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I/O Redirection and Pipes¶

Every process in Linux has three standard streams: stdin (0), stdout (1), and stderr (2). Redirection sends these streams to files, and pipes chain commands together. Understanding this is fundamental to shell proficiency.

Standard Streams¶

Stream	FD	Default	Description
stdin	0	Keyboard	Standard input
stdout	1	Terminal	Standard output
stderr	2	Terminal	Standard error

Redirection Operators¶

command > file          # stdout to file (overwrite)
command >> file         # stdout to file (append)
command 2> file         # stderr to file
command 2>> file        # stderr to file (append)
command < file          # file as stdin
command > /dev/null     # discard stdout

Combining Streams¶

command 1> out.txt 2> err.txt       # stdout and stderr to separate files
command 1> all.txt 2>&1             # both to same file
command &> all.txt                  # shorthand for above (bash)
command > /dev/null 2>&1            # discard all output

Examples¶

echo "text" > file.txt              # write to file
echo "text" >> file.txt             # append to file
echo "error" >&2                    # send to stderr
cat > file.txt                      # type from keyboard into file (Ctrl+D to end)
find / -name "*.sh" > found.txt 2> errors.txt  # separate stdout/stderr

Pipes¶

Send stdout of one command as stdin to the next:

command1 | command2                 # basic pipe
cat file.txt | grep "error"        # filter output
ls -la | less                      # page through output
cat file | grep 'pattern' | wc -l  # count matches
command | tee file.txt              # pipe AND write to file simultaneously

Command Chaining¶

cmd1 && cmd2    # run cmd2 ONLY if cmd1 succeeds (exit 0)
cmd1 || cmd2    # run cmd2 ONLY if cmd1 fails (non-zero)
cmd1 ; cmd2     # run cmd2 regardless of cmd1 result

Practical Examples¶

mkdir testdir && touch testdir/file    # create dir then file in it
ls ~/dir || mkdir ~/dir                # list or create if missing
cat file.txt || echo "error reading"   # print or report error
head file.txt && echo "Done"           # print then confirm

Exit Codes¶

echo $?        # last command's exit code
exit 0         # success
exit 1         # general error
exit 2         # misuse of builtins

Convention: 0 = success, non-zero = failure.

command && echo "success" || echo "failed"

Patterns¶

Log Errors Only¶

command 2>&1 | grep -i error

Capture Output and Display¶

command | tee output.log    # show on screen AND save to file
command 2>&1 | tee -a combined.log  # append both streams to log

Silent Execution¶

command > /dev/null 2>&1    # completely silent

Gotchas¶

> overwrites without warning - use >> to append
2>&1 order matters: cmd > file 2>&1 works, cmd 2>&1 > file does not do the same thing
Pipes connect stdout only - stderr still goes to terminal unless redirected
| creates a subshell - variables set inside a pipe don't persist outside
tee is useful for debugging pipes: insert it to see intermediate data

See Also¶

[[bash-scripting]] - Using redirection in scripts
[[text-processing]] - Tools commonly used in pipelines
[[terminal-basics]] - Shell fundamentals