Viewing and Editing Files

cat file.txt            # dump entire file
cat -n file.txt         # with line numbers
cat -A file.txt         # show non-printing chars (tabs as ^I, $ at EOL)
tac file.txt            # reverse (last line first)

less file.txt           # page through file (preferred)
more file.txt           # simpler pager (forward-only)

head file.txt           # first 10 lines
head -n 20 file.txt     # first 20 lines
head -c 100 file.txt    # first 100 bytes
tail file.txt           # last 10 lines
tail -n 50 file.txt     # last 50 lines
tail -f /var/log/syslog # follow live (stream new lines)
tail -F file.log        # follow + reopen if rotated

grep "error" file.log           # lines matching pattern
grep -i "error" file.log        # case-insensitive
grep -r "TODO" ./src/           # recursive
grep -n "def " app.py           # show line numbers
grep -c "fail" file.log         # count matching lines
grep -v "DEBUG" file.log        # invert (exclude matches)
grep -l "error" *.log           # filenames only
grep -w "exit" file             # whole-word match
grep -A 3 "error" file          # 3 lines after match
grep -B 3 "error" file          # 3 lines before match
grep -C 3 "error" file          # 3 lines before and after
grep -E "err|warn" file         # extended regex (ERE)
grep -P "\d{3}-\d{4}" file      # Perl-compatible regex

cut -d: -f1 /etc/passwd             # field 1, delimiter ':'
cut -c1-10 file.txt                 # characters 1–10
sort file.txt                       # alphabetical sort
sort -n nums.txt                    # numeric sort
sort -r file.txt                    # reverse
sort -u file.txt                    # sort + unique
sort -k2 -t: file                   # sort by field 2
uniq file.txt                       # remove adjacent duplicate lines
sort file | uniq -c                 # count occurrences
sort file | uniq -d                 # only duplicates
tr 'a-z' 'A-Z' < file              # transliterate
tr -d '\r' < file.txt > unix.txt   # strip carriage returns

sed 's/foo/bar/' file           # replace first occurrence per line
sed 's/foo/bar/g' file          # replace all occurrences
sed -i 's/foo/bar/g' file       # in-place edit
sed -i.bak 's/foo/bar/g' file   # in-place with backup
sed -n '5,10p' file             # print lines 5–10
sed '/^#/d' file                # delete comment lines
sed 's/^[[:space:]]*//' file    # strip leading whitespace
sed -n '/start/,/end/p' file    # range between patterns

awk '{print $1}' file           # print first field (whitespace-delimited)
awk -F: '{print $1}' /etc/passwd  # field delimiter ':'
awk '{print NR, $0}' file       # prefix with line number
awk '$3 > 100' file             # filter: field 3 > 100
awk '{sum += $1} END {print sum}' file  # sum column
awk '/error/ {print $0}' file   # grep-like filter
awk 'NR==5' file                # print line 5

nano file.txt

vim file.txt