Bash Using cat
Syntax[edit | edit source]
- cat [OPTIONS]... [FILE]...
Parameters[edit | edit source]
|-n||Print line numbers|
|-v||Show non-printing characters using ^ and M- notation except LFD and TAB|
|-T||Show TAB characters as ^I|
|-E||Show linefeed(LF) characters as $|
|-e||Same as -vE|
|-b||Number nonempty output lines, overrides -n|
|-A||equivalent to -vET|
|-s||suppress repeated empty output lines, s refers to squeeze|
Remarks[edit | edit source]
cat can read from both files and standard inputs and concatenates them to standard output
Concatenate files[edit | edit source]
This is the primary purpose of
cat file1 file2 file3 > file_all
cat can also be used similarly to concatenate files as part of a pipeline, e.g.
cat file1 file2 file3 | grep foo
Printing the Contents of a File[edit | edit source]
will print the contents of a file.
If the file contains non-ASCII characters, you can display those characters symbolically with
cat -v. This can be quite useful for situations where control characters would otherwise be invisible.
cat -v unicode.txt
Very often, for interactive use, you are better off using an interactive pager like
more, though. (
less is far more powerful than
more and it is advised to use
less more often than
To pass the contents of a file as input to a command. An approach usually seen as better (UUOC) is to use redirection.
tr A-Z a-z <file.txt # as an alternative to cat file.txt | tr A-Z a-z
In case the content needs to be listed backwards from its end the command
tac can be used:
If you want to print the contents with line numbers, then use
cat -n file.txt
To display the contents of a file in a completely unambiguous byte-by-byte form, a hex dump is the standard solution. This is good for very brief snippets of a file, such as when you don't know the precise encoding. The standard hex dump utility is
od -cH, though the representation is slightly cumbersome; common replacements include
$ printf 'HÃ«llÃ¶ wÃ¶rld' | xxd 0000000: 48c3 ab6c 6cc3 b620 77c3 b672 6c64 H..ll.. w..rld
Write to a file[edit | edit source]
It will let you write the text on terminal which will be saved in a file named file.
will do the same, except it will append the text to the end of the file.
N.B: Ctrl+D to end writing text on terminal (Linux)
A here document can be used to inline the contents of a file into a command line or a script:
cat <<END >file Hello, World. END
The token after the
<< redirection symbol is an arbitrary string which needs to occur alone on a line (with no leading or trailing whitespace) to indicate the end of the here document. You can add quoting to prevent the shell from performing command substitution and variable interpolation:
cat <<'fnord' Nothing in `here` will be $changed fnord
(Without the quotes,
here would be executed as a command, and
$changed would be substituted with the value of the variable
changed -- or nothing, if it was undefined.)
Display line numbers with output[edit | edit source]
--number flag to print line numbers before each line. Alternatively,
-n does the same thing.
$ cat --number file 1 line 1 2 line 2 3 4 line 4 5 line 5
To skip empty lines when counting lines, use the
--number-nonblank, or simply
$ cat -b file 1 line 1 2 line 2 3 line 4 4 line 5
Read from standard input[edit | edit source]
cat < file.txt
Output is same as
cat file.txt, but it reads the contents of the file from standard input instead of directly from the file.
printf "first line\nSecond line\n" | cat -n
The echo command before
| outputs two lines. The cat command acts on the output to add line numbers.
Show non printable characters[edit | edit source]
This is useful to see if there are any non-printable characters, or non-ASCII characters.
e.g. If you have copy-pasted the code from web, you may have quotes like
â instead of standard
$ cat -v file.txt $ cat -vE file.txt # Useful in detecting trailing spaces.
$ echo 'â ' | cat -vE # echo | will be replaced by actual file. M*bM*^@M*^] $
You may also want to use
cat -A (A for All) that is equivalent to
cat -vET. It will display TAB characters (displayed as
^I), non printable characters and end of each line:
$ echo 'â `' | cat -A M-bM-^@M-^]^I`$
Concatenate gzipped files[edit | edit source]
Files compressed by
gzip can be directly concatenated into larger gzipped files.
cat file1.gz file2.gz file3.gz > combined.gz
This is a property of
gzip that is less efficient than concatenating the input files and gzipping the result:
cat file1 file2 file3 | gzip > combined.gz
A complete demonstration:
echo 'Hello world!' > hello.txt echo 'Howdy world!' > howdy.txt gzip hello.txt gzip howdy.txt cat hello.txt.gz howdy.txt.gz > greetings.txt.gz gunzip greetings.txt.gz cat greetings.txt
Which results in
Hello world! Howdy world!
greetings.txt.gz is a single file and is decompressed as the single file
greeting.txt. Contrast this with
tar -czf hello.txt howdy.txt > greetings.tar.gz, which keeps the files separate inside the tarball.