This piece is really more of a transcription or an arrangement, rather than an actual piece composed by Alexandre Guilmant (1837-1911). It is a movement taken directly from "L'enfance du Christ", composed by Hector Berlioz (1803-1869).
"L'enfance du Christ" ("The Childhood of Christ"), Opus 25, is an oratorio based on the Holy Family's flight into Egypt. Berlioz wrote his own words for the piece. Most of it was composed in 1853 and 1854, but it also incorporates an earlier work La fuite en Egypte (1850). It was first performed at the Salle Herz, Paris on 10 December 1854. He described L'enfance as a Trilogie sacrée (sacred trilogy). The first of its three sections depicts King Herod ordering the massacre of all newborn children in Judaea; the second shows the Holy Family of Mary, Joseph, and Jesus setting out for Egypt to avoid the slaughter, having been warned by angels; and the final section portrays their arrival in the Egyptian town of Sais where they are given refuge by a family of Ishmaelites.
What Guilmant has successfully done is to make the movement work into an organ solo, which works beautifully on the Metz Cavaillé-Coll. The "rustic" sounds of the Positif Basson sound great as the "forte" tone of the shepherd's pipes, but is a bit too loud on the "piano" spots. Still, the overall effect is just about as nice as you could want.
This work, sung as "The Shepherds Farewell" used to be VERY popular. Perhaps TOO popular for some tastes... ;-)
After its long reign, it sort of "disappeared" for awhile as more "scholarly ears" found it to be "corny" or "outdated." I think these opinions came from hearing too many poor performances, rather than the music itself. Choirs like King's College Cambridge began including it in their Festival of Lessons and Carols (they sang it just a few days ago), and the piece has regained its status.
It is very pleasant when nicely performed! :-)