Redirecting output is a cardinal conception successful bid-formation environments similar Linux and macOS. Knowing however to manipulate streams, peculiarly utilizing the enigmatic /dev/null 2>&1, is important for immoderate developer oregon scheme head. This cryptic series controls wherever the output of your instructions goes, providing a almighty manner to negociate accusation travel. Whether or not you’re silencing pointless output, debugging analyzable scripts, oregon streamlining log records-data, mastering this method volition importantly heighten your bid-formation prowess.
Knowing Modular Streams
All bid-formation procedure interacts with 3 modular streams: modular enter (stdin), modular output (stdout), and modular mistake (stderr). Stdin sometimes represents enter from the keyboard, stdout represents the average output of a bid, and stderr represents mistake messages. By default, some stdout and stderr are displayed connected your terminal.
Manipulating these streams is indispensable for controlling the travel of accusation successful your scripts. For case, you mightiness privation to redirect the output of a bid to a record for future investigation oregon suppress mistake messages once you cognize they’re not captious. This is wherever /dev/null and the redirection operators travel into drama.
Deliberation of these streams similar pipes successful a home. stdin is the h2o coming successful, stdout is the cleanable h2o going retired, and stderr is the drain for wastewater. Redirecting them is similar altering the plumbing to direct the h2o wherever you demand it.
The Enigma of /dev/null
/dev/null is a particular record, frequently referred to arsenic the “spot bucket” oregon “achromatic gap.” Immoderate information written to /dev/null is discarded, efficaciously disappearing with out a hint. This is extremely utile for suppressing output that you don’t demand to seat. Ideate moving a book that generates a batch of verbose output; redirecting it to /dev/null retains your terminal cleanable and centered.
For illustration, if you tally ls -l > /dev/null, the elaborate record itemizing normally dispatched to your terminal is discarded. You’ll lone seat possible mistake messages. This is peculiarly adjuvant for scripts wherever you lone attention astir errors and not the average output.
Successful our plumbing analogy, /dev/null is similar a drain that disappears into the void. Thing you direct behind it is gone everlastingly.
Decoding 2>&1
The 2>&1 portion of the bid is what redirects stderr to the aforesaid determination arsenic stdout. The “2” represents stderr, the “&1” represents stdout, and the > performs the redirection. Truthful, 2>&1 efficaciously means “direct stderr to wherever stdout is presently going.”
This is generally utilized successful conjunction with redirecting stdout to a record oregon /dev/null. For illustration, bid > /dev/null 2>&1 sends some the modular output and immoderate mistake messages to /dev/null, efficaciously silencing the bid wholly. This tin beryllium utile for inheritance duties oregon conditions wherever you don’t privation immoderate output cluttering your terminal.
Persevering with our plumbing analogy, 2>&1 is similar connecting the wastewater tube (stderr) to the aforesaid outlet arsenic the cleanable h2o tube (stdout).
Applicable Examples and Usage Instances
Ftoβs exemplify with a existent-planet script. Say you’re moving a backup book. You privation to log immoderate errors to a record however suppress the average output. You might usage the pursuing bid: backup_script 2> errors.log.
To soundlessness some modular output and mistake messages, redirect some to /dev/null: backup_script > /dev/null 2>&1.
Different illustration: you privation to number the figure of strains successful a record however suppress immoderate mistake messages if the record doesn’t be. You might usage: wc -l record.txt 2>/dev/null.
- Soundlessness a bid wholly: bid > /dev/null 2>&1
- Log lone errors: bid 2> errors.log
- Place the bid you privation to tally.
- Find which streams you privation to redirect (stdout, stderr, oregon some).
- Usage the due redirection operators (>, 2>, &1) to redirect the streams.
For much successful-extent accusation connected ammunition redirection, mention to the Bash handbook connected redirections.
“Effectual watercourse redirection is a cornerstone of businesslike ammunition scripting.” - Linux Ammunition Scripting Cookbook
Larn much astir ammunition scripting.### Additional Speechmaking
Precocious Bash-Scripting Usher: Section 20. I/O Redirection
However to Redirect stderr and stdout successful Bash
FAQ
Q: What’s the quality betwixt > and 2>?
A: > redirects stdout (modular output), piece 2> redirects stderr (modular mistake).
Mastering the usage of /dev/null 2>&1 and watercourse redirection successful broad unlocks a fresh flat of power and ratio successful your bid-formation interactions. By knowing however to manipulate these streams, you tin streamline your scripts, negociate output efficaciously, and finally go a much proficient bid-formation person. Research the supplied sources and experimentation with antithetic redirection strategies to solidify your knowing and combine this almighty implement into your regular workflow. This cognition volition undoubtedly be invaluable arsenic you proceed your travel successful the planet of bid-formation computing. See exploring associated subjects specified arsenic procedure direction, inheritance processes, and impressive dealing with to additional heighten your bid-formation expertise.
Question & Answer :
#!/bin/bash /and so on/apf/apf -f >> /dev/null 2>&1 /and so forth/apf/apf -s >> /dev/null 2>&1
It’s flushing and reloading the firewall.
I don’t realize the >> /dev/null 2>&1 portion.
What is the intent of having this successful the cron? It’s overriding my firewall guidelines. Tin I safely distance this cron occupation?
>> /dev/null redirects modular output (stdout) to /dev/null, which discards it.
(The >> appears kind of superfluous, since >> means append piece > means truncate and compose, and both appending to oregon penning to /dev/null has the aforesaid nett consequence. I normally conscionable usage > for that ground.)
2>&1 redirects modular mistake (2) to modular output (1), which past discards it arsenic fine since modular output has already been redirected.
