ar: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/avr/bin/ar - from package: avr-binutils
/usr/aarch64-linux-gnu/bin/ar - from package: binutils-aarch64-linux-gnu
/usr/alpha-linux-gnu/bin/ar - from package: binutils-alpha-linux-gnu
/usr/arm-linux-gnueabi/bin/ar - from package: binutils-arm-linux-gnu
/usr/avr-linux/bin/ar - from package: binutils-avr32-linux-gnu
/usr/bfin-uclinux/bin/ar - from package: binutils-bfin-linux-gnu
/usr/c6x-uclinux/bin/ar - from package: binutils-c6x-linux-gnu
/usr/cris-linux-gnu/bin/ar - from package: binutils-cris-linux-gnu
/usr/frv-linux-gnu/bin/ar - from package: binutils-frv-linux-gnu
/usr/h8300-elf/bin/ar - from package: binutils-h8300-linux-gnu
/usr/hppa64-linux-gnu/bin/ar - from package: binutils-hppa64-linux-gnu
/usr/ia64-linux-gnu/bin/ar - from package: binutils-ia64-linux-gnu
/usr/m32r-linux-gnu/bin/ar - from package: binutils-m32r-linux-gnu
/usr/m68k-linux-gnu/bin/ar - from package: binutils-m68k-linux-gnu
/usr/microblaze-linux-gnu/bin/ar - from package: binutils-microblaze-linux-gnu
/usr/mips64-linux-gnu/bin/ar - from package: binutils-mips64-linux-gnu
/usr/am33_2.0-linux/bin/ar - from package: binutils-mn10300-linux-gnu
/usr/openrisc-elf/bin/ar - from package: binutils-openrisc-linux-gnu
/usr/powerpc64-linux-gnu/bin/ar - from package: binutils-powerpc64-linux-gnu
/usr/s390x-linux-gnu/bin/ar - from package: binutils-s390x-linux-gnu
/usr/score-elf/bin/ar - from package: binutils-score-linux-gnu
/usr/sh64-linux/bin/ar - from package: binutils-sh64-linux-gnu
/usr/sh-linux-gnu/bin/ar - from package: binutils-sh-linux-gnu
/usr/sparc64-linux-gnu/bin/ar - from package: binutils-sparc64-linux-gnu
/usr/tilegx-linux/bin/ar - from package: binutils-tile-linux-gnu
/usr/bin/ar - from package: binutils
/usr/x86_64-linux-gnu/bin/ar - from package: binutils-x86_64-linux-gnu
/usr/xtensa-linux-gnu/bin/ar - from package: binutils-xtensa-linux-gnu
/usr/i686-w64-mingw32/bin/ar - from package: mingw32-binutils
/usr/x86_64-w64-mingw32/bin/ar - from package: mingw64-binutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/aarch64-linux-gnu/bin/ar - from package: binutils-aarch64-linux-gnu
/usr/arm-linux-gnueabi/bin/ar - from package: binutils-arm-linux-gnueabi
/usr/bin/ar - from package: binutils-multiarch
/usr/bin/ar - from package: binutils
/usr/arm-linux-gnueabihf/bin/ar - from package: binutils-arm-linux-gnueabihf
/usr/lib/arm-none-eabi/bin/ar - from package: binutils-arm-none-eabi
/usr/lib/avr/bin/ar - from package: binutils-avr
/usr/h8300-hitachi-coff/bin/ar - from package: binutils-h8300-hms
/usr/i586-linux-gnu/bin/ar - from package: binutils-i586-linux-gnu
/usr/m68hc11/bin/ar - from package: binutils-m68hc1x
/usr/mips64el-linux-gnuabi64/bin/ar - from package: binutils-mips64el-linux-gnuabi64
/usr/mipsel-linux-gnu/bin/ar - from package: binutils-mipsel-linux-gnu
/usr/mips-linux-gnu/bin/ar - from package: binutils-mips-linux-gnu
/usr/msp430/bin/ar - from package: binutils-msp430
/usr/powerpc64le-linux-gnu/bin/ar - from package: binutils-powerpc64le-linux-gnu
/usr/powerpc-linux-gnu/bin/ar - from package: binutils-powerpc-linux-gnu

You may install the required package with command apt-get install {package}

Error in other languages:
ar: bevel nie gevind nie
ar: comandă negăsită
ar: comando não encontrado
ar: commande introuvable
ar: command not found
ar: käsku ei ole
ar: không tìm thấy lệnh
ar: komanda nerasta
ar: Kommando nicht gefunden.
ar: kommandot finns inte
ar: komut yok
ar: nie znaleziono polecenia
ar: níor aimsíodh an t-ordú
ar: no se encontró la orden
ar: no s'ha trobat l'ordre
ar: opdracht niet gevonden
ar: parancs nem található
ar: perintah tidak ditemukan
ar: príkaz nenájdený
ar: příkaz nenalezen
ar: команда не найдена
ar: командата не е открита
ar: 命令找不到
ar: コマンドが見つかりません