Reading and searching text

Files on a phone are mostly text. The terminal is the fastest way to read them. Here you build a small log file, print it in several ways, and search it for the lines that matter. These search skills are the ones you will reach for most often.

What you'll be able to do

• Print a whole file with cat, and number its lines.
• Read the start or the end of a file with head and tail.
• Page through a long file with less.
• Count lines, words and characters with wc.
• Search a file with grep, and search a whole tree with grep -r.

Step 1: Make a file to work with

You need a file to read. This command writes a small log file. It uses redirection, which lesson four explains in full. For now, read it as "put the text between the markers into app.log". There is no output, and the prompt returns.

$ cd ~
$ cat > app.log << 'EOF'
2026-01-16 09:01:12 INFO  server started on port 8080
2026-01-16 09:02:03 WARN  slow query took 3200ms
2026-01-16 09:02:44 ERROR failed to send email to user 42
2026-01-16 09:03:10 INFO  user 42 logged in
2026-01-16 09:03:55 ERROR failed to send email to user 91
2026-01-16 09:06:00 INFO  nightly backup finished
2026-01-16 09:06:30 WARN  error rate above threshold
EOF

The file now has seven lines. Every command in this lesson reads it.

Step 2: Print a file with cat

cat prints a file to the screen. The name is short for concatenate. Its everyday job, though, is showing you what is in a file.

$ cat app.log
2026-01-16 09:01:12 INFO  server started on port 8080
2026-01-16 09:02:03 WARN  slow query took 3200ms
2026-01-16 09:02:44 ERROR failed to send email to user 42
2026-01-16 09:03:10 INFO  user 42 logged in
2026-01-16 09:03:55 ERROR failed to send email to user 91
2026-01-16 09:06:00 INFO  nightly backup finished
2026-01-16 09:06:30 WARN  error rate above threshold

Add -n to print a line number in front of every line.

$ cat -n app.log
     1	2026-01-16 09:01:12 INFO  server started on port 8080
     2	2026-01-16 09:02:03 WARN  slow query took 3200ms
     3	2026-01-16 09:02:44 ERROR failed to send email to user 42
     4	2026-01-16 09:03:10 INFO  user 42 logged in
     5	2026-01-16 09:03:55 ERROR failed to send email to user 91
     6	2026-01-16 09:06:00 INFO  nightly backup finished
     7	2026-01-16 09:06:30 WARN  error rate above threshold

Step 3: Read the start and the end

head prints the first lines of a file. The -n flag sets how many.

$ head -n 3 app.log
2026-01-16 09:01:12 INFO  server started on port 8080
2026-01-16 09:02:03 WARN  slow query took 3200ms
2026-01-16 09:02:44 ERROR failed to send email to user 42

tail prints the last lines instead. Reach for it when you want the newest entries in a log.

$ tail -n 2 app.log
2026-01-16 09:06:00 INFO  nightly backup finished
2026-01-16 09:06:30 WARN  error rate above threshold

Step 4: Page through a file with less

less opens a file in a viewer you can scroll. It does not print and quit. It waits.

$ less app.log
2026-01-16 09:01:12 INFO  server started on port 8080
2026-01-16 09:02:03 WARN  slow query took 3200ms
:

The colon at the bottom left is the prompt from less, waiting for a key. Press Space to move down a screen. Press b to move back up, and q to quit. Type / then a word, then press Enter to jump to the next match.

Step 5: Count lines, words and characters

wc counts. With -l it counts lines.

$ wc -l app.log
7 app.log

With -w it counts words, and with -c it counts characters.

$ wc -w app.log
55 app.log

$ wc -c app.log
366 app.log

The character count is larger than you might expect. It includes every space and line break.

Step 6: Search with grep

grep prints only the lines that match a pattern. Give it a pattern, then a file.

$ grep "ERROR" app.log
2026-01-16 09:02:44 ERROR failed to send email to user 42
2026-01-16 09:03:55 ERROR failed to send email to user 91

Add -n to include the line number, so you can find the match again.

$ grep -n "WARN" app.log
2:2026-01-16 09:02:03 WARN  slow query took 3200ms
7:2026-01-16 09:06:30 WARN  error rate above threshold

Add -c to count the matching lines instead of printing them.

$ grep -c "INFO" app.log
3

By default grep is case sensitive, so ERROR and error are different words to it. Add -i to ignore case.

$ grep -i "error" app.log
2026-01-16 09:02:44 ERROR failed to send email to user 42
2026-01-16 09:03:55 ERROR failed to send email to user 91
2026-01-16 09:06:30 WARN  error rate above threshold

That found one extra line, the one with the lowercase error. Add -v to invert the match and print the lines that do not match.

$ grep -v "INFO" app.log
2026-01-16 09:02:03 WARN  slow query took 3200ms
2026-01-16 09:02:44 ERROR failed to send email to user 42
2026-01-16 09:03:55 ERROR failed to send email to user 91
2026-01-16 09:06:30 WARN  error rate above threshold

Add -r to search a whole directory tree. It prints the path in front of each match.

$ grep -r "ERROR" ~
/data/data/com.termux/files/home/app.log:2026-01-16 09:02:44 ERROR failed to send email to user 42
/data/data/com.termux/files/home/app.log:2026-01-16 09:03:55 ERROR failed to send email to user 91

Step 7: A realistic search

A user reports that confirmation emails are not arriving. Search the log for the word email.

$ grep -n "email" app.log
3:2026-01-16 09:02:44 ERROR failed to send email to user 42
5:2026-01-16 09:03:55 ERROR failed to send email to user 91

Both matches are errors, and both are about sending email. That points you straight at the email code. One search narrows the field, and the next confirms the pattern.

Quick reference

Command	What it does
cat FILE	Print a whole file
cat -n FILE	Print a file with line numbers
head -n N FILE	Print the first N lines
tail -n N FILE	Print the last N lines
less FILE	Open a file in a scrollable viewer
wc -l FILE	Count lines
wc -w FILE	Count words
wc -c FILE	Count characters
grep PATTERN FILE	Print matching lines
grep -i PATTERN FILE	Match without regard to case
grep -n PATTERN FILE	Show line numbers with matches
grep -c PATTERN FILE	Count matching lines
grep -v PATTERN FILE	Print lines that do not match
grep -r PATTERN DIR	Search a whole directory tree

Next steps

Now learn to link these commands into a single line in Pipes and redirection. For more practice finding things, revisit Navigating the filesystem. Then take on a real investigation in Logs Never Lie.