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[bash/en] More details about pipe and redirection (#4119)

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* Update bash.html.markdown

* More details about pipe and redirection
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Michael Chen 2022-01-03 12:02:22 -05:00 committed by GitHub
parent 5f118e3f61
commit 6467d88012
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1 changed files with 11 additions and 4 deletions
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@ -234,7 +234,8 @@ ls -l # Lists every file and directory on a separate line
ls -t # Sorts the directory contents by last-modified date (descending) ls -t # Sorts the directory contents by last-modified date (descending)
ls -R # Recursively `ls` this directory and all of its subdirectories ls -R # Recursively `ls` this directory and all of its subdirectories
# Results of the previous command can be passed to the next command as input. # Results (stdout) of the previous command can be passed as input (stdin) to the next command
# using a pipe |. Commands chained in this way are called a "pipeline", and are run concurrently.
# The `grep` command filters the input with provided patterns. # The `grep` command filters the input with provided patterns.
# That's how we can list .txt files in the current directory: # That's how we can list .txt files in the current directory:
ls -l | grep "\.txt" ls -l | grep "\.txt"
@ -274,7 +275,7 @@ cd # also goes to home directory
cd .. # go up one directory cd .. # go up one directory
# (^^say, from /home/username/Downloads to /home/username) # (^^say, from /home/username/Downloads to /home/username)
cd /home/username/Documents # change to specified directory cd /home/username/Documents # change to specified directory
cd ~/Documents/.. # still in home directory..isn't it?? cd ~/Documents/.. # now in home directory (if ~/Documents exists)
cd - # change to last directory cd - # change to last directory
# => /home/username/Documents # => /home/username/Documents
@ -289,9 +290,13 @@ mkdir -p myNewDir/with/intermediate/directories
# if the intermediate directories didn't already exist, running the above # if the intermediate directories didn't already exist, running the above
# command without the `-p` flag would return an error # command without the `-p` flag would return an error
# You can redirect command input and output (stdin, stdout, and stderr). # You can redirect command input and output (stdin, stdout, and stderr)
# using "redirection operators". Unlike a pipe, which passes output to a command,
# a redirection operator has a command's input come from a file or stream, or
# sends its output to a file or stream.
# Read from stdin until ^EOF$ and overwrite hello.py with the lines # Read from stdin until ^EOF$ and overwrite hello.py with the lines
# between "EOF": # between "EOF" (which are called a "here document"):
cat > hello.py << EOF cat > hello.py << EOF
#!/usr/bin/env python #!/usr/bin/env python
from __future__ import print_function from __future__ import print_function
@ -313,6 +318,8 @@ python hello.py 2> "error.err" # redirect error output to error.err
python hello.py > "output-and-error.log" 2>&1 python hello.py > "output-and-error.log" 2>&1
# redirect both output and errors to output-and-error.log # redirect both output and errors to output-and-error.log
# &1 means file descriptor 1 (stdout), so 2>&1 redirects stderr (2) to the current
# destination of stdout (1), which has been redirected to output-and-error.log.
python hello.py > /dev/null 2>&1 python hello.py > /dev/null 2>&1
# redirect all output and errors to the black hole, /dev/null, i.e., no output # redirect all output and errors to the black hole, /dev/null, i.e., no output