101 lines
1.9 KiB
Markdown
101 lines
1.9 KiB
Markdown
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# Linux I/O redirection and piping.
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1. Basic Concepts
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2. Input Redirection
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3. Output Redirection
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4. Error Redirection
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5. Piping
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6. Advanced Techniques
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## 1. Basic Concepts:
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In Unix/Linux, there are three standard streams:
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- Standard Input (stdin): 0
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- Standard Output (stdout): 1
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- Standard Error (stderr): 2
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By default, stdin is the keyboard, while stdout and stderr are both the terminal.
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## 2. Input Redirection:
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The < symbol is used for input redirection.
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Example:
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```bash
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sort < file.txt
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```
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This command sorts the contents of file.txt.
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## 3. Output Redirection:
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The > symbol is used for output redirection. It creates a new file or overwrites an existing one.
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The >> symbol appends to an existing file or creates a new one if it doesn't exist.
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Examples:
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```bash
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echo "Hello, World!" > greeting.txt
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echo "How are you?" >> greeting.txt
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```
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## 4. Error Redirection:
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You can redirect stderr using 2> or 2>>.
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Example:
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```bash
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ls /nonexistent 2> error.log
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```
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To redirect both stdout and stderr to the same file:
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```bash
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command > output.log 2>&1
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```
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## 5. Piping:
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The | symbol is used for piping. It sends the output of one command as input to another.
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Example:
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```bash
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ls -l | grep "\.txt"
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```
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This lists all files and then filters for those ending in .txt.
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## 6. Advanced Techniques:
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- a) Here Documents:
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```bash
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cat << EOF > file.txt
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Line 1
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Line 2
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EOF
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```
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- b) Process Substitution:
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```bash
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diff <(ls dir1) <(ls dir2)
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```
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- c) Redirecting stdout and stderr to different files:
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```bash
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command 1> output.log 2> error.log
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```
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- d) Discarding output:
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```bash
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command > /dev/null 2>&1
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```
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- e) tee command (writing to both file and stdout):
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```bash
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echo "Hello" | tee file.txt
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```
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- f) Named Pipes (FIFOs):
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```bash
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mkfifo mypipe
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command1 > mypipe & command2 < mypipe
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```
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These techniques allow for powerful data manipulation and process control in the Linux environment. They're essential for scripting, data processing, and system administration tasks.
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