README.md

Cat, tail, less and head

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💻 A collection of cat, less tail and head snippets, see Learnbyexample's Github page for more.

• For below examples, marks_201* files contain 3 fields delimited by TAB
• To avoid formatting issues, TAB has been converted to spaces using col -x while pasting the output here

Concatenate files

• One or more files can be given as input and hence a lot of times, cat is used to quickly see contents of small single file on terminal
• To save the output of concatenation, just redirect stdout

$ ls
marks_2015.txt  marks_2016.txt  marks_2017.txt

$ cat marks_201*
Name    Maths   Science
foo     67      78
bar     87      85
Name    Maths   Science
foo     70      75
bar     85      88
Name    Maths   Science
foo     68      76
bar     90      90

$ # save stdout to a file
$ cat marks_201* > all_marks.txt

Accepting input from stdin

$ # combining input from stdin and other files
$ printf 'Name\tMaths\tScience \nbaz\t56\t63\nbak\t71\t65\n' | cat - marks_2015.txt
Name    Maths   Science
baz     56      63
bak     71      65
Name    Maths   Science
foo     67      78
bar     87      85

$ # - can be placed in whatever order is required
$ printf 'Name\tMaths\tScience \nbaz\t56\t63\nbak\t71\t65\n' | cat marks_2015.txt -
Name    Maths   Science
foo     67      78
bar     87      85
Name    Maths   Science
baz     56      63
bak     71      65

Squeeze consecutive empty lines

$ printf 'hello\n\n\nworld\n\nhave a nice day\n'
hello


world

have a nice day
$ printf 'hello\n\n\nworld\n\nhave a nice day\n' | cat -s
hello

world

have a nice day

Prefix line numbers

$ # number all lines
$ cat -n marks_201*
     1  Name    Maths   Science
     2  foo     67      78
     3  bar     87      85
     4  Name    Maths   Science
     5  foo     70      75
     6  bar     85      88
     7  Name    Maths   Science
     8  foo     68      76
     9  bar     90      90

$ # number only non-empty lines
$ printf 'hello\n\n\nworld\n\nhave a nice day\n' | cat -sb
     1  hello

     2  world

     3  have a nice day

• For more numbering options, check out the command nl

$ whatis nl
nl (1)               - number lines of files