Master Linux Shell Tricks: Formatting Output, Loops, Sorting, and Powerful Xargs

1. Output formatting

For commands whose output is hard to read, such as mount, you can pipe the result to column -t to align columns. mount | column -t Similarly, the content of files like /etc/passwd can be formatted by specifying a delimiter:

cat /etc/passwd | column -t -s:

2. Repeat command until success

You can use an infinite while true loop to keep trying a command until it succeeds, for example pinging a server until it is live:

while true
 do
   ping -c 1 baidu.com > /dev/null 2>&1 && break
 done;

The redirection >/dev/null 2>&1 discards both standard output and error.

3. Sort process list by CPU or memory

The ps aux output shows CPU usage in column 3 and memory usage in column 4.

Sort by memory (column 4):

ps aux | sort -rnk 4

Sort by CPU (column 3):

ps aux | sort -rnk 3

4. Execute the previous command with root privileges

If you forget to prepend sudo to a command that requires root, you can re‑run the last command with sudo !!:

[vagrant@localhost ~]$ cat /etc/shadow
cat: /etc/shadow: Permission denied

Running: sudo !! will execute cat /etc/shadow as root.

5. Powerful Xargs command

Example (1)

Given a file urls.txt containing a list of URLs, you can download them all with a single pipeline:

cat urls.txt | xargs wget

xargs

takes the output of cat and passes each line as an argument to wget.

Example (2)

To kill all tomcat processes:

ps -ax | grep tomcat | grep -v grep | awk '{print $1}' | xargs kill -9

grep tomcat

selects lines containing “tomcat”, grep -v grep excludes the grep process itself, awk '{print $1}' extracts the PID column, and xargs kill -9 terminates those PIDs.

Example (3)

If the target command needs multiple arguments, such as copying files, you can use xargs -i to substitute each input line: ls *.txt | xargs -i cp {} /tmp This copies every *.txt file to /tmp.

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