Cat, Less, Tail and Head

Table of Contents

cat
less
- Navigation commands
- Further Reading for less
tail
head
Text Editors

cat

$ man cat
CAT(1)                           User Commands                          CAT(1)

NAME
       cat - concatenate files and print on the standard output

SYNOPSIS
       cat [OPTION]... [FILE]...

DESCRIPTION
       Concatenate FILE(s) to standard output.

       With no FILE, or when FILE is -, read standard input.
...

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

Viewing special characters

End of line identified by $
Useful for example to see trailing spaces

$ cat -E marks_2015.txt 
Name    Maths   Science $
foo     67      78$
bar     87      85$

TAB identified by ^I

$ cat -T marks_2015.txt 
Name^IMaths^IScience 
foo^I67^I78
bar^I87^I85

Non-printing characters
See Show Non-Printing Characters for more detailed info

$ # NUL character
$ printf 'foo\0bar\0baz\n' | cat -v
foo^@bar^@baz

$ # to check for dos-style line endings
$ printf 'Hello World!\r\n' | cat -v
Hello World!^M

$ printf 'Hello World!\r\n' | dos2unix | cat -v
Hello World!

the -A option is equivalent to -vET
the -e option is equivalent to -vE
If dos2unix and unix2dos are not available, see How to convert DOS/Windows newline (CRLF) to Unix newline (\n)

Writing text to file

$ cat > sample.txt
This is an example of adding text to a new file using cat command.
Press Ctrl+d on a newline to save and quit.

$ cat sample.txt 
This is an example of adding text to a new file using cat command.
Press Ctrl+d on a newline to save and quit.

See also how to use heredoc
- How can I write a here doc to a file
See also difference between Ctrl+c and Ctrl+d to signal end of stdin input in bash

tac

$ whatis tac
tac (1)              - concatenate and print files in reverse

$ seq 3 | tac
3
2
1

$ tac marks_2015.txt 
bar	87	85
foo	67	78
Name	Maths	Science

Useful in cases where logic is easier to write when working on reversed file
Consider this made up log file, many Warning lines but need to extract only from last such Warning upto Error line

$ cat report.log 
blah blah
Warning: something went wrong
more blah
whatever
Warning: something else went wrong
some text
some more text
Error: something seriously went wrong
blah blah blah

$ tac report.log | sed -n '/Error:/,/Warning:/p' | tac
Warning: something else went wrong
some text
some more text
Error: something seriously went wrong

Similarly, if characters in lines have to be reversed, use the rev command

$ whatis rev
rev (1)              - reverse lines characterwise

Useless use of cat

cat is used so frequently to view contents of a file that somehow users think other commands cannot handle file input
UUOC
Useless Use of Cat Award

$ cat report.log | grep -E 'Warning|Error'
Warning: something went wrong
Warning: something else went wrong
Error: something seriously went wrong
$ grep -E 'Warning|Error' report.log
Warning: something went wrong
Warning: something else went wrong
Error: something seriously went wrong

Use input redirection if a command doesn't accept file input

$ cat marks_2015.txt | tr 'A-Z' 'a-z'
name    maths   science
foo     67      78
bar     87      85
$ tr 'A-Z' 'a-z' < marks_2015.txt
name    maths   science
foo     67      78
bar     87      85

However, cat should definitely be used where concatenation is needed

$ grep -c 'foo' marks_201*
marks_2015.txt:1
marks_2016.txt:1
marks_2017.txt:1

$ # concatenation allows to get overall count in one-shot in this case
$ cat marks_201* | grep -c 'foo'
3

less

$ whatis less
less (1)             - opposite of more

$ # By default, pager is used to display the man pages
$ # and usually, pager is linked to less command
$ type pager less
pager is /usr/bin/pager
less is /usr/bin/less

$ realpath /usr/bin/pager 
/bin/less
$ realpath /usr/bin/less
/bin/less
$ diff -s /usr/bin/pager /usr/bin/less
Files /usr/bin/pager and /usr/bin/less are identical

cat command is NOT suitable for viewing contents of large files on the Terminal
less displays contents of a file, automatically fits to size of Terminal, allows scrolling in either direction and other options for effective viewing
Usually, man command uses less command to display the help page
The navigation commands are similar to vi editor

Navigation commands

Commonly used commands are given below, press h for summary of options

g go to start of file
G go to end of file
q quit
/pattern search for the given pattern in forward direction
?pattern search for the given pattern in backward direction
n go to next pattern
N go to previous pattern

tail

$ man tail
TAIL(1)                          User Commands                         TAIL(1)

NAME
       tail - output the last part of files

SYNOPSIS
       tail [OPTION]... [FILE]...

DESCRIPTION
       Print  the  last  10  lines of each FILE to standard output.  With more
       than one FILE, precede each with a header giving the file name.

