cat Command in Linux: Syntax, Options & Practical Examples

Aug 25, 2021 Jul 1, 2026 Deepak Prasad

Tested on	Ubuntu 25.04 (Plucky Puffin)
Package	`uutils-coreutils` `cat (uutils coreutils) 0.2.2`
Applies to	Ubuntu, Debian, RHEL, Rocky Linux, AlmaLinux, Oracle Linux, CentOS Stream, Fedora, Arch Linux, SUSE, openSUSE, Alpine
Privilege	read-only (no elevated privileges)
Man page	cat(1)
Scope	cat reads files or standard input and writes the bytes to standard output. Use it to view small text files, join files in order, or pipe stdin into a new file with shell redirection.
Related guides	Linux commands

cat — quick reference

Viewing and concatenating

Print file contents to the terminal or join several files in order.

When to use	Command
Print one or more files to the terminal	`cat file`
Print several files as one stream (order preserved)	`cat file1 file2`
Read from standard input (same as omitting files)	`cat -`
Create or overwrite a file from typed input (Ctrl+D ends input)	`cat > newfile`

Line numbers and blank lines

Number lines or collapse repeated empty lines when comparing configs or logs.

When to use	Command
Number every output line	`cat -n file`
Number only non-empty lines	`cat -b file`
Squeeze repeated blank lines into one	`cat -s file`

Invisible characters

Expose tabs, trailing spaces, and line endings when a file looks correct but diff or scripts fail.

When to use	Command
Show `$` at the end of each line	`cat -E file`
Show TAB characters as `^I`	`cat -T file`
Show ends, tabs, and non-printing chars (`-A` = `-vET`)	`cat -A file`
Same as `-A` using the long option	`cat --show-all file`

Help and version

When to use	Command
Show built-in usage text	`cat --help`
Print the installed package version	`cat --version`

cat — command syntax

cat concatenates each FILE to standard output. Synopsis from cat --help on Ubuntu 25.04 (cat uutils coreutils 0.2.2):

text

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

With no FILE, or when FILE is -, cat reads standard input. It does not edit files in place — use shell redirection (>, >>) or tee when you need to save output.

cat — command examples

Essential View a small text file

Use cat when you want the whole file on screen and it is short enough to read without a pager.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'line one\nline two\n\nline four' > sample.txt
cat sample.txt

Sample output:

output

line one
line two

line four

For multi-megabyte logs, prefer less so the terminal does not flood.

Essential Concatenate two files in order

Join files when you need one continuous stream — for example merging chapter files before archiving.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'alpha\n' > a.txt
printf 'beta\n' > b.txt
cat a.txt b.txt

Sample output:

output

alpha
beta

The second file always follows the first; order matters when building scripts or reports.

Essential Create a file from standard input

cat with > captures typed lines or piped data into a new file.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'hello\n' | cat - > created.txt
cat created.txt

Sample output:

output

hello

Press Ctrl+D after typing at an interactive prompt to close stdin. Use >> with cat when you want to append instead of overwrite.

Common Number every line with -n

Line numbers help when you discuss a config file with a teammate or match grep output back to a file.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'line one\nline two\n\nline four' > sample.txt
cat -n sample.txt

Sample output:

output

1	line one
     2	line two
     3	
     4	line four

Tabs between the number and text are normal on this build.

Common Collapse repeated empty lines with -s

Log files sometimes contain runs of blank lines. -s prints at most one consecutive empty line.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'line one\nline two\n\nline four' > sample.txt
cat -s sample.txt

Sample output:

output

line one
line two

line four

Pair with -n only when you need both features; -b overrides -n for numbering rules.

Common Reveal line endings and TAB characters

When diff reports a mismatch but the lines look identical, invisible characters are often the cause.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'tab\there\n' > tabs.txt
cat -E tabs.txt
cat -T tabs.txt

Sample output:

output

tab^Ihere$
tab^Ihere

-E adds $ at line ends; -T exposes TAB as ^I without showing other non-printing characters.

Advanced Debug trailing spaces with -A

-A combines -vET — useful when a line has trailing spaces that break parsers.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'trail ' > trail.txt
cat -A trail.txt

Sample output:

output

trail $

The space before $ is the trailing whitespace you were hunting.

Advanced Number only non-empty lines with -b

Use -b when blank separator lines should stay unnumbered but data lines need indexes.

