77 lines
2.1 KiB
Markdown
77 lines
2.1 KiB
Markdown
# 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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- `sort < file.txt` # 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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- `echo "Hello, World!" > greeting.txt` # Create or overwrite file called "greeting.txt" with "Hello World!"
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- `echo "How are you?" >> greeting.txt` # Append or addon to the end of "greeting.txt" with "How are you?"
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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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- `ls /nonexistent 2> error.log` # pipe the error log from the `ls` command into "./error.log"
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To redirect both stdout and stderr to the same file:
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- `command > output.log 2>&1`
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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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- `ls -l | grep "\.txt"` # 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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```
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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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#This will write "Line 1 Line2" into "file.txt" and only stop writing new data when it reaches the line "EOF" (End Of File)
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- b) Process Substitution:
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- `diff <(ls dir1) <(ls dir2)`
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- c) Redirecting stdout and stderr to different files:
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- `command 1> output.log 2> error.log`
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- d) Discarding output:
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- `command > /dev/null 2>&1`
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- e) tee command (writing to both file and stdout):
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- `echo "Hello" | tee file.txt` # This command will wrte "Hello" to stdout (screen) and to the file "file.txt" simultaneously
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- f) Named Pipes (FIFOs):
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- `mkfifo mypipe`
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- `command1 > mypipe & command2 < mypipe` |