       With no FILE, or when FILE is -, read standard input.
...

linewise tail

Consider this sample file, with line numbers prefixed

$ cat sample.txt 
 1) Hello World!
 2) 
 3) Good day
 4) How do you do?
 5) 
 6) Just do it
 7) Believe it!
 8) 
 9) Today is sunny
10) Not a bit funny
11) No doubt you like it too
12) 
13) Much ado about nothing
14) He he he
15) Adios amigo

default behavior - display last 10 lines

$ tail sample.txt 
 6) Just do it
 7) Believe it!
 8) 
 9) Today is sunny
10) Not a bit funny
11) No doubt you like it too
12) 
13) Much ado about nothing
14) He he he
15) Adios amigo

Use -n option to control number of lines to filter

$ tail -n3 sample.txt 
13) Much ado about nothing
14) He he he
15) Adios amigo

$ # some versions of tail allow to skip explicit n character
$ tail -5 sample.txt 
11) No doubt you like it too
12) 
13) Much ado about nothing
14) He he he
15) Adios amigo

when number is prefixed with + sign, all lines are fetched from that particular line number to end of file

$ tail -n +10 sample.txt 
10) Not a bit funny
11) No doubt you like it too
12) 
13) Much ado about nothing
14) He he he
15) Adios amigo

$ seq 13 17 | tail -n +3
15
16
17

characterwise tail

Note that this works byte wise and not suitable for multi-byte character encodings

$ # last three characters including the newline character
$ echo 'Hi there!' | tail -c3
e!

$ # excluding the first character
$ echo 'Hi there!' | tail -c +2
i there!

multiple file input for tail

$ tail -n2 report.log sample.txt 
==> report.log <==
Error: something seriously went wrong
blah blah blah

==> sample.txt <==
14) He he he
15) Adios amigo

$ # -q option to avoid filename in output
$ tail -q -n2 report.log sample.txt 
Error: something seriously went wrong
blah blah blah
14) He he he
15) Adios amigo

head

$ man head
HEAD(1)                          User Commands                         HEAD(1)

NAME
       head - output the first part of files

SYNOPSIS
       head [OPTION]... [FILE]...

DESCRIPTION
       Print  the  first  10 lines of each FILE to standard output.  With more
       than one FILE, precede each with a header giving the file name.

       With no FILE, or when FILE is -, read standard input.
...

linewise head

default behavior - display starting 10 lines

$ head sample.txt 
 1) Hello World!
 2) 
 3) Good day
 4) How do you do?
 5) 
 6) Just do it
 7) Believe it!
 8) 
 9) Today is sunny
10) Not a bit funny

Use -n option to control number of lines to filter

$ head -n3 sample.txt 
 1) Hello World!
 2) 
 3) Good day

$ # some versions of head allow to skip explicit n character
$ head -4 sample.txt 
 1) Hello World!
 2) 
 3) Good day
 4) How do you do?

when number is prefixed with - sign, all lines are fetched except those many lines to end of file

$ # except last 9 lines of file
$ head -n -9 sample.txt 
 1) Hello World!
 2) 
 3) Good day
 4) How do you do?
 5) 
 6) Just do it

$ # except last 2 lines
$ seq 13 17 | head -n -2
13
14
15

characterwise head

Note that this works byte wise and not suitable for multi-byte character encodings

$ # if output of command doesn't end with newline, prompt will be on same line
$ # to highlight working of command, the prompt for such cases is not shown here

$ # first two characters
$ echo 'Hi there!' | head -c2
Hi

$ # excluding last four characters
$ echo 'Hi there!' | head -c -4
Hi the

multiple file input for head

$ head -n3 report.log sample.txt 
==> report.log <==
blah blah
Warning: something went wrong
more blah

==> sample.txt <==
 1) Hello World!
 2) 
 3) Good day

$ # -q option to avoid filename in output
$ head -q -n3 report.log sample.txt 
blah blah
Warning: something went wrong
more blah
 1) Hello World!
 2) 
 3) Good day

combining head and tail

Despite involving two commands, often this combination is faster than equivalent sed/awk versions

$ head -n11 sample.txt | tail -n3
 9) Today is sunny
10) Not a bit funny
11) No doubt you like it too

$ tail sample.txt | head -n2
 6) Just do it
 7) Believe it!

Text Editors

For editing text files, the following applications can be used. Of these, gedit, nano, vi and/or vim are available in most distros by default

Easy to use

gedit
geany
nano

Powerful text editors

vim
- vim learning resources and vim reference for further info
emacs
atom
sublime

Check out this analysis for some performance/feature comparisons of various text editors

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tail_less_cat_head.md

tail_less_cat_head.md

Cat, Less, Tail and Head

cat

Concatenate files

Accepting input from stdin

Squeeze consecutive empty lines

Prefix line numbers

Viewing special characters

Writing text to file

tac

Useless use of cat

Further Reading for cat

less

Navigation commands

Further Reading for less

tail

linewise tail

characterwise tail

multiple file input for tail

Further Reading for tail

head

linewise head

characterwise head

multiple file input for head

combining head and tail

Further Reading for head

Text Editors

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tail_less_cat_head.md

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tail_less_cat_head.md

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Cat, Less, Tail and Head

cat

Concatenate files

Accepting input from stdin

Squeeze consecutive empty lines

Prefix line numbers

Viewing special characters

Writing text to file

tac

Useless use of cat

Further Reading for cat

less

Navigation commands

Further Reading for less

tail

linewise tail

characterwise tail

multiple file input for tail

Further Reading for tail

head

linewise head

characterwise head

multiple file input for head

combining head and tail

Further Reading for head

Text Editors