Bash Redirection and Pipes

Redirection sends output into files, and pipes send the output of one command into another. This is where Bash becomes powerful.

Saving Output to a File

> writes the output to a file and replaces what was there.
>> adds the output to the end of the file.

Example

bash

echo "Ali" > names.txt
echo "Ayesha" >> names.txt
echo "Bilal" >> names.txt
cat names.txt

Output

Ali
Ayesha
Bilal

Be careful with >: it empties the file before writing. Use >> when you want to keep the old contents.

Reading Input from a File

< gives a file to a command as its input:

Example

bash

printf "Ali\nAyesha\nBilal\n" > names.txt
wc -l < names.txt

Output

Errors: 2>

Commands have two output streams: normal output (stream 1) and error messages (stream 2). You can redirect them separately.

Symbol	Meaning
`> file`	Normal output into a file
`2> file`	Error messages into a file
`&> file`	Both into one file
`2> /dev/null`	Throw error messages away

Example

bash

ls missing-folder 2> errors.txt
ls missing-folder 2> /dev/null
echo "Done"

Output

Done

/dev/null is a special file that throws away everything written to it.

Pipes

The pipe symbol | sends the output of the command on its left into the command on its right. You can join many commands this way.

Example

bash

printf "mango\napple\nbanana\napple\nmango\napple\n" > fruits.txt
sort fruits.txt | uniq
echo "---"
sort fruits.txt | uniq -c | sort -rn | head -n 1

Output

apple
banana
mango
---
      3 apple

Read the second line from left to right: sort the fruits, count each unique one, sort by the count from high to low, and show the top line.

Useful Text Tools

These tools are most useful with pipes:

Tool	What it does
`grep word`	Keeps only lines that contain the word
`sort`	Sorts lines (`-n` for numbers, `-r` to reverse)
`uniq`	Removes repeated lines next to each other (`-c` counts them)
`wc -l`	Counts lines
`head` / `tail`	First or last lines
`cut -d, -f1`	Takes column 1 of comma-separated text
`tr a-z A-Z`	Changes characters, here to capital letters

grep

grep searches text. Add -i to ignore case and -c to count matching lines.

Example

bash

printf "Ali,Karachi,85\nSara,Lahore,92\nHamza,Karachi,78\n" > students.csv
grep "Karachi" students.csv
grep -c "Karachi" students.csv
grep "Karachi" students.csv | cut -d, -f1
cut -d, -f2 students.csv | sort | uniq

Output

Ali,Karachi,85
Hamza,Karachi,78
2
Ali
Hamza
Karachi
Lahore

Joining Commands

Besides pipes, Bash has other ways to join commands on one line:

a ; b runs a, then b, always
a && b runs b only if a succeeded
a || b runs b only if a failed

Example

bash

mkdir -p reports && echo "Folder ready"
ls no-such-file 2> /dev/null || echo "File not found"

Output

Folder ready
File not found

Exercise

What does echo "hi" >> log.txt do?

Deletes log.txtReplaces the contents of log.txt with hiAdds hi to the end of log.txtPrints the contents of log.txt