The linux cat command is one of those tools you use every single day without thinking about it. Then one day you realize you only know 20% of what it does. Here’s the other 80%.<\/p>\n\n\n\n
cat stands for concatenate. Its original purpose was to join files together, but it’s become the go-to command for quickly viewing file contents. It reads one or more files and writes them to standard output, which you can view in the terminal or redirect elsewhere.<\/p>\n\n\n\n
You’ll reach for cat when you need to quickly read a config file, check a log snippet, or verify a file was written correctly. For longer files, less or more are better. cat is for short files where you want the whole thing at once without paging.<\/p>\n\n\n\n
cat [OPTIONS] [FILENAME...]<\/code><\/pre>\n\n\n\nWithout a filename, cat reads from standard input until you press Ctrl+D. With one or more filenames, it reads and prints them in order.<\/p>\n\n\n\n
How to Use the cat Command<\/h2>\n\n\n\nDisplay File Contents<\/h3>\n\n\n\n
The most common use case:<\/p>\n\n\n\n
cat \/etc\/hostname<\/code><\/pre>\n\n\n\ncat \/var\/log\/syslog<\/code><\/pre>\n\n\n\nFor multiple files, cat displays them sequentially in the order provided.<\/p>\n\n\n\n
Concatenate Multiple Files<\/h3>\n\n\n\n
This is what cat was actually designed for. Combine files into one:<\/p>\n\n\n\n
cat file1.txt file2.txt > combined.txt<\/code><\/pre>\n\n\n\nThe > redirects the combined output to a new file. Without it, the combined content prints to the terminal.<\/p>\n\n\n\n
Add Line Numbers with -n<\/h3>\n\n\n\n
The -n flag numbers every output line, which is useful when referencing specific lines in config files or scripts:<\/p>\n\n\n\n
cat -n \/etc\/nginx\/nginx.conf<\/code><\/pre>\n\n\n\nThe -b flag numbers only non-blank lines if you want to skip counting empty ones.<\/p>\n\n\n\n
Suppress Blank Lines with -s<\/h3>\n\n\n\n
The -s flag squeezes multiple consecutive blank lines into a single blank line. Useful with verbose config files or log outputs that have excessive whitespace:<\/p>\n\n\n\n
cat -s verbose_config.txt<\/code><\/pre>\n\n\n\nShow Non-Printing Characters with -A<\/h3>\n\n\n\n
The -A flag shows tabs as ^I and line endings as $ characters. This is invaluable when debugging Windows-formatted files uploaded to a Linux server, where hidden carriage returns (^M) break scripts:<\/p>\n\n\n\n
cat -A script.sh<\/code><\/pre>\n\n\n\nIf you see ^M at the end of every line, the file has Windows line endings. Fix it with dos2unix.<\/p>\n\n\n\n
Create and Append to Files with cat<\/h2>\n\n\n\nCreate a New File<\/h3>\n\n\n\n
Use cat with redirection and a heredoc to create a file interactively:<\/p>\n\n\n\n
cat > newfile.txt<\/code><\/pre>\n\n\n\nType your content, then press Ctrl+D to save and exit. The file gets created with whatever you typed.<\/p>\n\n\n\n
Append to an Existing File<\/h3>\n\n\n\n
Use >> instead of > to append without overwriting:<\/p>\n\n\n\n
cat >> existing.txt<\/code><\/pre>\n\n\n\nType new content, Ctrl+D to finish. This adds to the end of the file.<\/p>\n\n\n\n
cat with Pipes and Redirects<\/h2>\n\n\n\n
cat works well as the start of a pipeline when you want to feed a file into another command:<\/p>\n\n\n\n
cat access.log | grep \"404\" | wc -l<\/code><\/pre>\n\n\n\nThat said, many commands accept filenames directly (grep “404” access.log), making the cat unnecessary. It’s a style choice for readability versus efficiency.<\/p>\n\n\n\n
When NOT to Use cat: Common Misuse<\/h2>\n\n\n\n
The ‘useless use of cat’ is a real thing in Linux scripting circles. Piping cat into commands that can read files directly adds a pointless process fork:<\/p>\n\n\n\n
\n- cat file.txt | grep pattern\u00a0 \u2192\u00a0 use grep pattern file.txt instead<\/li>\n\n\n\n
- cat file.txt | head -20\u00a0 \u2192\u00a0 use head -20 file.txt instead<\/li>\n\n\n\n
- cat file.txt | wc -l\u00a0 \u2192\u00a0 use wc -l file.txt instead<\/li>\n<\/ul>\n\n\n\n
These aren’t wrong, but they’re unnecessary overhead. On large files or in tight loops, it adds up.<\/p>\n\n\n\n
FAQ: Linux cat Command<\/h2>\n\n\n\nWhat does cat do in Linux?<\/strong> cat reads one or more files and writes their contents to standard output. You can view files in the terminal, concatenate multiple files, or redirect output to new files.<\/p> <\/div>
How do I display line numbers with cat?<\/strong> Use cat -n filename. This numbers every line in the output. Use -b instead to number only non-blank lines.<\/p> <\/div>
How do I concatenate files in Linux?<\/strong> Run: cat file1.txt file2.txt > combined.txt. This merges both files into combined.txt in the order specified.<\/p> <\/div>
How is cat different from less or more?<\/strong> cat dumps the entire file at once. less and more paginate the output so you can scroll through it. Use cat for short files, less for long ones.<\/p> <\/div> <\/div>\n","protected":false},"excerpt":{"rendered":"
The linux cat command is one of those tools you use every single day without thinking about it. Then one day you realize you only know 20% of what it does. Here’s the other 80%. What Is the Linux cat Command? cat stands for concatenate. 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