files in there).

Without -0 the output of locate can't be post-processed because the files are separated by newline characters while newline is a perfectly valid character in a file name.

cat without arguments writes to stdout what it reads from stdin, so locate | cat would be the same as locate, cat would just pass the output of locate along. What you need is to pass the list of files as arguments to cat.

That's what xargs is typically for: convert a stream of data into a list of arguments. -r is to not call cat if there's no input. Without -0 (which like -r is not standard but found on many implementations, at least those where xargs is useful to anything), xargs would just look for words in its input to convert into arguments, where words are blank separated and where backslash, single and double quotes can be used to escape those separators, so typically not the format locate uses to display file names.

That's why we use the -0 option for both locate and xargs which uses the NUL character (which is the only character not allowed in a file path) to separate file names.

Also note that locate is not a standard command and there exist a great number of different implementations with different versions thereof and different options and behaviours. The code above applies at least to relatively recent versions of the GNU locate and mlocate implementations which are the most common on Linux based operating systems at least.

Best Answer

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Why does pipe not work with cat and locate

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