Run the command:

bash
WORKDIR=/tmp/cslab
mkdir -p "$WORKDIR" && cd "$WORKDIR"
printf 'line one\nline two\n\nline four' > sample.txt
cat -b sample.txt

Sample output:

output

1	line one
     2	line two

     3	line four

The empty line stays blank; only lines with text receive numbers.

cat — when to use / when not

Use cat when	Use something else when
You need to view or join small text files quickly	The file is huge — use `less` or `head` / `tail`
You are creating or appending a short file from stdin with `>` or `>>`	You need an editor — `cat` does not modify lines in place
You must inspect invisible characters (`-A`, `-E`, `-T`)	You only need a pattern match — grep reads files directly
You are concatenating a fixed list of files in order	You would write `cat file \| grep` — run `grep pattern file` instead

cat vs less

	cat	less
Output	Dumps the entire file to the terminal	Pages through file interactively
Best for	Small files, concat, debugging whitespace	Logs, man pages, large configs
Scrollback	Terminal history only	Built-in search and navigation

Use less when output would scroll off screen.

cat — interview corner

What does the cat command do in Linux?

cat (concatenate) reads one or more files — or standard input — and writes the combined bytes to standard output. It does not change the source files unless you redirect stdout to a path.

Typical jobs:

Print a short config or script to the terminal
Join part1 and part2 into one stream
Feed stdin into a new file: cat > outfile

On Ubuntu 25.04, cat comes from uutils coreutils; flags match the traditional POSIX/GNU set (-n, -A, -E, …).

A strong answer is:

"cat sends file contents or stdin to stdout — view small files, concatenate them in order, or capture typed input with redirection. It does not edit files in place."

Why do people say 'useless cat' in shell pipelines?

In a pipeline like cat file | grep pattern, grep already opens file — the extra cat adds a process and a pipe with no benefit. The same applies to awk and sed: pass the filename as their last argument.

Good uses of cat are not about feeding filters — they are concatenation, stdin capture, and showing invisible characters with -A / -E / -T.

A strong answer is:

"cat file | grep is redundant because grep reads files directly. I use cat for concat, stdin redirects, or debugging whitespace — not as a universal pipe prefix."

What is the difference between cat -n and cat -b?

Both add line numbers to output, but they treat blank lines differently:

Flag	Blank lines
`-n`	Numbered like any other line
`-b`	Skipped — only non-empty lines get numbers

On Ubuntu 25.04, -b overrides -n when both are given.

A strong answer is:

"cat -n numbers every line including blanks; cat -b numbers only non-empty lines, which is handy when blank separators should stay unnumbered."

How do you show hidden characters in a file with cat?

Use visibility flags from the quick-reference deck:

Flag	Shows
`-E`	`$` at end of each line
`-T`	TAB as `^I`
`-A`	Combination of `-vET` — ends, tabs, and other non-printing notation

Example for trailing spaces:

bash

cat -A suspicious.conf

A strong answer is:

"I use cat -A for full visibility, or cat -E / cat -T when I only care about line endings or tabs — faster than guessing why diff disagrees."

What does cat - mean?

A lone - tells cat to read standard input instead of a path. It is equivalent to omitting filenames when stdin is already connected — common in somecommand | cat - > outfile so the same redirection pattern works in scripts.

A strong answer is:

"cat - reads stdin; I use it in pipelines when I want explicit stdin handling before redirecting to a file."

Troubleshooting

Symptom	Likely cause	Fix
Terminal floods with text	File is very large	Use `less file` or `head` / `tail`
`cat: file: No such file or directory`	Wrong path or typo	Check with `ls -l` or tab-complete
`Permission denied`	File not readable by your user	`ls -l file`; use `sudo` only when appropriate
Output looks empty but file has size	Binary file or NUL bytes	Use `file`, `hexdump`, or `less`
`Unknown option`	uutils vs GNU flag mismatch	Run `cat --help` on your host

Deepak Prasad

R&D Engineer

Founder of GoLinuxCloud with more than 15 years of expertise in Linux, Python, Go, Laravel, DevOps, Kubernetes, Git, Shell scripting, OpenShift, AWS, Networking, and Security. With extensive experience, he excels across development, DevOps, networking, and security, delivering robust and efficient solutions for diverse projects.

Expertise

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