Essential Linux File Management Commands (Part 2)

cat: view file contents

Displays the entire file and supports several options:

cat -n aa.txt    // show line numbers
cat -b aa.txt    // show line numbers for non‑empty lines
cat -A aa.txt    // show all special characters
cat -E aa.txt    // show $ as end‑of‑line marker

more: paginate text output

Interactive navigation while viewing:

b / space: page up / down
Enter: line break
q: quit

less: advanced pagination

Provides more navigation keys:

Up/Down arrows: scroll line by line
b / space: page up / down
Enter: line break
q: quit

head: show file beginning

Shows the first N lines (default 10). Example shows first 15 lines:

head -n 15 /var/log/message

tail: show file end

Shows the last N lines (default 10) and can follow a file in real time:

tail -n 15 /var/log/message    // last 15 lines
tail -f /var/log/message       // follow updates

du: disk usage of directories/files

Summarizes size; -s for current directory, -h for human‑readable output:

du -sh /app    // total size of /app in readable format

wc: word, line, byte count

Counts lines, words, bytes. Example counts lines in a file:

wc -l /app/aa.txt    // line count
-c    // byte count
-W    // word count

grep: search text patterns

Filters lines matching a pattern, with common flags:

grep -i "physicAl iD" /proc/cpuinfo    // case‑insensitive search
grep -i "^$" /proc/cpuinfo            // match empty lines
-v    // invert match
-i    // ignore case
-r    // recursive search
-E    // extended regex, e.g., -E "a|b|c"

cut: extract columns or characters

Extracts fields using a delimiter:

cut -d: -f1 /etc/passwd | head -3    // first field of passwd, first 3 lines
-d    // delimiter
-f    // field numbers (e.g., -f1,3 or -f1-3)

sort: order text lines

Sorts numerically, reverse, by specific columns, with custom delimiters:

sort -nr -k2 sort_demo.txt    // numeric reverse sort by column 2
sort -t: -nk3 /etc/passwd    // numeric sort by column 3 using ':'
-n    // numeric sort
-r    // reverse order
-k    // key (column)
-t    // field delimiter

uniq: filter duplicate lines

Counts or removes repeated lines:

uniq -c uniq.txt    // prefix each line with occurrence count
-c    // count occurrences
-d    // show only duplicates
-u    // show only unique lines
-i    // ignore case

which: locate executable path

Shows where a command resides in the system:

which cat
which ls

find: locate files and directories

Powerful search with many predicates:

find /etc -name init                     // exact name match
find / -name "pass"                     // exact filename
find / -size +204800k                    // files larger than 200 MB
find / -user root                        // files owned by root
find /etc -size +102400k -a -size -204800k // size between 100 MB and 200 MB
find /etc -name inittab -exec ls -l {} \; // show details of matching file
find / -size +200M -a -size -300M        // size between 200 MB and 300 MB
find / -name "*pass*"                   // fuzzy match (slower)
find / -type b                           // block devices
find / -type c                           // character devices

md5sum: compute and verify MD5 checksums

Used to detect file changes:

md5sum 111.txt    // prints MD5 hash for verification

tr: translate or delete characters

Works with pipelines to transform text streams:

cat test | tr 'A-Z' 'a-z'    // convert uppercase to lowercase
cat test | tr -d 'A-Z'      // delete all uppercase letters