Text Processing

Linux excels at text processing. The philosophy of small, focused tools that can be chained together with pipes makes the command line extraordinarily powerful for working with text data, log files, configuration files, and more.

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Viewing File Contents

cat — Concatenate and Display

cat file.txt                    # display entire file
cat -n file.txt                 # display with line numbers
cat file1.txt file2.txt         # display multiple files concatenated

less and more — Paged Viewing

less /var/log/syslog            # scroll through a large file
more /var/log/syslog            # simpler pager (forward only)

less navigation:

Key	Action
Space / f	Forward one page
b	Back one page
/pattern	Search forward
?pattern	Search backward
n	Next search match
N	Previous search match
g	Go to start
G	Go to end
q	Quit

head and tail — View Start or End

head -n 20 file.txt             # first 20 lines
tail -n 50 /var/log/syslog      # last 50 lines
tail -f /var/log/syslog         # follow (watch) new lines in real time
tail -f -n 100 app.log          # show last 100 lines then follow

Tip: tail -f is essential for monitoring log files in real time. Press Ctrl+C to stop.

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Pipes and Redirection

Pipes ( | )

Pipes send the output of one command as input to another:

ls -la | less                   # page through directory listing
cat /etc/passwd | wc -l         # count lines in passwd file
ps aux | grep nginx             # find nginx processes
history | grep "git commit"     # find past git commit commands

Output Redirection

Operator	Meaning
>	Redirect stdout to file (overwrite)
>>	Redirect stdout to file (append)
2>	Redirect stderr to file
2>&1	Redirect stderr to stdout
&>	Redirect both stdout and stderr
<	Use file as stdin

ls /home > users.txt            # save output to file
echo "log entry" >> app.log     # append to log file
find / -name "*.conf" 2>/dev/null  # discard error messages
command &> output.log           # capture both stdout and stderr
sort < unsorted.txt > sorted.txt   # read from file, write to file

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grep — Search Text

grep searches for patterns in files or piped input:

grep "error" /var/log/syslog           # find lines containing "error"
grep -i "warning" app.log              # case-insensitive search
grep -r "TODO" src/                    # search recursively in a directory
grep -n "function" script.js           # show line numbers
grep -c "404" access.log               # count matching lines
grep -v "debug" app.log                # invert — show lines NOT matching
grep -l "password" /etc/*              # list files containing "password"
grep -w "error" log.txt                # match whole word only
grep -A 3 "FATAL" app.log             # show 3 lines after match
grep -B 2 "FATAL" app.log             # show 2 lines before match
grep -E "error|warning|fatal" log.txt  # extended regex (multiple patterns)

Common grep Flags

Flag	Meaning
-i	Case-insensitive
-r	Recursive
-n	Show line numbers
-c	Count matches
-v	Invert match
-l	List filenames only
-w	Whole word match
-E	Extended regex (egrep)
-A N	N lines after match
-B N	N lines before match

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sed — Stream Editor