Bash Pipes and Redirects - |, >, >>, 2>, <, <<

Q: Bash Pipes and Redirects - |, >, >>, 2>, <, <<

## Pipes and Redirects in Bash Pipes and redirects are the mechanisms that make Unix shells so powerful. They allow you to connect commands together and control where input comes from and output goes. ### Pipes ( | ) A pipe connects the standard output (stdout) of one command to the standard input (stdin) of the next: bash # Chain commands together ls -la | grep ".log" cat server.log | grep "ERROR" | wc -l ps aux | sort -k3 -rn | head -5 Each command in a pipeline runs as a separate proces

Q: When is this useful?

Pipes and redirects are fundamental to shell scripting and daily command-line work. System administrators use them to filter and analyze log files, developers use them in build pipelines and data processing scripts, and DevOps engineers use them in deployment automation. Mastering these operators is essential for writing efficient bash scripts.

Learn how to use bash pipes to chain commands and redirects to control input/output. Covers stdout, stderr, heredocs, and process substitution.

Pipes & Redirects

Detailed Explanation

Pipes and Redirects in Bash

Pipes and redirects are the mechanisms that make Unix shells so powerful. They allow you to connect commands together and control where input comes from and output goes.

Pipes ( | )

A pipe connects the standard output (stdout) of one command to the standard input (stdin) of the next:

# Chain commands together
ls -la | grep ".log"
cat server.log | grep "ERROR" | wc -l
ps aux | sort -k3 -rn | head -5

Each command in a pipeline runs as a separate process. Data flows from left to right, with each command processing the output of the previous one.

Output Redirects ( > and >> )

The > operator redirects stdout to a file, overwriting its contents. >> appends instead:

echo "Hello" > greeting.txt      # overwrite
echo "World" >> greeting.txt     # append
ls -la > filelist.txt            # save directory listing
date >> log.txt                  # append timestamp to log

Error Redirects ( 2> and 2>> )

File descriptor 2 represents standard error (stderr). You can redirect it separately:

command 2> errors.log            # redirect stderr to file
command 2>/dev/null              # discard stderr
command > output.log 2>&1        # redirect both stdout and stderr
command &> combined.log          # shorthand for both (bash 4+)

Input Redirects ( < )

The < operator feeds a file as stdin to a command:

sort < unsorted.txt              # sort file contents
mysql < schema.sql               # execute SQL file
while read line; do echo "$line"; done < data.txt

Heredocs ( << )

Heredocs provide inline multi-line input:

cat << 'EOF'
This is a multi-line
text block that preserves
formatting and indentation.
EOF

Process Substitution ( <() )

Process substitution treats command output as a file:

diff <(sort file1.txt) <(sort file2.txt)
comm -13 <(sort list1.txt) <(sort list2.txt)

Tee - Split Output

The tee command writes to both stdout and a file simultaneously:

command | tee output.log         # display and save
command | tee -a output.log      # display and append

Use Case