Essential Linux Commands: pgrep, pstree, bc, split, nl, ldd, col, lsof Explained

pgrep

pgrep name starts with p, indicating it is related to processes; it lists process IDs.

Find SSH server PID

pgrep ssh
1529
28439
28442
28719

pgrep prints each matching PID on a separate line; the -d option lets you specify a delimiter.

pgrep ssh -d " "
1529 28439 28442 28719 28810 28813

-l option shows name and ID

pgrep -l ssh
1529 sshd
28439 sshd
28442 sshd
28719 sshd

pstree

This command displays the process hierarchy as a tree.

systemd─┬─NetworkManager───2*[{NetworkManager}]
        ├─VGAuthService
        ├─abrt-watch-log
        ├─abrtd
        ├─agetty
        ├─atd
        ├─auditd───{auditd}
        ├─crond
        ├─dbus-daemon───{dbus-daemon}
        ├─firewalld───{firewalld}
        ├─gssproxy───5*[{gssproxy}]
        ├─irqbalance
        ├─java───28*[{java}]
        ├─lsmd
        ├─lvmetad
        ├─master─┬─pickup
        │        └─qmgr
        ├─mcelog
        ├─nginx───nginx
        ├─ntpd
        ├─polkitd───6*[{polkitd}]
        ├─redis-server───4*[{redis-server}]
        ├─rngd
        ├─rpcbind
        ├─rsyslogd───2*[{rsyslogd}]
        ├─smartd
        ├─sshd─┬─sshd───sshd───sftp-server
        │      └─sshd─┬─bash───pstree
        │             └─sftp-server
        ├─systemd-journal
        ├─systemd-logind
        ├─systemd-udevd
        ├─tuned───4*[{tuned}]
        └─vmtoolsd───2*[{vmtoolsd}]

bc

This command provides high-precision arithmetic, such as square-root calculations. Example script (sqrt) uses bc.

#!/bin/bash

if [ $# -ne 1 ]
then
    echo 'Usage: sqrt number'
    exit 1
else
    echo -e "sqrt($1)
quit
" | bc -q -i
fi

./sqrt.sh 123.00
sqrt(123.00)
11.09
quit

split

Splits a large file into smaller pieces.

split -b 100m mysql.tar
ls -lh
-rw-r--r--. 1 root root 517M Jun 21 22:35 mysql.tar
-rw-r--r--. 1 root root 100M Jun 21 22:35 xaa
-rw-r--r--. 1 root root 100M Jun 21 22:35 xab
-rw-r--r--. 1 root root 100M Jun 21 22:35 xac
-rw-r--r--. 1 root root 100M Jun 21 22:35 xad
-rw-r--r--. 1 root root 100M Jun 21 22:35 xae
-rw-r--r--. 1 root root  17M Jun 21 22:35 xaf

File merging:

cat xa* > mysql.tar

nl

Similar to cat but adds line numbers.

nl rumenz.sh
     1  123
     2  345
     3  rumenz.com
     4  111
     5  222

ldd

Shows the shared libraries required by an executable.

ldd /usr/bin/ls
linux-vdso.so.1 =>  (0x00007ffdb51ba000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f4518ead000)
libcap.so.2 => /lib64/libcap.so.2 (0x00007f4518ca8000)
libacl.so.1 => /lib64/libacl.so.1 (0x00007f4518a9f000)
libc.so.6 => /lib64/libc.so.6 (0x00007f45186d1000)
libpcre.so.1 => /lib64/libpcre.so.1 (0x00007f451846f000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007f451826b000)
/lib64/ld-linux-x86-64.so.2 (0x00007f45190d4000)
libattr.so.1 => /lib64/libattr.so.1 (0x00007f4518066000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f4517e4a000)

col

Converts man pages to plain text.

PAGER=cat
man ls | col -b > ls.txt

lsof

Lists open files and the processes using them.

lsof rumenz.txt          # show processes that opened rumenz.txt
lsof -c nginx            # show files opened by processes whose name starts with nginx
lsof -p 1234             # list files opened by PID 1234
lsof -g gname/gid       # show processes belonging to group name or gid
lsof -u uname/uid       # show processes belonging to user name or uid
lsof +d /usr/local/     # show files opened under directory
lsof +D /usr/local/     # recursively search directory (slower)
lsof -d 4               # show processes using file descriptor 4
lsof -i                 # show network-related processes